The Hidden!: "High Brightness X-Ray Source for Directed Energy and Holographic Imaging Applications"

28 Jan 2020

"High Brightness X-Ray Source for Directed Energy and Holographic Imaging Applications"

PO Bo;08

Chicago, IL S F- CTF 60610-0084 APR 1 7 1992 00D S~C 00 31 March 1992

PHASE II FINAL REPORT

"High Brightness X-Ray Source for Directed Energy and Holographic Imaging Applications"

Prepared for: Dr. Paul Kepple Plasma Physics Division Naval Research Laboratory Code 4722 4555 Overlook Avenue S.W. Washington, D.C. 20375-5000

Prepared by: Dr. Armon McPherson, Principal Investigator and Dr. Charles K. Rhodes, President MCR Technology Corporation P.O. Box 10084 Chicago, IL 60601-0084

Contract // N00014-89-C-2274

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TABLE OF CONTENTS

ABSTRACT ................. ...............................

I. INTRODUCTION ................... ........................... 1

I1. PHASE I! RESEARCH ................. ........................ 4

A. DEVELOPMENT OF ALGORITHMS FOR HOLOGRAPHIC IMAGE RECON$TRUCTION ............... ....................... 4

1. 3-0 Algorithms .............. ...................... 4 2. Images of Drosophila Embryo ......... ................ 4

B. HOLOGRAPHIC MICROSCOPE ............ .................. 6

C. ELECTROMAGNETIC PROPAGATION ........ ................ 9

1. Capillary Guiding Structures ......... ................. 12 2. Dynamical Guiding of Electromagnetic Propagation ........ ... 12

III. CONCLUSIONS .............. ........................... ... 13

IV. REFERENCES ................... ............................ 14

APPENDICES ............... ............................... ... 16

Appendix A: "Charge-Displacement Self-Channeling as a Method for Energy Concentration" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Appendix B: Statement of Work ............ .................. 20

Appendix C: "Fourier Transform Holographic Microscope ......... 23

Appendix D: "Fourier-transform holographic microscope .. ....... 40

Appendix E: "Prospects for x-ray hoiography with free electron lasers" . ........... ....................... ... 48

Appendix F: University of Illinois at Chicago Final Technical Report ............ ........................ .. 59

Appendix G: "Prospects for X-Ray Amplification with Charge-Displacement Self-Channeling"....... . .................. .. 127

Appendix H: "Stabilization of Relativistic Self-Focusing of Intense Subpicosecond Ultraviolet Pulses in Plasmas .. ..... 135

Appendix 1: "Stable Channeled Propagation of Intense Radiation in Plasmas Arising from Relativistic and Charge-Displacement Mechanisms" . ............. .................... 139

Appendix J: "Observation of Relativistic/Charge-Displacement SelfChanneling of Intense Subpicosecond U'travlolet (248 nm) Radiation In Plasmas" . ......... .............. 150

iii

ABSTRACT

Advances in x-ray imaging technology and x-ray sources are such that a new

technology can be brought to commercialization enabling the three-dimensional (3

D) microvisualization of hydrated biological specimens.

- "--.

Statement A per telecon.. .

Dr. Paul Kepple NRL/Code 4722 . ........ Washington, DC 20375-5000 1 ___

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I. INTRODUCTION

The Company is engaged in a program whose main goal is the development of

a new technology for direct three dimensional (3-D) x-ray holographic imaging. It

is believed that this technology will have a wide range of important applications

in the defense, medical, and scientific sectors. For example, in the medical area,

it is expected that biomedical science will constitute a very active and substantial

market. The basis of this view, is represented directly below.

The application of physical technologies for the direct visualization of biological

entities has had a long and extremely fruitful history. The invention of the light

microscope in the 17th century and the development of the electron microscope shortly

before World War II, have obviously been enormously successful scientifically.

Equally significantly, these two landmark advances, in addition to revealing radically

new physical features of the human environment, have also had a profound and

unexpected influence on man's spiritual perception of his world. The light microscope

opened up an unseen universe, not only of strange plant and animal life, but also

one embodying new shapes and forms, serving to challenge and stimulate the

mind. The electron microscope, by greatly enhancing the spatial resolution achievable,

led to further seminal findings, such as the first views of viral particles and the

complex cytoskeletal structure of cells. As a consequence of the broad biological

and medical applications of these two technologies, both now represent valuable

basic and widely used tools in biological research and clinical medicine.

Presently, the opportunity exists to take another step in this historical

process with the development of a powerful new means of visualization, three

dimensional 'x-ray micro holography.1 Importantly, unlike electron microscopy.

which generally requires stained and desiccated specimens, this new technology

will uniquely combine a main feature of the light microscope, the ability to observe

hydrated living matter, with the principal capability of the electron microscope,

the power to resolve very small spatial elements, such as those defining the

cytoskeletal architecture.

This micro holographic technology is composed of two basic and separate

components. They are (A) an x-ray laser and (B) an x-ray holographic camera

designed specifically for biological specimens. The suitability of x-ray laser

technology for this type of imaging arises very naturally, since x-rays (i) can

readily penetrate hydrated matter, (ii) intrinsically embody a short spatial scale

length permitting high resolution, a- (iii) can attain extraordinarily high exposure

rates as a direct consequence of the very high source brightness

The holographic camera has been sufficiently well developed by th,- Company

that a prototype instrument 2- 4 is currently under development. Indeed, since the

capability of this technology is quite general, it has been shown by the Company

that it is possible to develop a technique of direct imaging for the sequencing and

mapping of the human genome based on a modification of this prototype. However,

as noted above, the full potential of holography fo. hydrated specimens, can only

be realized when it incorporates a suitable x-ray laser. The resulting 3-D images,

on account of the rate of radiation damage, require a sour'7e with a brightness

that can only be achieved with an x-ray laser. It is important to note that the

use of such a laser uniquely provides two important new functions, atomic specificity

and "stop-action" imaging, By appropriately selecting the laser wavelength, it is

possible to selectively enhance the imaging of a particular atomic species of

biological interest such as nitrogen, phosphorus, sulfur, calcium, and potassium,

among others. Finally, the exposure time, predicted to be - 10" 3 seconds, is

faster than most biological or chemical processes.

Special conditions are required to produce the amplification needed for laser

action at x-ray wavelengths. Amplification In the x-ray region mainly requires

prodigious energy deposition rates spatially organized in a high-aspect-ratio volume

of material. The fundamental requirement, therefore, reduces to the efficient

deposition of energy at very high specific powers in a spatially controlled and nonthermal manner. Canonically, a deposition rate in the 10"7 - 10i 9 W/cm 3

range is necessary.

To meet this requirement, we have developed an approach that uniquely

combines three different components: (1) an energy deposition mechanism based

on a highly nonlinear coupling 6 '7 to the source of energy used for excitation of the

x-ray amplifying medium, (2) a mode of channeled propagation8ý- 10 capable of guiding

both the ultraviolet and x-ray radiation, and (3) a new extremely high brightness

ultraviolet pulsed laser technology 1 1 ,12 to provide the source of energy for excitation.

It can be seen that these three elements, which appear capable of producing

and controllinq the most extreme conditions of power density yet realized with

laboratory-scale apparatus, fit together in a remarkably congenial way, as outlined

in Appendix A. Importantly, the conditions needed to produce the (1) strong

nonlinear coupling observed experimentally are exactly (2) those required for

channeled propagaton and (3) that these conditions can be achieved by present

laser technology. The confluence of these three conditions is the key feature of

this method for the efficient attainment of the very high-energy-density states of

matter needed for x-ray amplification.

This report deals with two central aspects of the developing technology

which cirectly affect the feasibility of commercial application. They are (1) the

technology for ,iolographic image reconstruction and (2) the x-ray source technology

in particular, th,. issue associated with the optimum spatial control of the deposition

of energy. The effort on both of these questions is aimed principally at reducing

the complexity, cost, and physical scale of the instrumentation needed to serve

the targeted applications, Most significantly, the results show indeed that a small

laboratory instrument is possible. Full satisfaction of the Statement of Work

represented in the Phase II proposal has been accomplished.

I. PHASE II RESEARCH

The Statement of Work for the Phase II effort is presented for reference in

Appendix B.

A. DEVELOPMENT OF ALGORITHMS FOR HOLOGRAPHIC IMAGE RECONSTRUCTION

1. 3-D Algorithms

Algorithms suitable for the reconstruction of 3-D images in the x-ray range

have been developed and tested by appropriate simulations at visible wavelengths.

Appendix C gives a description of the important aspects of this work.

2. Images of Drosophila Embryo

Other subsequent experiments have shown that a properly prepared Drosophila

embryo can serve as a very convenient and informative model system for examining

the 3-D imaging properties of the microscope. Specifically, since we desire images

with (1) high spatial resolution, (2) biological structural specificity, (3) 3-D informa

tion, and (4) high contrast, a staining procedure involving silver-enhanced immuno

gold complexes 13 was used. This stain was used to visualize the segmentation of

the Drosophila embryo as displayed by using an antibody against the engrailed

protein.14

A reconstructed image of a Drosophila embryo, at three successive depths,

(z), is shown in Fig. (1) taken under dark-field illumination. The study of such

images is informing on the operational characteristics of the system such as (1)

t,,e sensitivity of the image to the conditions of exposure, (2) the staining procedure,

and (3) the numerical procedures used in the reconstruction. These characteristics

represent simulations of their counterparts in the x-ray regime. Of course, the

high contrast that can be naturally obtained in the x-ray region will eliminate the

need for staining.

Several important aspects of the imaging system are revealed by examining

the reconstructions shown in Fig. (1). They are (1) the high contrast shown by

the banding, (2) the sharp edges, demonstrating the high spatial resolution, shown

most clearly at the edge of the embryo. (3) the absence of speckle, and (4) the

3-D character of the images. The latter point is apparent through examination of

the region of invaginatlon toward the bottom right of the embryo.

Fig. (1): Reconstructed images of Drosophila embryo stained with silver-enhanced Immuno-gold complexes. The image reveals the banded pattern governed by the segmentation genes. The three panels represent reconstructions corresponding to three different depth (z) planes in the specimen. In the left panel, z = -t54 pm: in the center panel, z = 0 pm; in the right panel z = -54 pm. The full size of the reconstructed image is - 256 pm, as indicated by the black edges at the left and right of the embryo.

B. HOLOGRAPHIC MICROSCOPE

Two basic types of holographic microscopes have been constructed using the

Fourier configuration. One version utilizes a glycerol microdrop as the reference

wave scatterer. This instrument is described in Appendix D.

A second basic system we have used for the studies conducted with visible

radiation is shown in Fig. (2). A convenient configuration is a modification of a

Mach-Zehnder (MZ) interferometer geometry. In this configuration have we

replaced the spherical reference scatterer with a suitable microscope objective,since,

for visible wavelengths, this permits facile and flexible operation. The primary

beam of coherent radiation is split into two beams with a beam splitter. Advantage

is taken of the inherent polarization of the coherent laser radiation, and a polarizing

beam splitter is used. For these studies, the polarization is utilized to control

the relative intensity of the two subbeams. In the x-ray case, this relative

intensity would be governed by the radius of the spherical reference scatterer

One subbeam of the MZ interferometric section of the microscope is used to

illuminate the biological specimen, while the other subbeam is brought to a focus

by illumination of a compound microscope (CM) objective. Both subbeams are

then recombined in a beam-combiner cube and these recombined beams, which

form the holographic interference of the specimen, diverge to fill the aperture

subtended by the detector (CCD). The fourth direction of the beam combiner 1s

occupied by a CM to permit alignment of the specimen and the reference beam

This configuration of the microscope functions as a dark-field microscope and has'

performed exceptionally well at NA - 0.50 with highly scattering biological specimens

C izi e ti v eM ic . 15sC c c

900 Ieani Combining 0..ce Rotation -ci7nlPlate Seie ld --~~~1 ~-Cylindrical L~ens

Polarizimg E4asm Splitter

Retardation Plate

Et!iNarrowed SinglIe Mode Ar

Fig. (2): Schematic of Mac~h-Zehnder holographic microscope.

A photograph of the existing instrument, which corresponds to the sichemnatic

presented in Fig. (2), which was used for the studies at visible wavelengths is

given in Fig. (3a). The specific design incoiporating the same concepts that has

already been built for work at x-ray wavelengths is shown in Fiq. (3b). As

described In Appendix E, this x-ray camera is sufficiently flexible that it can be

used with various sources, such as a free-electron laser.

Studies of the characteristics of operation of this instrument have been

performed on several biological materials These include (1) parascarls univalens larva.

(2) a fish melanoma histopathology specimen (Xiphophorus maculalus x heileri), (3)

a section through duodenum Including a nerve ganglion, and, as noted above, (4) a

Drosophila embryo. The latter was used to examine specifically the three-dimensional

character of the imaging.

Fig. (3): (a) Photograph of the visible holographic microscope. (b) Photograph of x-ray holographic system.

C. ELECTROMAGNETIC PROPAGATION

As outlined in Section I, the .patial control of the energy deposited for

excitation of an x-ray amplifier plays an important role in the fundamental scaling

relationship between the required ,nergy, the gain and the wavelength. The

critical governing issue, which determines the scaling relationship between the

required excitation energy (E) and the amplifier gain (G) of x-ray lasers, is the

spatial control of the deposited energy. The information presented in Fig. (4)

shows that optimizing the gain (G) per unit energy (E) calls for the guided mode

of propagation in order to optimaLly control the deposition of the energy. Overall,

in comparison to traditional forms of excitation, for a fixed x-ray energy output

(Ex) arid wavelength (Xx), a reduction of several orders of magnitude in the

necessary energy (E) results, as shown in Fig. (5), if this forri of confined (channeled)

propagation can be achieved. Therefore, if this scaling holds, a relatively small

and useful laboratory-scale technology becomes feasible. One joule of energy is

seen as sufficient up to a wavelength of several kilovolts (- 1.6 A).

At least two types of conditions can lead to the desired confined propagation

They involve (1) the use of guiding structUres, s'ich as static waveguides, or (2)

propagation that is dyna.mic_.aly confined 15 Both aspects nave been considered.

X-Ray Laser Scaling Spatial Distribution/Amplifying Volume

S/ (densities P, p*, P,) particle,inversion electron) S\ ,>E

X~N

>_t- \ '

4 6

= G =- t*r

Laboratory Scale Technology-.G ru.O

E - WV small 1E = " (5 > d• Large 1/6, but 6 N> I free space Really want to set-- propagation I>> Rayleigh range t F 6 / -•,loss length 6small p iarge as possible *. Guided Mode of Propagation

Fig. (4): Spatial distribution of energy of excitation (E) for an x-ray (hwx) amplifier. Parameters are the same as in Fig. (5) with X the wavelength of the excitation energy, assumed longitudinally delivered, and with p, p*, and p. representing the particle, inversion and electron densities, respectively. The analysis shows that optimization of G/E requires a guided mode of propagation so that high concentrations of power can be organized into high-aspect--ratio spatial volumes.

Energy (E)/Wavelength (N) Scaling

GThco 62 •. "O× 0-x1G --- A (5

100

10-1 -- G = 102

ii 10-2 6 = 3 Am "> lo-3 a- X 1/4 (rXR

0) C: 10-4

10-5

I J

S 1 O-Bli

0.1 1.0 10.0

Quantum Energy (fhcwx) keV

Fig. (5): Scaling relationship between required excitation energy (E) and quantum energy (hwx) characteristic of the amplifier. Parameters: total gain exponent G - 100, energy efficiency rx = 10"3, channel diameter 6 = 3 pim, x-ray (hwx) cross section for stimulated emission ax, x-ray cross section for stimulated emission for radiatively-broadened transition oXp.

*11

1. Capillary Guiding Structures

A static guiding structure, assuming that the wave can be properly launched

and that the losses are sufficiently low, could potentially serve to provide the

desired confined propagation. This possibility was experimentally explored, under

subcontract to the Company, by the University of Illinois at Chicago. Basically,

it was found that the use of startc guiding structures is not feasible for the

range of parameters required for x-ray amplification. The report of the subcontractor

is given in Appendix F.

2. Dynamical Guiding of Electromagnetic Propagation

Theoretical work 8 ,16- 19 indicates that dynamical guiding of very intense pulsed

radiation may be possible under conditions suitable for x-ray amplification. The

essential aspects of this theoretical work are given In Appendix A and Appendices

(G - 1).

Significantly, experimental evidence now exists, as described in Appendix J,

that demonstrates for the formation of such a mode of propagation. Specifically,

the experimental studies have examined a new relativistic regime of high-intensity

short-pulse propagation in plasmas which indicates the development of a stable

mode of spatially confined (channeled) propagation. For an electron density of

1.35 x 1021 cm" 3 and a power of - 3 x 10l W, the results indicated a channel

radius < 1 pm and a peak intensity - 10"9 W/cm 2. Comparison of these findings

with a dynamical theory yield agreement for both the longitudinal structure and

the radial extent of the propagation observed.

The most important outcome of these findings is that it enables the scaling

relationship shown in Fig. (5). The existence of this mode of propagation makes

possible a small laboratory-scale x-ray laser technology that can serve a multitude

of applications including the importance biomedical ones of interest to the Company.

III. CONCLUSIONS

The implications of developments achieved under the support of this work

for general applications to x-ray imaging and the microcharacterization of condensed

matter are extremely important and propitious. In terms of the x-ray source,

they are (1) that a properly controlled energy deposition rate, sufficient for the

production of stimulated x-ray sources up to a few kilovolts in quantum energy,

can now be achieved with an excitation energy of - 1 J, (2) that an x-ray output

energy of - 1 mJ per pulse is achievable with laboratory-scale technology, and (3)

that an x-ray beam diameter (- 2 - 3 pm) arises as a natural consequence of the

physics. These parameters represent an exceptionally high peak brightness figure

that permits a new and completely unexplored range of physical measurements to

be made. Indeed, a high-brightness source of this nature is ideal for the microimaging

of condensed matter. In particular, an x-ray source with these parameters is perfectly matched to the requirements for holographic imaging of biological materials 1

3,21 in terms of all its relevant properties, specifically, wavelength (10 - 40 A), pulse

energy (- 1 mJ), pulse length (- 10-'3 s), beam diameter (- 2 - 3 Pm), and divergence

(- 1 mrad). Finally, the 3-D camera technology matched to this source is available

IV. REFERENCES

1. J. C. Solem and G. C. Baldwin, "Micro Holography of Living Organisms", Science 218, 229 (1982).

2. W. S. Haddad, D. Cullen, K. Boyer and C. K. Rhodes, "Design for a Fourier Transform Holographic Microscope", X-Ray Microscopy II, edited by D. Sayre, M. Howe!ls, J. Kirz, and H. Rarback (Springer-Verlag, Berlin, 1988) p. 284-287.

3 W. S. Haddad, J. C. Solem, D. Cullen, K. Boyer, and C. K. Rhodes, "A Description of the Theory and Apparatus for Digital Reconstruction of Fourier Transform Holograms", in Electronics Imaaina '87, Advanced Printing of Paper Summaries, Vol. II (Institute for Graphic Communication, Inc., Boston, 1987) pp.. 683--688.

4. W. S. Haddad, D. Cullen, J. C. Solem, J. W. Longworth, A. McPherson, K. Boyer, and C. K. Rhodes, "Fourier Transform Holographic Microscope", manuscript submitted to Applied Optics.

5. W. Haddad, K. Boyer, R. M. Moriarty, J. C. Solem, and C. K. Rhodes, "Genome Sequencing by Direct Imaging X-Ray Color Holography", Proceedings of the Workshop on X-Ray Micro Mapping for the Life Sciences, May 24-26, 1989 (Lawrence Berkeley Laboratory, Berkeley, CA, 1989) LBL Report No. 27660, p. 81-84.

6. K. Boyer, H. Jara, T. S. Luk, I. A. McIntyre, A. McPherson, R. Rosman, and C. K. Rhodes, "Limiting Cross Sections for Multiphoton Coupling", Revue Phys. Appl. 22, 1793 (1987).

7. C. K. Rhodes, "Multiphoton Ionization of Atoms", Science 229, 1345 (1985).

8. J. C. Solem, T. S. Luk, K. Boyer, and C. K. Rhodes, "Prospects for X-Ray Amplification with Charge-Displacement Self Channeling", IEEE J. Quantum Electron. QE-25, 2423 (1989).

9. A. B. Borisov, A. V. Borovskiy, 0. B. Shiryaev, V. V, Korobkin, A. M. Prokhorov J. C. Solem, T. S. Luk, K. Boyer, and C. K. Rhodes, "Relativistic and Charge Displacement Self-Channeling of Intense Ultrashort Laser Pulses in Plasmas", submitted to Physical Review A. accepted 9 January 1992.

10. A. B. Borisov, A. V. Borovskiy, V. V. Korobkin, A. M. Prokhorov, 0. 6 Shiryaev, X. M. Shi, T. S. Luk, A. McPherson, J. C. Solem, K. Boyer, and C K. Rhodes, "Observation of Relativistic/Charge-Displacement Self-Channeling of Intense Subpicosecond Ultraviolet (248 rim) Radiation in Plasmas", submitted to Phys. Rev. Lett., 12 November 1991.

11. A. P. Schwarzenbach, T. S. Luk, I. A. McIntyre, U. Johann, A. McPherson, K Boyer, and C. K. Rhodes, "Subpicosecond KrF*-Excimer-Laser Source", Opi Lett. 11, 499 (1986).

12. T. S. Luk, A. McPherson, G. Gibson, K. Boyer, and C. K. Rhodes, "Ultrahigh Intensity KrF* Laser System", Opt. Lett, 14, 1113 (1989).

13. G. Danscher and J. Norguard, "Light Microscope Visualization of Colloidal Gold on Resin Embedded Tissue", J. Histochem. Cytochem. 31, 1394-1398 (1983).

14. B. Alberts, D. Bray, J. Lewis, M. Raff, K. Roberts and J. D. Watson, Molecular Biology of the Cell, Second Edition (Garland Publishing, Inc., New York, 1989( p. 928.

15. K. Boyer, A. B. Borisov, A. V. Borovskiy, 0. B. Shiryaev, D A. Tate, B. E. Bouma, X. Shi, A. McPherson, T. S. Luk, and C. K. Rhodes, "Methods of Concentration of Power in Materials for X-Ray Amplification". Appl. Optics, in press.

16. P. Sprangle, E. Esaray, and A. Ting, "Nonlinear Theory of Intense Laser Plasma Interactions", Phys. Rev. Lett. 64., 2011 (1990).

17. A. B. Borisov, A. V. Borovskiy, V. V. Korobkin, A. M. Prokhorov, C. K. Rhodes, and 0. B. Shiryaev, "Stabilization of Relativistic Self-Focusing of Intense Subpicosecond Ultraviolet Pulses in Plasmas", Phys. Rev. Lett. ., 1753 (1990).

18. A. B. Borisov, A. V. Borovskiy, 0. B. Shiryaev, V. V. Korobkin, A. M. Prokhorov, J. C. Solem, T. S. Luk, K. Boyer, and C. K. Rhodes. "Relativistic and Charge Displacement Self-Channeling of Intense Ultrashort Laser Pulses in Plasmas", Phys. Rev. A, in press.

19. A. B. Borisov, A. V. Borovskiy, V. V. Korobkin, A. M. Prokhorov, 0. B. Shiryaev, X. M. Shi, T. S. Luk, A. McPherson, J. C. Solem, K. Boyer, and C. K. Rhodes, "Observation of Relativistic/Charge-Displacement Self-Channeling of Intense Subpicosecond Ultraviolet (248 nm) Radiation in Plasmas", Phys. Rev. Lett., submitted.

20. W. S. Haddad, D. Cullen, J. C. Solem, K. Boyer, and C. K. Rhodes, "X-Ray Fourier-Transform Holographic Microscope", in Short Wavelength Coherent Radiation: Generation and Applications, Vol. 2, R. W. Falcone and J. Kirz, eds. (Optical Society of America, Washington, D.C., 1988) pp. 284-289.

APPENDIX A Reprinted from the

OSA Proceedings on

Short Wavelength Coherent Radiation: Generation and Applications

Volume 2

Charge-Displacement Self-Channeling as a Method for Energy Concentration

K Boyer, T. S. Luk, J. C. Solem, nnd C. K Rhodes Laboratory forAtomic, Molecular, and Radiation Physics, Department of Physics University of Illinois at Chicago, P.O. Box 4348, Chicago, Illinois 60680

Proceedings of the OSA Topical Meeting on Short Wavelength Coherent Radiation: Generation and Applications September 26-29, 1988, North Falmouth, Cape Cod, MA

Charge-Displacement Self-Channeling as a Method for Energy Concentration

K. Boyer, T. S. Luk, J. C. Solem,t and C. KI Rhodes

Laboratory for Atomic, Molecular, and Radiation Physics, Department of Physics University of illinois at Chicago, P.O. Boa 4348, Chicago, Illinois 60680

Abstract Indeed, preliminary analysis 171 supports the presence of the following mechanism. For a The concentration of energy arising from charge- sufficiently short pulse (- 100 fis), the massive displacement self-channeling Is discussed. Since Ions remain spatially fixed while the relatively high energy deposition rates are expected to mobile electrons are expelled by the ponderomotive arise from multiphoton coupling and the channel force from the high Intensity zone. Thereby, a can also serve as an effective waveguide for state of equilibrium can be established between secondary radiation, such circumstances are the ponderomotive and the electrostatic force ideal for generating coherent short wavelength densities owing to the charge displacement. radiation. Since the electrons, which embody a negative contribution to the Index. are expelled, and Channeled Propagation axis region of relatively high refractive Index Is formed which supports the channeling. In a Amplification In the x-ray region requires prodigious limiting case, It appears that the behavior approaches energy deposition rates (1] spatially organized In that characteristic of a metallic waveguide. a high-aspect-ratio volume of material. The Interestingly, a focusing mechanism of this type fundamental question, therefore, centers on the is not describable In terms of the conventional [8) controlled deposition of energy at high specific nonlinear Index parameter nr. A further unusual powers. We show that the use of extremely aspect of this process, since there Ip a strong Intense (1013 - 10" W/cm') short pulse (- 100 tendency to locally reduce the plasma frequency fis) radiation may be able to produce bo=h the 41, in the region where the Intensity Is high, is necessary deposition rates and spatial control by that it may enable propagation for appreciable combining (1) the energy deposition 12,31 arising distances in plasmas that would normally be from high-order multiphoton processes with (2) a considered as overdense. new mode of channeled propagation Involving a The essence of the Idea can be understood In charge-displacement mechanism. A significant the following manner. The steady-state force point that will emerge Is that the conditions belance between the radially outward ponderomotive needed for the strong multiphoton coupling are force and the oppositely directed electron-ion identical to those found required for the confined attraction Is represented, in cylindrical symmetry propagation, for a completely ionized tenuous (,w >> ip) It appears 14-41 that a fundamentally new regime plasma, by of electromagnetic propagation will develop In plasmas for subpIcosecond radiation of sufficient 2-,e' p(r) (r- r') Intensity. In the high Intensity case of Interest, 7V(r) + e2nJ n. d'r' - 0. (1) in which processes generating ionization dominate Zrc the coupling, multiphoton ionization Is expected to produce a substantial reduction of both the For a cylindrical gaussian Intensity distribution linear and nonlinear refractive Indices of the I(r), the total charge density p(r) can be written medium. For a suitably Ionized plasma, this in exact form as combination of high charge state Ions with the electron density produced by the multiphoton p(r) - 25 (1 - r2/rD exp(--f/rD (2) coupling appears capable of producing a new mode of radiative channeling. In which 8 - I,(mwicnr )-' and m, c, ne, and

233

r, denote the electron mass. speed of light. Controlled Energy Concentration - Summary quiescent plasma density. and radius (HWHM) of the assumed gausslian Intensity profile., respectively. In terms of the basic question of the controlled The form of this solution Is shown In Fig. (1) for deposition of energy at high specific powles, the a value of 1 - 0.4. The expulsion of the electrons answer that emerges from these considerations from the central region Is apparent. involves three separate components. They are A charge distribution of this nature can lead (1) a new ultraviolet pulsed power technology. (10] to channeled propagation. An estimate of the (2) an energy deposition mechanism based on condition necessary :,an now be made by describing highly nonlinear coupling, 1111 and (3) a condition the charge displacement by two regions, such for channeled propagation. (71 It can be seen that that p(r < r$)-p(0) and p(r ), r")-p(r.^), these three elements, which appear capable of and equating the angle of total Internal reflection producing conditions comparable to, or possibly to the angle corresponding to the first minimum exceeding those of a thermonuclear environment, of diffraction. This procedure yields a critical (10 fit together In a remarkably congenial way. 1ntgnslt1 Figure (2) Illustrates these relationships. A principal finding Is that the radiative conditions vne'c m•Zc, needed for the strong multliphoton coupling Ic 64(1 + e)re (4.3 x 10") 2. (3) governing the energy transfer rate (11] are essentlally 1re identilcl to those require6. for the channeled propagation (7] &aW that the laser technology (101 In which r. denotes the classical electron radius. can readily produce the regime of Irradiation Significantly, this Intensity Is Independent uf the necessary. The compatibility of these three electron density ne and contrasts with the power factors Is a key feature of this method for the threshold [8) normally arising from Induced Index attainment of very high energy density states of charges In transparent dielectrics. Analytic matter. approximations extending to high electron density show a significant dependence of Ic on ne only quite near the critical density. Furthermore, It Is easily demonstrated that !c normalized to the Compton Intensity 191 Is a constant (0.54) Independent Multiphoton Energy Transfer Cross Section of frequency. a finding which shows that the charg.-dlsplacement mechanism Is associated with Ny + A --- .vring (ionization) relativist'c conditions. For a wavelength of 248 10`7 o'y nm, 1c - 2.4 x 10" W/cm". Finally, a striking aspect of the analysis 17] Is ths gxtremoly strong Limit frequency dependence favoring the use of ultraviolet wavelengths to establish the conditions for channeling. Scaling laws are derived (7] Indicating "PERTL., variations as rapid as tSYTM

C-...ONTICtN4 -/AJi/•ir/I80I

Charge-DI placement Focusing (Low Ortsity Plasma) 1 0" 0 Z

o=248

10-ftI poll 10" 1o" low 10"1 10 lol ' olo Interwity (W/cmN)

0 .----------- Figure 2. Illustration of how (1) the energy transfer cross section a. (2) the channeling condilion, and (3) the pulse power technology allI it together, 0 1 I• 2 3 The significant fact Is that the conditions needed for the strong multiphoton coupllnq governing the energy transfer (I = Ice "- 10 ' W/cm 2) are Idenica to those required for the channeled Figure 14 Illustration of the total charge density propagation (1f = Ico - 10" W/cru), and that the p as a function of normalized radial distance r/r, developing large aperture ultraviolet laser technology for 8 , 0.4. moves us Into the desired range.

234

Acknowledgements

This work was supported by AFOSR, LLNL, ONR, and SDI.

t permanent address: Theoretical Division, MS.B210, Los Alamos National Laboratory, Los Alamos, NM 87545

References and Notes

1. A. V. Vinogradov and 1. I. Sobel'man, 'The Problem of Laser Radiation Sources In the Far Ultraviolet and X-Ray Region,* Soy. Phys.-JETP 2§, 1115 (1973). 2. C. K. Rhodes, *Physical Processes at High Field Strengths,' Physics Scripta M.7, 193 (1987). 3. C. K. Rhodes, 'Multiphoton Ionization of Atoms,' Science 22W, 1345 (1985). 4. W. 8. Mori, C. Joshl, J. M. Dawson, 0. W. Forslund, and J. M. Kindel, 'Evolution of SelfFocusing of Intense Electromagnetic Waves in Plasma,' Phys. Rev. Lett. 6Q, 1298 (1988). 5. G.-Z. Sun, E. Ott, Y. C. Lee, P. Guzdar, "Self-Focusing of Short Intense Pulses in Plasmas,' Phys. Fluids 2Q, 526 (1987). 6. Ya. L. Bogomolov, S. F. Llrin, V. E. Semenov, and A. M. Sergeev, 'Ionization Self-Channeling of Extremely Intense Electromagnetic Waves in a Plasma,' PIs'ma Zh. Eksp. Teor. Fiz. AA, 532 (1987) [Engl. transi.: Soy. Phys. - JETP Lett. 45, 680 (1987)). 7. J. C. Solem, T. S. Luk, K. Boyer, and C. K. Rhodes, *X-Ray Amplification with ChargeDisplacement Self-Channeling,' Phys. Rev. Lett., submitted. 8. P. L. Kelley, "Self-Focusing of Optical Beams,' Phys. Rev. Lett. n5, 1005 (1965). 9. The Compton Intensity Is given by 1co m,,,c/41tcO% with Vc the Compton wavelength of the electron and cs the fine structure constant. The intensity is defined by the condition that the average nonrelatlvlstlcally calculated quiver speed of the electron be equal to the speed of light. 10. K. Boyer, G. Gibson, H. Jars, T. S. Luk, I. A. McIntyre, A. McPherson, R. Rosman, J. C. Solem, and C. K. Rhodes, 'Strong-Field Processes In the Ultraviolet Region,' this volume. 11. K. Boyer, H. Jars, T. S. Luk, I. A. McIntyre, A. McPherson, R. Rosman, and C. K. Rhodes, 'Limiting Cross Sections for Multiphoton Coupling," Revue Phys. Appl. 22, 1793 (1987).

235

Appendix B: Statement of Work

NRL STATEMENT OF WORK FOR MCR TECHNOLOGY CORPORATION PHASE It SBIR

1. Background Advances in x-ray imaging technology and x-ray sources are such that a new technology enabling the microvisualization of condensed materials can be brought to commercialization. In order to accomplish this goal, further development of the computational technology of holographic image reconstruction and corresponding aspects of source technology, particularly concerning the spatial control of the energy deposition, are needed. This Phase II SBIR is directed at these objectives.

2. Scope This is an SBIR Phase II contract, therefore it is anticipated that a commercially viable product will be developed under this contract.

3. Applicable Documents None.

4. Requirements Development of a high brightness x-ray source for directed energy and holographic imaging applications.

4.1.1A The contractor shall develop advanced algorithms for multidimensional holographic reconstruction from a holographic microscope operating in the visible, but able to simulate the parameters of an x-ray holographic microscope. This task to be performed in conjunction with a subcontract to Robotronix, Inc.

4.1.1B The contractor shall explore techniques for low resolution optical reconstruction of selected portions of the hologram as a means for rapid screening of data prior to numerical reconstruction.

4.1.1C The contractor shall begin the technological developments that will enable the performance of experimental studies to examine channeled propagation in capillaries with N2 serving as the medium. In these studies, the contractor shall place emphasis on (1) the launching of the wave in the capillary and (2) determining the relationship gýverning the length of propagation that can be achieved and the density of the medium. This task to be performed on a subcontract basis with the University of Illinois at Chicago.

4.1.1D In conjunction with a subcontract to Robotronix, the contractor shall develop supporting theoretical capability for the analysis of the experiments being performed for in task 4 1.1C. The contractor shall place particular emphasis on (1) the dynamics of channel formation and (2) channel stability.

4.1.1 Deliverable products

Preprints of any articles submitted to Journals for Publication, which cover any of the work under this contract. A letter type or formal progress report.

4.1.2 Schedule Tasks 4.1.1A - 4.1.1D are to be completed twelve months after start of work.

4.2.1A The contractor shall design, build, test and operate a holographic microscope operating in the visible region suitable for simulaion of the parameters governing the x-ray holographic application.

4.2.1B The contractor shall continue development of the algorithms of task 4.1.1A.

4.2.1C The contractor shall evaluate techniques for microfabrication of target systems for both visible wavelength simulation and x-ray exposures.

4.2.1D The contractor shall continue the development of the low resolution

optical reconstruction techniques started in task 4.1.11.

4.2.1E The contractor shall continue the experiments noted in task 4.1.1C.

4.2.1F The contractor shall experimentally explore the possibility of self channeling in N2. The contractor shall evaluate the suitability of other materials, both atomic and molecular, for the generation of considerably shorter wavelengths. This task to be performed on a subcontract basis with the University of Illinois at Chicago.

4.2.1G The contractor shall continue the theoretical developments of 4.1.1D,

4.2.1H The contractor shall develop a prototype of the target conversion system based on the work of tasks 4.1.1C, 4.1.1D, 4.2.1E, and 4.2.1F.

4.2.1 Deliverable products Preprints of any articles submitted to Journals for Publication, which cover any of the work under this contract. A final report containing designs, calculations, source code of and uters guide for any software developed wholly under this contract, drawings, cost estimates, test plans and conclusions resulting wholly from tasks 4.1.1A - 4.1.1D and 4.2.1A - 4.2.1H.

4.2.2 Schedule Task 4,2.1A - 4.2.1H to be completed twenty-four months after start of work.

5. Progress report3 The contractor shall submit a letter type or formal progress report at the end of twelve months and a final report by the end of twenty-four months, in addition, as directed by the COTR, the contractor shail submit viowgraphs and other material for use in program reviews.

Contract No. N00014--89--C-2274

STATEMENT OF WORK REVISIONS

Tasks 4.2.1B, 4.2.1D, 4.2.1E and 4.2.1G are revised to read as follows:

4.2.1B The contractor shall develop the algorithms c,f task 4.1.1A.

4.2.1D The contractor shall develop the low resolution optical reconstruction techniques started in task 4.11B.

4.2.1E The contractor shall do the experiments noted in task 4,1.10.

4.2.1G The contractor shall do the theoretical developments of 4.1.1D.

APPENDIX C

FOURIER TRANSFORM HOLOGRAPHIC MICROSCOPE

Waleed S. Haddad, David Cullen, Johndale C. Solem, James W. Longworth,

Armon McPherson, Keith Boyer and Charles K. Rhodes

MCR Technology Corporation

P. 0. Box 10084, Chicago, IL 60610-0084

ABSTRACT

We describe a holographic microscope with spatial resolution approaching the

diffraction limit. The instrument uses a tiny drop of glycerol as a lens to create

the spherically diverging reference illumination necessary for Fourier-transform

holography. Measurement of the point-spread function, obtained by imaging a

knife edge in dark-field illumination, indicates a transverse resolution of 1.4 Hrn

with wavelength X = 514.5 nm. Longitudinal resolution was obtwined from the

holograms by the numerical equivalent of optical sectioning. We describe the

method of reconstruction and demonstrate the microscope's capability with selected

biological specimens. The instrument offers two unique capabilities; (1) it can

collect three-dimensional information in a single pulse of light, avoiding specimen

damage and bleaching; and (2) it can record three-dimensional motion pictures

from a series ui lig:tt pulses. The conceptual design is applicable to a broaci

range of wavelengths anri wv- d;.qcuss extension to the x-ray regime.

Kay Words; Microholography, F ofuiE.--TrPrsforrn Microholography, 3iologicai

Samples, Glycerol Micro;rop, P:es'lution, Numerical Reconstruction

Refeience Scatvrper

SSubmitted to PApolied Optics, 22 April 1991, r.gvised 21 January '992

1. INTRODUCTION

Although Gabor invented holography1 with microscopy applications In mindt,

the problem of recording medium resolution 2 was not solved until the introduction

of the Fourier transform3 technique. Shortly after the technique was proposed,

Leith and Upatnieks 4 attempted Fourier-transform microholography and were able

to resolve a bar pattern spaced at 7 - 10 pm using a 632.8 nm laser. Although

their report is sketchy, VanLigten and Osterberg 5 claimed an optimum resolution

of detectability of 12 pm and very serious difficulties in obtaining a resolution of

- 5 p1m. Their holographic microscope formed a hologram from a pre-magnified

image rather than using the Fourier method. Apparently because initial results

were disappointing, visible-light Fourier-transform microholography was generally

abandoned. Most papers6 of the 70's and 80's are concerned with Its application

to x-ray microscopy.

In the present work, we describe an instrument that is capable of resolution

approaching the diffraction limit. The camera design discussed herein represents

a new technology for three-dimensional visualization that is applicable to a spectral

range spanning from visible (- 700 nm) to x-ray (- 0.6 nm) wavelengths. Descriptions

of the Fourier-transform technique, biological target design and related numerical

reconstruction simulations have been given in previous work.7 The new camera

system has been operated with visible radiation (X = 514.5 nm) from an argon-ion

laser. The experiments in the visible region have enabled the imaging properties

of the camera to be studied and optimized as well as demonstrating that this

camera design can be employed for applications in embryology where three-dimensional

information is of particular value.

t The original concept was for lensless electron microscopy.

II. MICROHOLOGRAPHY WITH VISIBLE RADIATION

A. Camera Design

Figure (1) is a schematic of the microholographic camera. The target holder

is positioned horizontally so gravity will hold the sample and a glycerol droplet in

place. A side view schematic of the target design used for visible light exposures

is shown in Fig. (2). The glycerol droplet proved to be a superb lens for generating

the reference illumination. It can spread the illumination to the large angles necessary

for good resolution and can be placed very close to the specimen. Furthermore,

its size can be varied to match the reference intensity to the intensity of radiation

scattered from the specimen.

The basic functions of the camera, such as shutter timing and triggering, are

controlled by a personal computer (PC). The holograms are recorded electronically,

u•gitized and stored by the PC. The holographic data are then transferred by an

ethernet link to a Stardent GS 2000 supergraphic work station where the numerical

image reconstruction Is performed. The reconstructed Image can then be manipulated,

scaled, and checked for three-dimensional information and displayed on a high

resolution graphics monitor.

The holograms were registered on a customized charge-coupled device (CCD)

having a sensitive area of 18.43 mm x 18.43 mm containing 2048 x 2048 square

pixels each with a dimension of 9 pm. To protect the CCD from the direct laser

beam, we installed a cover glass witth a central opaque square.

B. Liquid Microdrop Lenses

The drop of glycerol Is placed on the microscope cover slide, close to the

edge of the sample and nearest the region of interest in the sample, as shown in

FIg, (2). The droplet acts as a lcns intercepting a portion of the illuminating

plane wave front and converting it into a diverging, nearly spherical wave. The

focal point of this lens acts as the point source of reference illumination. Glycerol

is a particularly suitable liquid for the microdrops because of Its low vapor pressure.

high surface tension, and polarity. To improve the characteristics of the microdrops.

It is often necessary to pre-coat the glass cover slide with a thin layer of silicone.

Dricote, a standard anti-wetting agent for PH electrodes, proved to be adequate

for this purpose. Both the reduced wetting and the surface tension help the

glycerol to form a round lens with a relatively short focal length. This conveniently

produces a system with a low f number. The microdrop of glycerol is positioned

on top of the sample cover slide so that the illumirs :ing beam first impinges on

the object, then on the microdrop - 200 pm closer to the detector. Since the

microdrop is a positive lens, the reference point source is formed even closer to

the detector. The transverse and longitudinal resolution of the camera system is

dependent on the angular aperture subtended by the hologram. Consequently, it is

necessary that the cone angle of the diverging reference and object waves be

large enough to produce interference over an aperture which is sufficient to give

the desired resolution in the image. Hence, the f number of the microdrop lens

determines the usable f number of the camera

The microdrops are formed using a glass micropipette which is mounted on an

X-Y-Z micropositioner. The sample and micropipette tip are viewed through a

dissecting microscope while manipulating the micropipette into position with the

micropositioner. When the tip of the micropipette has been positioned properly, it

is brought almost in contact with the glass slide. A slight pressure at the back

of the pipette dispenses the desired quantity of glycerol. The tip is then slowly

lifted away from the slide, allowing the droplet to form.

The droplet size is an Important factor in the formation of a good hologram

because it is primarily the diameter of the drop that determines the brightness of

the reference wave. The larger the diameter of the drop, the larger the portion

uf the illuminating wavefront which is intercepted, and consequently, the brighter

the ruference wave. In order to form a hologram with a good signal-to-noise

ratio 6, it is necessary to have the reference wave brightness approximately equal

to the object wave brightness. Depending on the type of object being Imaged, we

have used microdrops ranging between 20 and 200 p~m in diameter.

C. Numerical Reconstruction

For the sake of clarity, it is necessary to define a series of parallel planes:

(1) the hologram plane is the sensitive surface of the CCD; (2) the reference

plane contains the focal point of the droplet; and (3) an object plane is any plane

we wish to reconstruct, generally a plane intersecting the specimen.

The basic method of image reconstruction used for all results presented in

this article is a fast Fourier transform (FFT). However, some pre-processing of

the holographic data is performed in order to form the best image. Since the

hologram contains three-dimensional data. a specific object plane must be chosen

as the focal plane for the reconstructed image. If an FFT is performed on the

raw holographic data, the object plane will be the same as the reference plane.

Therefore, the hologram formed in the Fourier-transform geometry can be thought

of as containing an implicit lens which causes it to form a focused image at the

reference plane. If a different object plane is chosen, it is necessary to add a

second lens to the hologram such that the combined action of the implicit lens

and the additional lens cause the image to be in focus at the desired object

plane. This second lens can be added numerically by multiplying the hologram by

a phase factor P given by

(x,y) = exp [iT X Y] ,

(1)

where x and y are coordinates In the hologram plane whose origin is at the

center of the I-ologram and fa is the desired focal length of the additional lens.

The focal length a can be determined from knowledge of the position of the reference plane and the position of the desired object plane by8

bc fa = - (2) b-c(2

where b is the distance from the reference plane to the hologram plane and c is

the distance from the object plane to the hologram plane.

The next preprocessing step is to numerically clip the corners of the hologram

and remove the central region, thus converting the originally square hologram into

an annulus. Removal of these portions of the data reduces noise in the reconstructed

image and involves a negligible loss of image quality. The holographic data are

also scaled and converted to a floating point format prior to performing the FFT.

Post-processing of the reconstruction can also improve the quality of the

image. First, the central portion of the image is eliminated, since it contains

only a reconstruction of unwanted scattering sources on or near the reference

point source and is overwhelmingly bright. Secondly, the image brightness can be

rescaled. The best images are given by the power spectrum of the holographic

data, namely, the natural log of the sum of squares of the real and imaginary

parts of .... Fourier transform. The image data must then be converted from

floating point to integer format for display.

D. Resolution

We measured the microholograph transverse resolution by direct observation

of the spread in the image of a very fine line, i.e., the transverse point spread

function. The line was produced by side illumination of a sharp edge, specifically,

the edge of one of the figures in a microscope test reticle. The image of this

6 28

edge appears as a line because, for the camera geometry presented here, only

light scattered by the object is collected to form an image (a form of dark-field

illumination). The width of this line as observed at high magnification with the

same sort of dark-field illumination in a compound microscope, was seen to be

much less than 1 pm. The hologram of the line was recorded and reconstructed

in the usual manner. An intensity distribution at a representative cross section

perpendicular to the line was then generated at small spatial increments. This

intensity distribution is shown in Fig. (3). Remarkably, the full width at half

maximum is about 1.40 pm, less than three times the wavelength.

The standard expression for diffraction limited resolution of a microscope is:

1.22 X

(3)

2n sin G

where (n sin 9) is the numerical aperture (N.A.) of the imaging system. This

expression is valid since, for the camera geometry described herein, the hologram

fringe spacing is always greater than the pixel size of the detector, and hence,

the image resolution is not limited by the detector pixel size. In this case, the

refractive index is n = 1, and at f/2 the N.A, = 0.24. Thus, using Eq. (3), 6 =

1.29 pm. This compares favorably with our measured value for the point spread

function of 1.40 pm.

E. Image Reconstructions

Although we have examined a wide variety of specimens, we show here an

image of a Parascaris univalens (Ascaris) larva because it demonstrates several of

the capabilities of our microholograph simultaneously.

Figure (4) shows the reconstructed image from a hologram taken at f/2 of a

section of the Ascaris larvae in its maturation stage. Since this sample is not a

strong scatterer, the reference microdrop had to be only - 25 pm in diameter in

order to make the reference wave and object wave brightness comparable. Visible

in the image is the cuticle of the Ascaris as well as individual cells. Cell walls

are clearly visible and In some cells nuclear structure Is evident.

A detailed section of the Ascaris containing 10' spatial resolution elements

(102 x 102 X 102) in the volume is shown In Fig. (5) for four different positions

in depth of the focal plane. The set of 100 slices was reconstructed from a

single hologram. The spatial range covered by the full set of 100 slices (depth

wise) is 72 pm; consequently, each slice is separated by 0.72 lim. However, the

actual thickness of the sample is - 15 - 16 pm as measured with a research-grade

compound microscope. The two slices shown in Fig. (5b) and (5c) represent the

lower and upper surfaces of the sample. respectively, and are separated by 16.56

pm. Figure (Sa) and (54b) correspond to focal planes well below and above the

boundaries of the object. For Fig. (Sa) the focal plane is 24.48 pm below Fig.

(5b), and for Fig. (5d) the focal plane is 24.48 pm above Fig. (5c). This is because

longitudinal blurring of the image introduces ambiguities which can be partially

alleviated by focusing well outside the sample volume. This quality of longitudinal

resolution is comparable to what we observe with a comrpound microscope with an

0.25 N.A. (- f/2) objective.

Blurring of the image is primarily due to two factors: (1) a longitudinal

spatial resolution which is lower than that governing the transverse coordinates

and (2) aberration from coma that is inherent in the use of a Fourier-transform

for image reconstruction. The changing character of the image is clear. The

central nuclear region is visible and the structure in the double wall can also be

seen. These features vary with the frame shown, since they change with depth.

We have found that a video representation assembled from the separate 100 slices.

which shows the continuous variation of these images, is a more informative

8 30

method to demonstrate the three dimensional character of the microholographic camera.

Those familiar with the lore of holographic microscopy will note that our

reconstructions are conspicuously devoid of speckle, Speckle is not a problem

with dark-field illumination because the image is produced by scattered light.

I11. CONCLUSION

A microholographic camera capable of three-dimensional imaging has been

constructed, tested and applied to the imaging of biological materials. This system

has recorded holograms of test patterns and biological specimens with transverse

resolutions near the diffraction limit at f/2 with 514.5 nm light. The three

dimensional character of the recorded images has also been demonstrated. This

instrument can be directly applied to imaging in the x-ray range when suitable

sources become available.

IV. ACKNOWLEDGEMENTS

This research was supported in part by the Department of Energy (DOE)

under Grant #DE-FG02-86ER13610 and #DE-FG02-89ER60898, and by the National

Cancer Institute (NCI) under grant number I R43 CA53917-01. Such support does

not constitute an endorsement by DoE or NCI of the views expressed in this

article. Additional support by the SDIO/IST and managed by NRL, under SBIR

contract #N00014-89--C-2274 is acknowl dged. Support by Eastman Kodak Company

and Maxwell Laboratories Is also acknowledged.

V. REFERENCES

1. D. Gabor, *A New Microscopic Principleo, Nature 161, 777-778 (1948); D.

Gabor, Proc. R. Soc. London Ser. A197, 454-487 (1949).

2. G. Stroke, "Attainment of High Resolutions in Image-Forming X-Ray Microscopy

with 'lensless' Fourier-Transform Holograms and Correlative Source-Effect

Compensation", Optique des Rayons X et Microanalyse (Hermann, Paris, 1966),

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3. J. Winthrop and C, Worthington, "X-Ray Microscopy by Successive Fourier

Transformation," Phys. Lett. 15, 124-126 (1965); G. Stroke and R. Restrick.

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Reconstruction Imaging - II," J. Opt. Soc. Am. a5 595 (1965).

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Microscopy and Lens Aberration Compensation by the Use of Holograms", J

Opt. Soc. Amer. .5, 595 (1965).

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283 (1966).

6. J. C. Solem, G. C. Baldwin, and G. F. Chapline, "Holography at X-Ray Wave

lengths," in Proc. of Intl. Conference on Lasers 1981 (STS Press, McClean, VA.

1981) pp. 296-235; J. C. Solem and G. C. Baldwin, "Microholography of Living

10 32

Organisms," Science 218, 229-235 (1982); J. C. Solem and G. F. Chapline, "X

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Boyer, and C. K. Rhodes, "A Description of the Theory and Apparatus for

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Imaaina '87, Advanced Printing of Paper Summaries, Vol. II (Institute for

Graphic Communication, Inc., Boston, 1987) pp. 683-688; W. S. Haddad, D

Cullen, J. C. Solem, K. Boyer, and C. K. Rhodes, "X-Ray Fourier-Transform

Holographic Microscope," in Short Wavelength Coherent Radiation: Generation

and Applications, Vol. 2, R. W. Falcone and J. Kirz, eds. (Optical Society of

America, Washington, D.C., 1988) pp. 284-289.

8. F. A. Jenkins and H. E. White, in Fundamentals of Optics, Fourth Edition

(McGraw-Hill, New York, 1976) pp. 71-72.

FIGURE CAPTIONS

Fig. (1): Schematic of Fourier-transform microholographic camera configured for

operation in the visible range.

Fig. (2): Side-view schematic of the target system prepared for visible microholog

raphy.

Fig. (3): Transverse point spread function (PSF) measured by imaging the edge

of a microscope test reticle. The curve is comprised of 250 data points

reconstructed with a transverse spacing of 10 nm.

Fig. (4): Reconstructed image of an Ascaris in maturation stage. Visible are the

cuticle and individual cells. Cell diameter is - 40 pm. The nuclear

region visible within some cells is - 20 pm in diameter.

Fig. (5): Local region of Ascaris image involving 10' resolution elements (102 x

102 x 102). The region is centered around one cell adjacent to the

cuticle. Features arising from the depth resolution are visible as the

focal plane is positioned at different depths (a) - (d). See text for

discussion.

- .-DShutter

Ar Ion Laser

L Z-l2 m f.t.

_____ _____lens UCCDU

xYz Stage Target

xYz Stage Iris

FIG;URE. (I 35

Microscope Cover Slide

iycerol Droplet 20-200 /Jzm Biological diameter Specimen

- L -i

, Mount ircý Microscope M Slide Medium

FLGURE (2)

S i

04_._

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j, % f

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FIGURE 5

APPENDIX D

Fourier-transform holographic microscope*

W.S. Haddad, D, Culien, J.C. Solem. J. W. Longworth, A. McPherson, K. Boyer, and C.K Rhodes

MOR Technology Corporation P.O. Box 10084, Chicago, IL 60610-0084

1. ABSTRACT

A camera system suitable for rnicrohoiography has been constructed, tested, and applied to the imaging of biological materials, The design of this instrument Is compatible with operation over a very wide spectral range spanining from visible to x-ray wavelengths. In order to evaluate its properties, visible light Fourier transform microholograms of biological samples and other test targets have been recorded and digitally reconstructed using a glycerol mlcrodrop as a reference wave scatterer. Current results give a resolution of - 4 X with I a 514.5 tim.

2. INTRODUCTION

Advances in high speed computation and electronic detection have accelerated the development of practical instruments for microholograplhy. The concept discussed herein promIse3 a new technology for threeý-dimensional visualization of condensed matter that Is applicable to a soectral range spanning from visible (-700 rim) tox-ray (~- 0.6 nm) wavelengths, Several Important results in this area have Jready been achieved by How/eis 'et al.1, D. Joyeux and F. Polack 2, McNulty et al.3, j. C. Solem 5?t aik4 6. and London et al.7.

Description,- of the Four ier-transf orm system. target design, and related numerical simulations have been given in previous work8- 10. Tests of the operation of the system, which l1.venabled the details of its imaging properties to be studied, have been performed with visible radiation, The exp~eriments in the visible region, In addition to allowing optimization of tho instrument, have independently demonstrated that this Is an attractive techniq~ue for visualizations in fields such as embryology where three-dimensional information Is of particular Interest.

j. VISIBLE LIGHT MICROHOLOGRAPHY

3.1. Apparatus

A schernalic for the holographic micrcmcope, configured for operation with visible rad1Intiois shown in Fig. (1), The beam path through the target Is vertical so that the target hoidar *s horizontal and gravity can be used to hold the sample and reference wave scatterer In pla'-A Several objects have been used as refervince wave scatterers. These Include GRIN rod tensas glycerol microdrops, and reflecting microspheres, The microspheres represent the optimum chr'-: for use with soft "-rays4,6, and the corresponding x-ray target design Is described elsewhere&0 Glycerol microdrops have performed very weti in experiments with visible light, A magnitod( side view schematic of the target used for visible light exposures Is shown In FIg. (2).

The basic, functions of the apparatus. -,ijc:h as shutter timing and triggering, are conlrfillaj by a P. C. The holograms are recorded electronically, digitized, and stored by the P.C. T'.0 dala are then transferred by an ethernot link to a Stardent GS 2000 supergraphlc workstatlu,-where the imnage reconstruction Is performed numerically, The reconstructed Image Can than 2-, namnipuiil'ed, scaled, and displayed on a high resolution graphics monitor. A schematic of lh-i computer system Is shown In Fig. (3).

*m~nhec r Ipt sutriettted fr Ll~ ri g/P ;v m :;ynp,sin on cctroii I imaging: Science ' Tocno Logy, 2/4 Felbrua ry -- I March 1iI991 ý, ir, tiw,;f , (;A, Lo, be pub lished in Hie Pr~~lI. of thef, Syinposium on f:iectr,,ntc imaging; science and Technology, SPIE, Bellingham,.~A

- Glycerol erope

pecImon diameter

L Mcrscoe lid M~~~in cdix

Fig (): Maniie sdeviw sheatc f hetage -sstm prprdU. vsbl irhtgah

len

Serial Interface for Remote Control

Exposure Hologram Display Trigger

Ethernet Mainframe

Video Signal

Carnera -Shutter Driver

Fig. (3): Diagram of computer system and electronics for acquisition and reconstruction of

microholograms.

3.2. Hologram detector

The holograms are recorded on a charge-coupled device (CCD). The chip has a sensitive area of 18 mm x 18 mm containing - 4 x 10" pixels (2048 x 2048). The pixels are square with a dimension of 9 im. For these experiments, a protective window Is placed over the CCD which carries a small central opaque square serving as a blocker for the unscattered incident beam. Specially modified drive electronics were designed for this system Including a separate headboard which carries the CCD and allows for remote mounting of the detector. Access through the headboard to the back of the CCD also is available to permit cooling of the chip. The piX.1 clock rate (data readout rate) has also been reduced from - 20 MHz to 2 MHz to allow lhq system to function well when cooled to - -401 C. Such cooling would allow direct Integral"In of photons on the chip for periods of up to about one hour without a significant accumulation of dark counts.

3.3. Liquid microdrop lenses

A liquid microdrop of glycerol has been found to serve well as a reference wave sca~te;r for visible radiation. The drop is placed on the glass slide, close to the edge of the sarlpl and nearest the region of interest in the sample, as shown In Fig. (2). The droplet acts al a lens Intercepting a portion of the Illuminating plane wavefront and converting It Into a divergi1re. nearly spherical wave, The focal point of this lens acts as the reference point source. Thq imaginary plane parallel to the detector plane (hologram plane), and containing the refereica point source, will be referred to as the reference plane. The object can be divided up into a series of planes (object planes) which lie parallel to the hologram plane. In most cass. l•-, reference plane is located a significant distance away from the object planes. The usual sltullton is that the microdrop is placed on top of the sample coversllde, so that the Illuminating bea,'

first impinges on the object, then a short distance later (- 200 p.m closer to the detector), the microdrop. Sinc the microdrop is a positive lens, the reference point source Is then formed even closer to the detector.

Glycerol Is a suitable material for the microdrops because of its low vapor pressurp. high surface tension, and polarity. To improve the characteristics of the microdrops, it is ýfren necessary to pre-coat the glass coverslide with a thin layer of silicone. Dricote, a standard anti-wetting agent for PH electrodes, has been found useful for this purpose. The redujc=,? welting and the viscosity help the glycerol form a round lens with a relatively short focal length. This produces a system with a low F number.

The microdrops are formed by the use of a glass micropipette. The microplpette Is mounted on an X-Y-Z micropositloner and the operator views the sample and micropipette tip throligh a dissecting microscope while manipulating the micropositloner, When the tip has been pos:t~onnrc properly, it is brought almost into contact with the glass slide, and a slight pressure Is pill It the back of the pipette, thus dispensing the desired amount of liquid. The tip Is then slowly brought vertically away from the slide, allowing the drop to form,

The droplet size is an important factor in the formation of a good hologram. This is because it is primarily the diameter of the drop that determines the brightness of the reler,,,-. wave. The larger the diameter of the drop, the larger the portion of tho Illuminating waveforrt which is intercepted, and therefore, the brighter the reference wave. In order to lo- .a hologram with a good signal-to-noise ratio, it is necessary to have the reference wave brlghtr,-ss approximately equal to the object wave brightness. Depending on the type of object, we have? used microdrops ranging between 20 pm and 200 pm in diameter..

3,4. Reconstruction technique

The method of image reconstruction used for all results presented In this article is i. a fast Fourier transform (FFT) However, some pre-processing of the holographic data is pOO. in order to form the best image. A specific plane parallel to the hologram plane must he Q'. as the focal plane for the reconstructed image. This plane will be referred to as th, plane. If an FFT is performed on the raw holographic data, the Image plane will be the as the reference plane. Therefore, the hologram formed in the Fourier transform georn",, be thought of as containing a lens which causes it to form a focused image al the r.4 s'.' plane. If a different Image plane is chosen, it is then necessary to add a second lens , , hologram such that the combined action of the implicit lens and the additional lens cage*' image to be in focus at the desired plane. To do this, the hologram must be multipled , phase factor L(xh, Yh) given by

L(xh, Yh) - ei(x yh

where xh and Yh are coordinates in the hologram plane whose origin is at the center # hologram.

The next preprocessing step is to clip the corners of the hologram and remove the region, thus converting the originally square hologram into an annulus. Removal of portionmr of the data reduces noise in the reconstructed image without loss of image The holographic data are also scaled and converted to a floating point formal prior to pereo-the FFT.

Post processing of the reconstruction can also improve the quality of the image . the central portion of the image Is eliminated, since it contains only a reconstruction unscattered beam and would be overwhelmingly bright. Secorndly, the image brightness -"°

re-caled. The best Images are given by the "power spectrum* of the holographic data, namely, the natural log of the sum of squares of the real and Imaginary parts of the Fourier transform. The image data must then be converted from floating point to Integer format for display.

3.5. Results of Image reconstructions

3.5.1. Swordtail melanoma

All the holograms described below were produced with the use of 514.5 nm radiation from an argon ion laser. Figure (4) Shows a hologram of swordtall (Xlphophorus maculatus x hellerl) malignant melanoma and the reconstructed image. Also shown are Images taken with a standard low power optical microscope with both white light and laser Illumination. Clearly visible In Fig. (4a) is the - 130 pm glycerol droplet used to produce the reference wave. This sample was provided to us by R. B. Setlow of Brookhaven National Laboratory. o

(a) (b)

Fig. (4): Fish melanoma histological sample with glycerol microdrop reference wave source (a) Image of the target taken with a standard optical microscope and white light Illuminatiom (b) Same as (a) except with Ar' (514.5 nm) laser illumination. (c) Digital recording of hologram. (d) Numerical reconstruction of the Image; resolution M 4 Prm.

3.5.2. Air Force target Figure (5) is a reconstructed Image from a hologram of a U.S. Air Force resolution tarqrpl The full reconstruction Is shown In Fig. (Sa), and a zoomed Image of the region containing 11-0 smallest features is shown in Fig. (Sb). These stots have width equal to ihe slot spacing arnd for the smallest, this dimension Is - 2 pm. Since the features of the target are holes In a metallic coating, they scatter the incident radiation by diffraction. Therefore, It Is only ih', edges of the features that show up in the reconstruction. In principle, the Information about the central regions of the target features is present at the cgnter of the hologram, how.Vr this information would not be available because of the overwhelming brightness of the unscailered

laser beam. In Fig. (5b), it is possible to resolve the edges of both the vertical and horizontal

sets of slots for the smallest group (- 2 pm separation). Since I - 514.5 Mm, a conservative estimate for the resolution of this Instrument is - 41, The theoretical resolution, limit for such an Imaging system operated at f/2 Is - 1 )., a value which compares favorably with our result.

a(b)

Fig. (5): Reconstructed Image from a hologram of a U.S. Air Force resolution target taken at f/2 with 514.5 nm light. A glycerol microdrop was used as the reference wave scatterer. (a) Full-size reconstructed image, (b) 16x zoom from (a) of the sma!lest slots (2 pm separation between each edges).

3.5.3. Ascaris

Figure (6) shows the reconstructed image from a hologram taken of an Ascarls In Its maturation stage, also at 1/2. Since this sample Is not a strong scatterer, the reference microdrop had to be only - 25 pm In diameter, Visible In the Image Is the cuticle of the Ascarls as well as individual cells, Cell walls are c0early visible and In some cells nuclear structure Is evident.

4. CONCLUSIONS

A holographic microscope capable has been constructed, tested, and applied to the Imaging of biological materials. This system has recorded holograms of test targets and biological specimens with resolutions in the range of - 4 Xk with 514.5 nm light, This Instrument can be directly applied to Imaging in the x-ray range when suitable sources become available.

5. ACKNOWLEDGEMENTS

We are grateful to Or. Richard S. Setlow of Brookhaven 'National Laboratory for providing samples of fish melanoma. This research was supported In part by the Department of Energy under Grant #DE-FG02-86ER13610 and #DE-F"GO2-89ER60898. Such support does not constitute an endorsement by DoE of the views expressed In this article. Additional support by the SDIO/IST and managed by NRL, under SBIR contract #N00014-89-C-2274 Is acknowledged.

Support by Eastman Kodak Company and Maxwell Laboratorles Is also acknowledged.

Fig, (6): Reconstructed image of an Ascarls in maturation stage. Visible Is the cuticle and individual cells. Cell diameter is ~ 40 prm. Nuclear region visible within some cells Is - 20 pm In diameter.

6. REFERENCES

1. M. Howells, C. Jacobsen, J. Klrz, R. Feder, D. Quald, and S. Rothman, "X-Ray HoIqrivrhv at Improved Resolution: A Study of Zymogen Granules*, Science, Vol. 238, pp. 514 ,.PI; C. Jacobsen. M. Howells, K. Quald, and S. Rothman, 'X-Ray Holographic Microqr,-nv Improved Images of Zymogen Granules', OSA Proceedings on ShorthWQvelenath' C,"•e,-'j Radiation, Vol, 2, R. Falcone and J. Kirz, eds., Optical Society of America. Washingtor D.C., pp. 295, 1988.

2. 0. Joyeaux and F. Polack, "Progress In Optical Reconstruction of Submlcron X-Ray Holoqr'"" OSA ProceedIons _nQShortWh Vol. 2, I. Falcone and J I#2 eds., Optical Society of America, Washington. D.C., pp. 295, 1988. 3. 1. f-AcNulty, J. Klrz, C. Jacobsen, M. R. Howells, and E. H. Anderson, 'First Results ,-,,, 4 Fourier Transform Holographic Microscope', X-P.y MIlcroscopy III, eqdited by A. G. Oc-P',e et al., Sprlnger-Verlag, Berlin, 1991, to be published.

4. J. C. Solem, G. C. Baldwin, and G. F. Chapline, "Holography at X-Ray Wavelengths" r--. of Intl. (onflrerce on Lasers 19Q1, STS Press, McClean, VA, pp. 296.

5 J. C Salem and G. C. Baldwin, "Microholography of Living Organisms". Science Vol r ,O pp. 229, 1982.

6. J. C. Solem and G. F. Chapline, "X-Ray Blomicroholography', Opt. Engineering Va. 23 . 193 1984,

7. R. London, "Radiation Damage and Its Influence on Source Requirements for High Resolution X-Ray Holography*, X-Ray Mlcrolmaolno for the Life Sciences. Lawrence Berkeley _Laboral..ry Report. LBL-27660. UC,-p60. CQNF-8905192, p. 51. August, 1989; R. London, M. Rosen. and J. Trebes, 'Wavelength Choice for Soft X-Ray Laser Holography of Biological Samples*, Applied Optics, Vol. 28, p. 2297, 1989.

8. W. S. Haddad, D. Cuilen, K. Boyer, C. K. Rhodes. J. C. Solem and R. S. Weinstein. 'Design for a Fourier-Transform Holographic Microscope', X-Ray •_..j scRa. I[._, edited by 0. Sayre. M. Howells, J. Kirz and H. Rarback, Springer-Verlag, Berlin, 1988, pp. 284-287.

9. W. S. Haddad. J. C. Salem, D. Cullen. K. Boyer, and C. K. Rhodes, *A Description of the Theory and Apparatus for Digital Reconstruction of Fourier Transform Holograms", Electronics olno R. Advanced Printing of Paper Summaries, Vol. 2, Institute for Graphic Communication, Inc., Boston, 1987 p. 693.

10. W. S_ Haddad. 0. Cullen. J. C. Solem, K. Boyer, and C. K. Rhodes, *X-Ray Fourier-Transform Holographic Microscope", Short Wavelength Coherent Radiation: Generation arid Applications, Vol. 2, edited by R. W. Falcone and J. Kirz, Optical Society of America, Washington, D.C., 1988, p. 284.

APPENDIX E

Prospects for x-ray holography with free electron lasers

J,C. Solem*, K. Boyer, W. Haddad, and C. K. Phodes

MCR Technology Corporation P.O. Box 10084 Chicago, IL 60610-0084

1. ABSTRACT

We review the technical advantages offered by x-ray holographic microscopy for imaging the structure of living biological specimens. We discuss the wavelength, coherence, energy, and pulse-length requirements and conclude that these could be met by free-electron laser architectures of the near future. We also show that Fourier-transform holography using a reference scattering sphere is the best optical configuration for a practical instrument.

2. INTRODUCTION

X-ray microholography will ultimately allow three-dimensional imaging of living organisms with a spatial resolution far beyond the reach of optical microscopes. In the wavelength region between the K--edges of carbon and oxygen, we find a high contrast between protein and water that will allow Imaging of the structural elements of cells without staining. 1 Because of the quantum nature of any probe on the < 100-A level of resolution, the specimen will be severely damaged in the imaging process.2 If the x-ray energy car. be delivered in a very short pulse, however, a snapshot view of the specimen In the living state can be obtained, even though the specimen is obliterated in the process.3 An ultrashort pulse x-ray laser could provide the illumination for such snapshot microholography. Another option is the free electron laser, if the coherence and energy requirements can be met.

3. WATER WINDOW

Figure 1 shows the absorption length (e-folding distance) in normal density protein and water." Between the K-edges of carbon and oxygen, there is a region of high contrast; water is transparent and protein is opaque. This wavelength interval has become known as the water window and the maximum contrast appears at the K-edge of nitrogen. Most of the structural elements of a cell are made of protein. Therefore. within the water window, most biologically Interesting features are visible without staining. This compelling fact has provided the principal impetus for the emergence of the field of x-ray microscopy.

Determining exposure requirements for x-ray microscopy from the data In Fig. (1) is fairly straightforward. For holography, however, the problem is a bit more subtle. About a decade ago one of the present authors2,3 calculated the exposure requirements from the cold opacities alone. An approximation to the Mie scattering problem for a semitransparent sphere in a vacuum revealed that the scattering cross section could be estimated by the geometric cross section times the square of the beam extinction. 5 Since that time, quantum dispersion theory has been applied to the soft x-ray regime to give both real and imaginary atomic scattering factors and researchers have solved the Mie problem for protein sphere Immersed in water, including both absorption and dispersion. 6 The results have revealed that the best wavelength for holography is not at the nitrogen K-edge, as in microscopy, but just below the carbon Kedge, a region actually below the threshold of the water window.

There appears to be no Impediment to free-electron lasers producing coherent radiation within

* Consultant to MCR Technology Corporation

SPIE Vol. 122 7 free-Electron Lasers end Applications (1990) / 105

104

I -•o I I

Io I

0.5 1.0 1.5

hv (C(ov)

Fig. 1. X-Ray absorption lengths for protein and water at 1 gm-cm-"=

the water window. The shortest wavelength achieved to date Is 2,400 by the Soviet storagering FEL at Novosibirsk,7 but many of the requisite technologies to permit extension below V00 A, using RF linear accelerators8 are now In hand.9 The foremost of these is the photoelectron injector.8

Projecting from current rf linac and x-ray optical technology,8 either an FEL oscillator or a single-pass, self-amplitied-spontaneou s--emiss Ion amplifier may qualify.8"11 Both conceptual designs could be operated at the third harmonic, for which high gain could be achieved with sufficiently large undulator parameter. In one speculative single-pass design,12 tuning to 132 A•, and using the third harmonic, gives about three orders of magnitude more gain than tuning directly to 44 A. An oscillator can produce the requisite power with higher emittance than required for a single--pass amplifier. For example, an FEL oscillator using a 7_50 period undula;or (1.6 tin-period"•) and 35%-reflectance multilayer mirrors could produce 44 A, radiation at 5 x 10 & W In 2.5 x 10-11 s pulses with peak beam current of 400 A and normalized electron beam emittance of 10ir rmmmr. A single-pass laser with the same characteristics would require an emittance of 41r mmomr and an undulator twice as long.11

4. HOLOGRAPHY

Holography uses interference with a reference wave to record the amplitude and phase of the specimen-scatitered wave. From the symmetry properties of Maxwell's equation3 it can be shown that, "• the fields are known on a surface. they are also known in three-dimensional space. Thus, it is possible to reconstruct rather complete three-dimensional information about • the geometric structure of the specimen from its scattered wave. Although the configurationsm

of holographic apparatus are diverse and varied, all recording techniques divide into two basic

106 / $PIE Vol, 1227 Free-flectron Losers ard Applications oI 990)

categories: Fresnel transform and Fourier transform.

4.1. Fresnel-transform holography

In Fresnel-transform holography,10 the reference waves are planar. The specimen is Illuminated by the same bearn of radiation that supplies the reference waves:. the ipecimen simply stands betweeni the radiation sourciv and the recording surface. Tile wider the anQ,'e fiom the specimen to the edge of the reference Illuminated area on the tecording surface, th~e better the resolution. The same ccne-angle dependence applies to an optical micrcscope. The cone-tingle dependence further Implies that the re.Tolutio.'i dependis on the coheience of the radiation. Spatial coherence must be maintained over 06t extent of thle reference illuminaled area of the recording sujrface and the temporal coherence lerngth must be larger than the difference between the perpendicular distance frcm the recording surface to the spokrien, an'i the distance from the edge of the refetince i~llumninated area to the speciff-eii.111

A mo~re severe filmitation derives from the intrinsic resolvtion )f the recording medium. The resolution of a Fresno! transform holograph is limited to twice the gain siz.6 of the recording -neditum.12 This limitation makes -...ray hciloý;rarshy 'axiceedingi", difficult, oven with the finest grain commercial films. The deus P'A machirli is photoresist. Ovveloped for micro.litho raph'j, photoresi,*1s are photocensrtIve etchaole polymers with no significant Craiin structure.q, X-rays will break cros~s lin'<s In chesG polymers making them mer.-e suisceptir~e to etching in the regions ci x-ray expnsure. Aftev etching, the resist resembies a relief map. with eowest elevatiomis corre~sponding to greats.at exposo'ras. The spatial resolution of photoresists (more properly/, x-ray resists) Is dependeriý on the wavelength of the x-rays to which it Is exposed. For long wavelengths, it i3s timiteJ by diffraction wit~iin the resist Itself. ror short wavelengths, it Is limited by the iartgfe of sacondary electrons, whic-h carn also break cross links and thereby cause blurring. The optimum wavelength 14 Is about 50 A, serendipitously close to the lower edge of the water window. Thq best spatial resolution of rpsiat at the optimumr wavelength is 50 to 100 A.

Two groups are prsaontly making hologram-, of blo~ogical specimens using the Presnel transform technique, An Arrerican ýItoup' 5 'working at OR~ National Synchrotron Light Source of Brookhaven Nationa) Laboratory uies a spoc~ally equipped beamline with an undulator for high brightness In the watcr window'. After exposing and etching the resist on which their hologram Is recot-Jod. they shadow *he contours with gold. They then read out the hologram with a transmiss~on electron microscooe, digitize it, and reconstruct It with a computer. A French group16 using a synchrotron at C'rsay records their holograms on photoresist In a simila. manner. They r.-constyuct the huloGram by pass~ing a He-NoL (6,328 A) laser I' rough tte etched resist, whose contours prodw..a locally differing phase shifts. Thi3 produces .magnified image of the specimen, They can accompiish the same phase shifting effect by aluminizing the surface of the resist and reflecting the )&!car beam.

4.2 Fourier transform holography

Fourier transform holography 17 doi.s not suffe.r frm tho resolution limits of the reco~rding medium that affect Fresnel transform holography. Th3 Fourier technique uses cw.rved rather than planar refererce waves. In the far field Js.rig ideaily spherical reference wavcs, the technique maps increments of d!stance In the specimen Into spatial frequencies at the recording surface - thus the name "Fourier transform'. In the far field, the spatiai relationship amonq the fringes simply scales with distance from the specimen. Thus, thl' hologram can be faithfully registered by recording media of Aniy resolution by simply adjusting the ccistance from the specimen to the recording surface. A charge-coupled device (CCD) array Is particularly attractive in this application. CCDs can be made to work with high efficle-icy In the water windcsA.18 and their signal can be digitized directly for simplified computer reconstruc~lon.

SVIE Vol. 1227 Free-Electron Losers and Applications (199011 107V 50

CurlouGly, small increments of distance (minute features of the specimen) are mapped into low spatial frequencies, while large Increments of distance (gross features) are mapped into high spatial frequencies. This coun~terintuitive result can be reconciled with familiar optics by noting that the diffractive resolution limit of the holograph is again determined by the angle over which the hologram Is recorded. A larger angle will allow a better measurement of lower spatial frequencies.

The Fourier-transform technique has less stringent coherence requirements than the Fresnel transform technique.4 Spatial coherence is reduced because the coherent part :' the wavefront need only span the specimen and the lens that is creating the reference way-; and these may be placed quite close together. Temporal coherence Is reduced because the difference in pathlength from the reference source and the specimen is always small. Figure 2 compares the coherence requirements for the two techniques as a function of specimen mass over 11 orders of magnitude. A free-electron laser will have no difficulty meeting these coherence requirements, but the Fourier-transform technique Is clearly less demanding.

-E CL *E CE

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'IoI -" ,.

00.. 4.

r0 r102

10- 6 g0 O.? - • 10.6 ~ ' - ' '0 - , 0i-- 1O- g 100 g0 I

Ma$t (ng) Fig. 2 Coherence requiremnents as a function of specimen mass: (A) F'resnel transform wI~f high contrast', (8) Fourier tr.ansform. Solid fines and temporal Loherence and dashed lines are spatial coherence. The specime~ns at •he top we•re used as models.

IOB 'SPIE Vol 1 "•on Lasers and A~phc~nions/199OJ

A principal difficulty in Fourier transform holography is the generation of spherically diverging reference waves. A straightforward technique involves the use of a lens. The water window is too far from any strong resonances for a refractive lens to be used, so we must use a diffractive lens in the form of a Fresnel zone plate. It can be shown, however, that the resolution is then limited to the ý.lame order as the finest spacing of the zone plate. In other words, we need to bW able to fabricate x-ray optics to the same precision as we wish to resolve the specimen. Present technology is limited to a few hundred angstroms.19 If a pinhole with a diameter less than the finest zone spacing is placed at the fccus, then the resolution is on the order of the pinhole diameter. It may be possible to fabricate pinholes with diameters somewhat smaller than the finest zone-plate spacing. The real solution to the reference wave dilemma Is a reflecting sphere, which will be discussed in detail later.

Despite these difficulties, the Brookhaven group is now generating Fresnel transform x-ray holograms using a zone plate and a CCD as a recording surface. 20 The CCD output can be directly digitized for computer reconstruction.

5. EXPOSURE

Holography results from the symmetry properties of Maxwell's equations-classical electrodynamics. We know this is only an approximation to quantum theory. While the wavefunction is aaalytic and deterministic, any measurement is inherently statistical. We must count particles just as in nuclear physics experiments. Since the energy carried by a sirgle photon in the water window Is more than a hundred times the energy carried by an optical photon, the damage to the specimen is enormous. Furthermore, we are trying to resolve much finer features than in the optical range and each of these features must scatter a statistically significant number of photons. Thus for diffraction limited resolution, the damage. is increasing very roughly as the fourth power of wavelength. For an electron microscope, the damage for the same resolution is even greater. 1 1,14

Table I lists some specimens and the dosage required to image them to a resolution of 100 Awith signal-to--noise of 5, using x-rays at the nitrogen K-edge. Somewhat higher dosages are required !or the virons because there is less contrast between DNA and protein.1" Higher doses are required for the larger cells because of their physical thickness. For most cells, a few times 109 rad Is the required dosage. Using the optimal wavelength Just below the water window would reduce these exposures about an order of magnitude, but the enormity of the close is very clear. To put this in perspective, 102 rad would make yw)u very sick and 10' rad would be surely fatal. The sturdiest known living thing is the bacterium Micrococcus radiodurens which has been alleged 2 1 to survive 3 x 10" rad. Surely no specimen can survive the exposure The best we can do is try to record its true Image when it was in the living state, and this Is best accomplished by taking a snapshot.

6. SNAPSHOT HOLOGRAPHY

X-ray flash pictures or snapshots seem essential to imaging living specimens. The requirpd dosages are so high that we can expect the specimen to exp!ode owing to the sudden heating 22 If the pulse is short enough, the image can be captutod from ýhe plasma ghost of the specimen before there is sufficient bulk motion to blur the features we are seeking to resolve.1 Analytic expressions for hydrodynamic blurring owing to the ex, losion of semiopaque features are useful for estimating pulse length requirements. To achieve a resolution 6, a specified number of photons must be coherently scattered from a volume da during the exposure time At. and no dimension of the specimen can be allaved to change by more than 6 during Lit. To examine a feature with resolution 6 << d, where d is the transverse dimension of that fealure. 1he maximum exposure time is

SPIE Vol. 1227 Free-Elec'tron Lasers and Applications ( 19901 9

Specimeii Mass(g) Dosage(rad) Specimen Mass(g) Dosage(rad)

Echerichia colt Anthrax phase *x174 10" 1 ' 2 x 1012 bacterium 10"11 4 x 10'

Herpes 16 1 2 Red blood cell viron 10 2 x 10 (human) 10, 5 x 10'

Vaccinla White blood cell viron 10"16 6 x 10"' (human) 10'. 4 x 10'

Mycoplasma Amoeba pneumonlae 10"1, 3 x 10' (dysentery) 10' 2 x 10'

Escherichla coil Smooth muscle (immature) 10-"1 3 x 10' cell 10'7 4 x 10'*

Escherichla coil Paramecium (mature) 10"12 3 x 10' (protozoan) 10's 2 x 10*

Table I. Dosage to Obtain 100 A Resolution with S/N = 5 at N K-Edge.

r 62M 1/a

At=[ = ]

where m is the mass of the feature being examined, ca Is its absorption cross section. I s ,ý, x-ray intensity, and F is a dimensionless coefficient accounting for equation-of-state paramrelr', and the rate at which transport processes remove energy from the exploding feature to ,'e surrounding material. If 6 >> d is acceptable, then tit should be multiplied by 6,d aV-, substituting a different dimensionless coefficient G.

The number of useful photons elastically scattered from a single resolution eleme,,t approximated by

EX N - ple6 2 la firm

where p is the density, oe Is the elastic scattering cross section of the featLre, . wavelength, and c is the quantum efficiency of the recording medium, This number rwo .. statistically significant, compared to photon noise. Combining Eqs. (1) and (2) gives the '. intensity as a function of resolution.

N [ (Na a 1 /2 F /2 6 < 3d/8 I x I G" (d/6)/ ,6 > 3d/8

Intensity increases very rapidly as better resolution is sought. For typical b,o, ,. specimens, intensities on the order of 1012 W-cm' with pulse lengths on the order of

110 / SPIE Vol. 1227 Free.Eieuron Lasers and Appficarions (1990)

are required for a resolution of 1OO A. The projected designs for free-electron lasers will

have no difficulty meeting these requirements.

7. REFERENCE SCATTERING SPHERES

The resolution dilemma encountered with the Fresnel zone plate reference source in Fourier transform holography Is solved by using a reference scattering sphere.23 The zoneplate holograph is limited to a resolution about the same as the finest zone-plate spacing because the largest angle covered by the reference Illumination is determined by that spacing. A tiny, highly reflective, smooth sphere, which can be used to replace the zone plate as the reference source, will not suffer this limitation.

Reference-sphere-Fourier-transform holography also offers a host of advantages when an x-ray laser is used: (1) the sphere can provide reference illumination over a sufficiently large angle to realize diffraction-limited resolution; (2) the sphere is appropriate to the narrow beam of a laser in that it is small and can be placed quite close to the specimen; (3) orientation of the sphere is unimportant; and (4) the sphere is rather Inexpensive to fabricate in large quantities, so unlike the zone plate, it is unimportant whether the sphere Is damaged or destroyed or eaoch exposure. Figure 3 Is a schematic of how the reference sphere might be used in practice. The most logical recording device is a CCD, the output of which can be digitized for direct computer analysis. The scattering sphere will not produce spherical reference waves, there will always be an angle-dependent phase shift. Thus it will not generate a true Fourier, transform, even in the far field. However, any scattering shape can be used as long as we know exactly what It Is and how it Is oriented, A set of basis functions, analogous to the sinusoldal basis functions of Fourier reconstruction which incorporates the appropriate correction, can be generated by the computer and used to recover the Image.2 4 The image can also be reconstructed optically using a scaled reflecting sphere and a computer modified hologram that compensates for the angular dependence of reflectivity. The hologram might even be an array of liquid crystal optical gates for direct interfacing with the computer and "real-time* reconstruction.

X-Ray

2.:36 ))fl C. < 4.47 rrn 1

Spherical Reference

Currpute r

GraptIcs

/Bioiogical sanrmpe

Fig. 3. Schematic of a reference-sphere-Fourier--transform holographic microscope.

SPIF Vol. 1227 Free.Fleatnn Laset$ &ndApp/iratvnsI199'O/

That It Is possible to manufacture a sphere with the requisite surface smoothness was not obvious In the beginning of our research program. We found that many of the spheres manufactured by laser ablation had rather crystalline surfaces resulting from slow cooling. However. in examining some hollow spheres that were manufactured at laser fusion targets, we found surfaces so smooth that no structure could be resolved with an electron microscope. The smoothness was attributed to rapid cooling of the thin layer, which left the metal In a glassy state rather than forming crystals.2 3,25 Smooth spheres have also been manufactured In very small sizes at Lawrence Livermore Laboratory. Finally, if there Is a problem with the metallic spheres, a clear alternative exists. For example, we can use readily available glass spheres, which will have the necessary smoothness, and then coat them by metal evaporation, a process which Is known. to produce amorphous layers with a smoothness better than 3 A. Actually. recent work26 on x-ray mirrors has demonstrated the ability to achieve a microroughness of 2 A. Processes to provide uniform coating have been developed. The spheres also turn out to be exceedingly round owing to rapid damping of mullpole oscillations on the time scale of cooling. This removes completely the necessity for measuring departure from sphericity and for orientation of the sphere.

We surveyed the periodic table for elements of optimum specular reflectivity to serve as reference scatterers in the water window. Unless a compound can significantly enhance the density of an element, the pure element wilt be the best reflector. We used the tables of real and Imaginary atomic scattering factors 2 5 calculated from quantum dispersion theory2 7 to obtain the reflectivities shown in Fig. 4. Nickel Is found to be the best reflector at the water window threshold (carbon K-edge). If we are able to precisely select our wavelength, it would behoove us to use the optimum wavelength Just below the threshold, so nickel is the element of choice. Furthermore, osmium Is only slightly superior to nickel and suffers from a variety of fabrication and handling difficulties. Figure 5 shows the reflectivity of nickel as a function of grazing angle at 44.7 A. It provides adequately uniform reference Illumination out to an angle of about 280. This Is sufficient to obtain good resolution In both the transverse and longitudinal directions. The transverse resolution is given by 6t = X/20, while the Iongitudinai resolution28 is given by 6d )=/482. Table 11 shows the resolutions that can be obtained at the carbon K-edge and the nitrogen K-edge.

Assuming an average specimen diameter and as compact a spacing as possible between the sphere and specimen, the laser beam would have to illuminate about an area of about 10" cm2. This means the freelectron laser would have to supply about 10-6 J in a single pulse, which is quite reasonable,

Nickel Osmium 44.7 A 11.6 A

0 300 220

6t 43A 41 A

61 41 53

Table 11. Spatial Resolution at carbon and nitrogen K-edges.

112 / SPIE Vol. 1227 FPee-Electron Lesers andApplicotions (1990Q

0.40

0.1,

2,O% ''

4 0 gO 30 0 40 40 "'• 70 00 60 -I00

Fig. 4. Specular reflectivityes of various eaerncto at oelected grazing angles.

0.COCLSIN

0.4

I.8

Fig. 5. Specular reflectivity of nickel as a function of grazing angle.

8. CONCLUSIONS

If the proposed design 7,9 can be reduced to practice, no impediment is seen to using a free electron laser to obtain water window, x+-ay holographtc images of biological specimerns The best arrangement Is to use the Fourier tranmdorm technique with a nickel reference scatter ing, sphere and a CCD to register and record the interference pattern.

9. ACKNOWLEDGEMENTS

This research was supported In part by the Department of Energy under Grant #DE-FG0?86ER13610 and #DE-FG02-.89ER60898. Sutch support does not constitute an endorsement tv DoE of the views expressed in this article. Additional support by the SDIO/IST and manag,,1 by NRL, under SBIR contract #N00014-89-C-2274 i5 acknowledged.

SPIE Vol 1227 Free-.Eectron Lasers r•nd Application* (19901' 1 I

10. REFERENCES

1. J. Solemn and G. Baldwin, 'Microholography of Living Organisms, Science, Vol. 218, p. 229, 1982. 2. J. So1am, 'High Intensity X-Ray Holography: An Approach to High-Rsolution Imaging or Biological Specimenm., Los &Almo. -National Laboatgry Renot LA-95SOA.-MS, 1982. 3. J. So1am, G. Baldwin, and G. Chapline, alHolography at X-Ray Wavelengths' Proceefdinga of the International Conference on Lasiers '81, STS Press. McLean, VA, p. 296, 1982. 4. J. Solemn and G. Chapline, 'X-Ray Biomicroholographyo, Optical Engineering, Vol. 23, p. 193, 1984. 5. R. Glauber, LeCtUreg InI Theorotgical Physics, W. BrIttln and A. Dunham, eds., Vol. 1, p. 315, Interacience, New York, 1959. 6. R. London, Ofadiation Damage and Its Influence on Source Requirements for High Resolution X-Ray Holography', X-Raly Microimaging for the Life Sciences. Lawrence flericeley Laboratory Regort. LBL-27660. UQ,-QQ0. CONEF-8905192, p. 51, August. 1989; also R. London, M. Rosen. and J. Trebes, 'Wavelength Choice for Soft X-Ray Laser Holography of Biological Samples', Applied Optics, Vol. 28, p. 2297, 1989. 7. V. Litvenenko, 'Results of the USSR Storage Ring FEL', Nuci. Instr. and Mothods in Physics Research, to be published, 1990. 8. B. E. Newnam, 'Projected Performance of RF-Linac-Oriven Free--Electron Lasers In the Extreme-Ultraviolet Spectral Region', Nuci. Instr. and Methods. In Physics Research, Vol. B40/41. p. 1053-1057, 1989, 9. J. Fraser and R. Sheffield, 'H-igh-Brightness Injectors for RF-Oriven Free-Electron Lasers', IEEE Journal of Quantum Electronics, Vol. OE-23, p. 1489, 1987; and R. Sheffield, E. Gray and J. Fraser, *The Los Alamos PhotoInjector Program', Nuclear Instruments and Methods In Physics Research, Vol. A272, p. 222, 1988. 10. J. C. Goldstein, T. F. Wang, 12. E. Newnam, and B. D. McVey, "A Single-Pass Free-Electron Laser for Soft-X-Rays with Wavelengths 4 10 nm', In Prgicpedlnas of-tDA 1987 IEEE Particle Accelerator Conference, E. R. Lindstrom and L. S, Taylor, eds., IEEE Cat. No. 87CH2387-9, pp. 202-204, 1987; Abstract In Bull. Am, Physical Soc., Vol. 32, p. 206, 1987; also, T. F Wang, J. C. Goldstein, B. E. Nownarn, and B. D. McVey. YGeneration of Coherent Soft XRays Using A Single-Pass Free.-Electrort Laser Amplifier%. Intl. J. Electronics, Vol, 65, pp 589-595, 1988. 11. B. Newnam and J, Goldstein, bFree-Zlectron Lasers as Potential Sources for Soft X-Ray Holographic Microscopy of Biological Structures', Abstract for Migrom-agIng. for the L"f Sciences. May 24--26, 1989, Lawrence Berkeley Laboratory, Berkeley, CA, LA-UR-89.-1797 12. H, Shay, W. Barletta, S. Yui, E. Sctherlemann, R, Schlueter. and G. Deis, 'Sphort Wavelength FELs as High Brilliance Sources', X=fiAyMicrolmagIno for the Lifel Sciences. Lar Bgrkeley LaboratoryRgpgE~iL L . 760 J 1 Qf.8i12 p. 170, August, 1989. 13. D. Gabor, OA New Microscope Princip.le', Nature, Vol. 161, p. 777, 1948; also D. Gabor erlgdLg~~ tgRylýQ2y Series A, Vol.197, p. 454, 1949. 14. J. Solem, 'X-Ray Imaging of Biologicail Specimens', Prgoceedings o~f thI Intgroatlgnal Conference gnL.geii2:g STS Press, McLean. VA, p. 635, 1984, 15. G. Stroke, Optique des Rayons X et Microanalyse, Herman, Paris, p. 3, 1968. 16. D, Sayre, J. Kirz, R. Feder, D. Kim, arid E. Spiller, 'Potential Operating Region for Ultrasoff X-Ray Microscopy of Biological Materials'. Science, Vol. 196, p. 1339, 1977. 17. D. Sayre, J. Kirz, R. Feder, D. Kim, and E. Spiller, 'Transmission Microscopy of Unmodified Bilological Materials: Comparative Radiation Dosages with Electrons and Ultrasoft X.-Ray Photons*, Ul tramilcroscopy, Vol. 2, p. 337, 1977. 18. M. Howells, C. Jacobsen, J. Kirz. A. Feder, D. Quaid, and S. Rothmarn, 'X-Rlay Holograph i at Improved Resolution: A Study of Zymogen Granules', Science, Vol. 238, p. 514 (1987), and C. Jacobsen, M, Howells, K. Quaid, and S. Rothman, 'X-Ray Holographic Microscopy Improved Images of Zymogen Granules*, OSA Proceedings on Short -Wavelenoth Coher~er' Raitin Vol. 2, R. Falcone and J. Kirz, eds., Optical Society of America, Washingtonl

11f4 / $PIE Vol. 1227 fro#-Electron Lasers andApplications (7.990)

D.C., p. 295. 1988. 19. D. Joyeaux and F. Polack, 'Progress In Optical Reconstruction of Submicron X-Ray Holograms", OSA Proceedinas on Short Wyavelenoth Cohelrent Radiation, Vol. 2, R. Falcone and J. Kirz, eds.. Optical Society of America, Washington. D.C., p. 295. 1968. 20. J. Winthrop and C. Worthington. *X-Ray Microscopy by Successive Fourier Transformation'. Physics Letters, Vol. 15, p. 124. 1965; and G. Stroke and R. Restrick, 'H-olography with Spatially Noncoherent Llghte, Applied Physics Letters, Vol. 7, p. 229, 1966. 21. J. Janesick, *Potential of CCI~s for UV and X-Ray Plasma Diagnostics'. Reviews of Scientific Instruments, Vol. 56. p. 795. 1985; K. Marsh, "Nondispersive Spectroscopy and Imaging of Plasmas using a Charge-Coupled Device%, Reviews of Scientific Instruments, Vol. 56. p. 837, 1985, and 0. Lumb, G. Hopkinson. and A. Wells, Advances In Electronics and Electron PlagmAA. Vol. 648. p. 487, Academic Press. London. 196. 22. Y. Viadimiraky, 0. Kern. W. Meyer-11se, and D. Attwood, 'X-Ray Imaging of Nanostructure Patterns". Applied Physics Letters, Vol. 54. p. 285, 1989. 23. 1. McNulty. J. Kirz, C. Jacobsen. E. Anderson, M. Howells, and H. Raeback, 'Soft X-Ray Microscope Using Fourier Transform Holography", to be published In the Proceedings o-f the 6th Synchrotiron Radiation Instrumentation Conference. August, 1989. 24, 1990 QGuijnnes Book of World Roca-Ld, 0. MoFarlan and N. McWhirter, eds, Sterling Publishing Co., New York, p. 78, 1989. 25. J. Solem, "Imaging Biological Specimens with High-Intensity Soft X-Rays", Journal of the Optical Society of America B. Vol. 3, p. 1551. 1986. 26. W, Haddad, J. Solem, 0. Cullen, K. Boyer, and C. K. Rhodes, OA Description of the Theory and Apparatus for Digital Reconstruction of Fourier Transform Holograms', Electronics IMaging '87. Advanced Printino of Paoer Summaries, Vol. 2 (Institute for Graphic Communication, Inc., Boston, 1987) p. 693. 27. W. S. Hadidad, D. Cullen, K. Boyer, C. K. Rhodes, and J. C. Solem, *D~esign for a FourierTransform Holographic Microscope', Pr(.",eedlnas of the International Syvmoosium on X-Ray Microscog x edited by 0. Sayre. Springer -Verlag, Berlin, In press; W. S. Haddad, 0. Cullen. J. C. Salem, K. Boyer, and C. K. Rhodes, 'X-Ray Fourier-Transform Holographic Microscope'. O§A Proceedings on3 Short Wavelength- Coherent Radiation: Getneration and -A1onlicavion, edited by R. W. Falcone and J. Kirz (Optical Society of America, Washington, D.C., 1980) p. 28428. B. Henke, P. Lee, T. Tanaka, R. Shimabukure, and B. Fujikawa, 'Low-Energy X-Ray Interaction Coefficients: Photoabsorption, Scattering, and Reflection', Atomic Data and Nuclear Data Tables, Vol. 27, p. 1, 1982. 29. B. Aschenbach, 'Design, Construction, and Performance of the ROSAT High-Resolution XRay Mirror Assembly', Applied Optics, Vol. 27, p. 1404. 1988. 30. A. Compton and S. Allison, X-Ravs In Itheory and Experiment, Van Nostrand, 1935; and U Fano and J. Cooper, 'Spectral Distribution of Atomic Oscilla*.or Strengths', Reviews of Modern Physics, Vol. 40, p. 441, 1968. 31. A. Kondratenko and A. Skrinsky, Ootilca Information Processing, Vol. 2, p. 1, Plenum Press, New York, 1978.

SHE Vol. F227 free-Electron Laaara and Applications (19901 ' 58

APPENDIX F

UIC The University of Illinois at Chicago

Laboraoy for Atomic, Molecular, and Radiation Phyfics (M/C 273) Department of Physics College of Liberal Arts and Sciences Box 4348, Chicago, Illinois 60680 (312) 996-4868

Aug. 6, 1990.

MCR Technology Corporation P. 0. Box 10084 Chicago, IL 60610-0084

Dear Dr. Rhodes:

Enclosed please find the report for the *Propagation Studies for High Brightness X-Ray Sources* (subcontract 090189-SD1-1).

The main findings of this study are (1) capillary tubes are effective In enhancing harmonic conversion as well as parametric processes, (2) propagation properties of intensity laser beam are influenced by electrons produced from ionizations to the extend that the laser fluence Inside the capillary tube falls off rapidly.

One of the aspects in this study, namely large fluctuation in the harmonic intensity, have not been studied In great detail. However, one can speculate that it is directly related to the focal intensity distribution of the pump beam. Recently, we have developed a method of examining the focal Intensity distribution from any focussing system. From these Investigation, although incomplete, reveals that unless the laser beam is properly aligned to the focussing optics, the focal Intensity distribution can be distorted severely by astigmatism and spherical aberration. This Immediately suggests that the performance of harmonic and parametric conversion can be expected to improve substantially when these optical distortions can be controlled judicially. Furthermore, availability of this diagnostic tool will allows us to perform better controlled experiments to investigate spectral stability Issue of harmonic radiation in the future.

It has been a pleasure working on this contract. If there Is any questions about the report, please feel free to contact me.

Sincerely yours,

Ting Shan Luk

UIC The University of Illinois at Chicago

Laboratory for Atomic. Molecular, and Radiation Physics (M/C 273) Department of Physics College of Liberal Arts and Sciences Box 4348. Chicago. Illinois 60680 (312) 9964868

13 July 1990

FINAL TECHNICAL REPORT "Propagation Studies for High Brightness X-Ray Sources 5 September 1989 - 5 February 1990 Subcontract No. 090189-SDI-I

Principal Investigator: Ting Shan Luk

Prepared for: Charles K. Rhodes, President MCR Technology Corporation P. 0. Box 10084 Chicago, Illinois 60610-0084

Approved:

Ting Shan Luk

Abstract

The advent of subpicosecond laser systems capable of achieving focal intensities exceeding 1019 W/cm 2 allow consideration of a fundamentally new mode of electromagnetic propagation, namely channeled propagation, In which both relativistic electronic motions and electron expulsion from the region of high laser intensity produces a channel which could be useful as a high-aspect-ratio longitudinally pumped x-ray laser source. Two classes of experiments to Investigate longitudinal pumping were performed. The first used small bore capillary tubes to investigate the propagation of the pump laser through an extended gas target and to examine the radiation emitted from this target. The second class of experiments Investigated the laser beam propagation through a static gas by photographing the breakdown of the target gas in the laser focal region in order to determine the spatial location of the emitted radiation.

CONTENTS

I. Introduction

II. Capillary Transmission Studies A. Low Power B. High Power C. Capillary Reflections

III. Spectral Studies A. Normal Incidence Spectrometer B. Grazing Incidence Spectrometer

IV. Photographic Studies of Propagation in Static Gases

V. Conclusions

VI. References

VII. Appendices

Channeled Pr.9paoation Simulation in Capillary Tube Targets

I. Introduction

The Interaction of high intensity subpicosecond laser systems with gaseous

targets allow the production of plasmas that are characterized by a low

temperature, perhaps as low as a few tens of electron-volts, and a density that may

approach the critical value. A fundamentally new mode of electromagnetic propaga

tion Is theorized to arise in this species of plasma, namely, channeled propagation

produced when the laser intensity exceeds a value approximately one half of the

Compton intensity, 1 which for a KrF* laser is 4.5 x 10'9 W/cm'. Because of

charge displacement, the boundary of a channel can be associated with a sharp

change of index of refraction of the target medium resulting from relativistic

motions and the expulsion of electrons from the region of high laser Intensity. If a

channel is formed, a self-contained cylindrical region of target material would be

created which could serve as a high-aspect-ratio source of stimulated radiation.

Recent dynamical calculations 2 have Indicated the existance of such a stable mode

of propagation. The ability to propagate high intensity radiation via channeled

propagation, and thus, longitudinally pump a target material is the main motivation

for pursuing the Investigations described below.

It has been established that atoms can be put Into high charge states through

a multiphoton absorption process when gaseous atomic targets are exposed to strong

electromagnetic fields created by high intensity laser systems,3,4 Experiments in

this laboratory have demonstrated that the resulting ion can be left In an excited

state, although whether the excitation Is a direct consequence of the multiphoton

process or is the result of collisional recombination processes is yet to be

determined. What is certain Is that, depending upon the target material, copious

quantities of short wavelength radiation are produced from the small interaction

volume of target gas produced from a pulsed gas jet and the focal volume of the

laser system, the lighter gases up through Ne being the noted exceptions.5

The following premise was proposed to investigate longitudinal pumping of a

gaseous target medium. Assume that the observed radiation from a pulsed gas jet

target is produced by either a direct multiphoton process or by collisional

recombination processes. Further, assume that this radiation can be made to

originate from an extended source rather than the typical focal volume of the

laser. Can or will this extended emission region, which would be generated by

confining the target gas inside a small bore capillary tube, exhibit arm~plification?

Longitudinal pumping is accomplished by utilizing a small bore capillary tube to

extend the length of the target gas to a cylindrical volume. In the event that

recombination is the primary method of producing excited state Ions, the capillary

tube walls will assist in rapidly cooling 6 the plasma produced. Small bore glass

capillary tubes are used because the smooth interior wall has a high reflectivity at

grazing Incidence, a fact which allows for the reflection of the laser beam rather

than Its absorption, much like the anticipated refractive index boundary resulting

from channeled propagation at high laser intensity in a gaseous target. Emission

will then presumably occur along some fractional length of the capillary tube. Only

radiation emitted in the direction parallel to the capillary bore, or parallel to the

axis of propagation and not absorbed by residual gas, will be able to escape the

capillary tube and be detected. Whether the detected radiation is the result of a

direct multlphoton absorption process yielding an excited state, and therefore

prompt, or If It Is due to collislonal recombination mechanisms, amplification parallel

to the capillary tube bore should result if absorption from the ambient material Is

sufficlent!y low.

II. Capillary TransmIssion Studies

A. Low Power

Preliminary studies considered the transmission of a HeNe laser beam focused

Into and through a small bore capillary tube. These studies established the

methodology o* focusing and aligning a laser through a small bore capillary tube.

Essentially, the capillary tube was found to be correctly aligned when the

transmitted beam produced a TM, 0 image on a screen positioned beyond the output

side of the capillary tube. Figure 1 illustrates the initial setup for these studies.

The capillary tube was positioned Inside a small vacuum chamber equipped with CaF2

input and output windows and pumped to a base pressure of a few millitorr. The

exact base pressure was not important as long as it was much lower than the target

gas pressure and sufficient not to affect the laser beam propagation, especially

when thu KrF* laser was used. Alignment of the HeNe beam, and the KrF*, was

facilitated by placing the vacuum chamber with the enclosed capillary tube upon an

X-Y-Z-"-- translation stage. A HeNe laser was focused into a 250 pm bore

capillary tube which has a length of approximately 10 cm. The focusing optic was a

56 cm focal length lens. Since the HeNe laser beam was expanded and collimated to

a 1.3 cm diameter beam, an effective f/42 focal system resulted. The transmission

of the HeNe beam through this capillary tube was measured using an empty capillary

tube as well as the capillary tube with N2 flowing through it. Gas was supplied to

tne capillary tube by attaching another tube to a small hole which had been drilled

into the side of and at the center of the capillary tube. A constant gas flow was

mainta 1-d while measuring the transmission. The capillary tube was translated

along the laser path to assure that the entrance of the capillary tube was

positioned at the focal position of the laser beam. The target gas density was

varied by changing the gas line pressure to the capillary tube from 10 psi up to 70

psi. The transmission of HeNe through the capillary tube for a target gas line

pressure of 50 psi is plotted In Figure 2 and compared to a static air fill of 760

torr or less. Nearly 90% of the HeNe beam was transmitted through the empty

capillary tube. However almost 97% was transmitted through the capillary tube

Vacuum Gauge Gas Inlet Wido

Pump Out

Window 250 Aim Bore * Capillary Tube

Figure 1. initial experimentlal setup to measure the transmission of laser light through a small bore capillary tube.

GAS: N. LwonsFboellLenth m56c CM f* 42 Be=m Dfsmter 1 3 cm

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31 Tb t070Tr

I~ 27

-5-4-3-2-1 0 12 34 56 7 891 011

Micromtetr Position (Relative Lhits)

Figure 2. A relatively high percentage of HeNe laser light was transmitted through the 250 Vim bore capillary tube. When gas was flowed through the tube, the transmission Increased.

when N2 was flowing through it and being pumped out of the encompassing vacuum

chamber. This increase in transmission was attributed to a lensing effect of the

gas jet escaping out the end of the capillary tube. The escaping gas served to

favorably deflect the laser beam into the capillary bore. Increasing the gas flow

above a characteristic maximum value, typically 40 psi, but which depended upon the

diameter of the capillary bore, yielded no additional transmission due to this lensing

effect. Changing the bore diameter of the capillary tube to 150 prm resulted in a

reduced transmission, perhaps because of the focusability of the HeNe laser beam.

However, the lensing effect became more pronounced as indicated in Figure 3 with

tne transmission Increasing from 72% to 83%. Smaller bore capillaries were not

considered because of an inability to remove all debris from the bore resulting from

the side hole duilling process.

Hav;n~j demonstrated thet a HeNe laser beam could be transmitted through a

small bore capillary tube of considerable length, propagation of the KrF* beam

through the capillary cube was considered. Initially, a KrF* beam of approximately

100 pJ was focused into the capillary tube using the same' setup as described in

Figure 1. Alignment of the capillary tube bore with respect to the KrF* laser beam

was found to be critical as evident by the reduced quality of the TM,,0 mode. Data

was taken after the best TM.0 mode image was obtained. Transmission through an

empty 10 cm long capillary tube with a 250 pm bore diameter was determined to be

approximately 40% to 50%. The transmission dropped rapidly as the capillary tube

was rotated away from the maximum transmission orientation.

B, High Power

After performing several experiments involving the acquisition of emission

spectra, which are discussed below, the transmission of the small bore capillary

tubes was tested using approximately 100 mJ of KrF* energy from the amplifier

Prometheus.7 The focusing optic used was a 10 cm diameter lens with a focal

41 44- 1 t 171 .1

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Figure 3. When the capillary bore was reduced to 150 pm, the transmitted HeNe energy was reduced. However, the effect of gas flowing through the tube was Increased.

length of 100 cm. Since the KrF* laser beam from Prometheus has a rectangular

2" by 4" shape, the focal system was f/10 to f/20. Two sizes of capillary tubes

were used, 40 mm long with a 0.75 mm bore and 30 mm long with a 0.25 mm bore.

Gas was admitted into the capillary tube through a pulsed valve gas jet assembly

similar to that -shown in Figure 4. Later modifications allowed capillary tube

lengths from 2.5 cm up to 23 cm to be tested. Target density in the capfilary tube

could be varied in two ways. One could change the backing pressure to the pulsed

valve or one could change the time delay between the laser fire signal and the

pulsed valve open signal. The gas flow through the capillary tube was monitored

and calibrated using a differential microphone. Typical responses for two capillary

tube lengths and bores are illustrated in Figure 5.

For the transmission test using the Prometheus amplifier, the timing was set

so that the capillary tube was completely filled with gas and the gas flow was

nearly a maximum at the capillary tube exit. Figures 6-9 Illustrate the results. As

noted In Figures 6 and 7, some fraction of the KrF* laser beam could be

transmitted through al of the test gases even at high pulsed valve backing pressure

when using a 750 pm bore 4 cm long capillary tube. However, when the capillary

tube bore was reduced by a factor of 3, to 250 pim, and the length was decreased to

3 cm, the transmission of the KrF* beam dramatically changed. Transmission

through the empty capillary tube dropped. While transmission through He and Ne

remained high, even at high pulse valve backing pressure, transmission through the

heavier gases rapidly dropped to zero. This effect cannot be attributed only to a

refractive effect due to the abundance of electrons occuring from ionizing Ar and

Kr because H2 behaved like Ar and Kr in attenuating the laser beam as Illustrated

In Figure 8. The transmission of the KrF* beam through an empty 30 mm long

capillary tube with a bore of 250 iprr was measured to be approximately 60%, or

roughly equal to the transmission through a much longer capillary tube using 100 pJ

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Figure 8. When the capillary tube bare was reduced to 250 pm and the length decreased to 3 cm, the KrF* beam transmission was significantly reduced. While He and Ne still transmitted the beam H 2 , Ar and Kr did not.

I2 II

-Z Z- - -4

Figure 9. Schmatic illustrating the laser being focused into a capillary tube of length I and bore diameter b. The focusing optic has a diameter of d and a focal length f. The first reflection Occurs at position r Into the capillary tube.

of energy and a larger f-number focal system. Clearly, the KrF* beam is being

absorbed into the capillary tube walls at each reflection. Finally, note that addition

of gas does not produce a lensing effect. Rather, the laser transmission through

the capillary tube decreases, for all gases, as the gas flow through the capillary

tube is increased. Only by increasing the capillary tube bore diameter to 750 pm

does the laser beam transmission impove. A defocusing action due to the electron

density could, at least in part, account for these observations.

C. Capillary Reflections

Internal reflections are important since the laser beam is partially absorbed at

each reflection. Consider Figure 9 In which a lens Is used to focus laser light

and at the bore of a capillary tube. Using light rays to define similar tridaI,

the first reflection is found to occur at the position

r = bf/d

where b is the capillary bore diameter, d is the laser beam diameter and f is the

focal length of the lens. Additional reflections will occur at (2n-1)r along the

length of the capillary tube where n is an integer. The total number of reflections,

N, in a capillary tube is found from

(2N -1)r = I

where I is the total length of the capillary tube. In the two transmission examples

cited above, there will be 2 reflections for the 40 mm long capillary tube and 4

reflections for tho 30 mm long capillary tube, the small bore capillary tube

experiences more reflections. Following several light rays through the capillary

tube indicates that beyond the point 2r, the laser beam establishes an average

intensity over a diameter of approximately b/2. The angle of reflection Is given by 6 = tan-1 (d/2f).

If the focusing optics result in an angle of incidence which is too large, absorption

will be severe. Denote the absorption of the laser beam at a reflection as Y. For

the examples of Figures 6-8, the laser beam travelijlg tnrodgIh the 250 pm bole

capillary tube mould experience considerabily more absorption, -Y-2, than the laser

beam traveling through the 750 pm bore capil!ary tube.

Sin•ce the gas flow through the capillary tube will be Important, as indicated

by the strong attenualion of the KrF* beam when using H,, Ar and Kr, as well as

-the need to know the target density, a model was established which assumes that

the gas flow is viscous. The validity of th!s model was check:ed using an

interferometer to measure the rata of gas flow in the capillary tube and iound to

be within 10% of the calculated value. For details of the Interferometer

measurement see Appendix A. Further testing of 'he gas flow was accomplished

using a microphone to monitor the flow through the capillary tube as illustrated in

Figure 5. From the model, the pressure at the end of a tube is related to the

pressure at the input of thq tube by

P' = -.(/2 + (132 + 4p1)0i/2

where

c= 16rqvmL/a",

T1 is the viscosity of the gas, vm is the molecular velocity of the gas, L is the

length of the tube, and a is the bore radius, A worked example is presented in

ApDendix B Indicating that target densities as large a- 101" cm'' are encountered.

With modest ionization of the target gas along the cap;llary tube, rather large

electron densities could result.

III. Spectral Studies

A. Normal Incidenca Spectrometer

Spectra from various gase, were obtained using the capillary iarget

geometry. Two seperate data acquisition systems were employed, a 1-meter normal

Incidence spectrometer (NIS) with a 1200 I/mm (,rating and a 1-meter grazlnt

incidence spectrometer (GI6) with a .300 I/mm grating. Each spectrometer employed

a microchannel plate (MCP) detector mounted at the exit focal plane of the

spectrometer. For the NIS, a charge coupled device (CCD) camera was coupled to

the MCP allowing 21-nm regions of the spectrum to be simultaneously collected.

For the GIS an optical multichannel analyser (OMA) was coupled to an MCP

mounted tangenlally to the Rowland Circle and allowed 6-nm regions of the

spectrum to be collected. Spectral resolution on the NIS was 0.9 nm due to using

the capillary tube bore as the spectrometer entrance slit. The GIS gave 0.2 nm

resolution. Alignment of the capillary tube bore with the optical axis of the

spectrometer was achieved using an X-Y-Z manipulator which allowed tilt motion.

The gasee H2, He, N2, Ne, Ar, Kr and Xe were used as target materials.

Spectra between 35 nm and 90 nm were obtained for all gases, with a few extending

up to 130 nm. The gases Ar and N2 will be discussed seperately with the remaining

gases being summarized together for completeness. Three general areas of

investigation were examined: (1) the spectrum as a function of cap'llary tubg

length and bore diameter; (2) the spectrum as a function of target density; and (3)

the spectrum as a function of laser energy and confocal parameter. Most of the

experimential effort is limited to the first two areas. Carefid studies requiring

changing the laser energy and the confocal parameter were not possible, only large

changes were attempted to clearly determine if an effect was present.

The first experlmential configuration utilized the NIS. A 23 cm long capillary

tube with a 0.75 mm bore diameter was positioned at the Image plane and served as

the entrance aperture to the spectrometer. Argon was selected as the Initial target

material because of familiarity with the VUV spectrum. 5 Figure 10 Illustrates the

typical spectrum obteined. Two features are noted. First, the third, fifth and

reventh harmonics are evident aithough the seventh Is weak. These three lines can

be used as an apf~roximate wavelength calibration to compensate for errors

Int(oduced by using the capillary bore as the spectrometer entrance sOlt. Second,

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Figure: 10. Arglon spectrurn, obtained using a 23 cm long capillary tube with a 750 jar bore as• the target geometry. The KrF-laser energy was approxi-nTwel,, 10- mJ- Focusing was with an f/25 to f/10 optical system.

all of the remaining spectral lines are unidentified lines, and the spectrum Is not

nearly as rich as that obtained previously using only a gas jet target.5 The

strengths of the signals vary considerably with the third harmonic, at nearly 2000

times stronger than tho fifth harmonic, being the strongest feature. No lines were

observable below 33 nm because scattered light from zeroth order prevented

observation below this region.

Argon was searched at longer wavelength. Figure 11 Illustrates an interesting

feature observed at =124 nm which Is thought to be Ar2*. Since this transition has

a natural line width of = 10 nm, the signal in Figure 11a would represent a gain of

*=3.5 cm-1, the strongest gain yet recorded for this excimer transition.8- 10 As

should be expected, this line exhibited a strong dependence upon laser energy and

confocal parameter as illustrated In Figures 11a-d.

Figure 12 illustrates the lines grouped around the third harmonic In the Ar

spectrum as the target pressure is increased by increasing the backing pressure of

the pulsed valve. Figure 13 plots the signal strength as a function of backing

pressure as the pressure is increased and then deo gased and illustrates that the

effect is reproducible. Two features are svident, First, the third harmcnic has a

strong target density dependence as evidenced by the rapid Increase In signal when

the backing pressure is increased from 15 to 25 psi. Second, whon the target

density reached a critical value, the third harmonic signal drops rapidly and a new

signal at 87 nm rises. The association between thse two spectral signals will be

discussed later.

As noted earlier, the target dunsity within the capillary tube could be varied

other than by changing the pulsed valve backing pressure. By changing tIne timing

between the laser fire command and the pulse valve open command, the laser pulse

could be made to arrive at any time durig tie gas flow. Figure 14 Illustrates the

change In signal which Could be achieved by altering the target density In this

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Figure 11. Argon spectrum illustrating the. laser energy dependence and confocai parameter dependence of the Ar2* 124--nm line. (a). Laser energy c 25 mJ. Large confocal parameter achieved by aperaturing down the laser beam. (b). Lase= energy - 25 mJ. Full laser beana aperature. (c). Laser energy =100 mJ. Full la.ser beam aperature, (d). Laser energy =140 mJ, Full laser beam aperature.

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Figure 14. Using a 23 cm long capillary tube with a 750 pm bore, the region of Ar spectrum around the third harmonic changes as the target density Is changed by varying the delay between the laser fire signal and the turn on of the pulsed valve. (a). 500 psec. (b). 1000 psec. (c). 1500 psec.

fashion. Note the presence of the 87 -nm feature as seen in Figure 12 and that

the third harmonic has decreased significantly while lines at lower wavelength have

appeared to replace it. Changing the timing was a more accurate and reproducible

method of changing the target density than changing the pulsed valve backing

pressure on the present setup.

When a 9 cm long capillary tube with a 0.75 mm bore was used with Ar as the

target gas, the basic spectrum was not changed. However, the variation witti target

density did become more pronounced. Figure 15 illustrates the variation in the

third harmonic region and the 87-nm region as the target density was changed.

Note the sharp growth of the signal in the 87-nm region and its near equal

intensity with the third harmonic region once it !s established. More about this

variation with target density will be discussed later in association with the observed

change in the N2 spectrum.

B. Grazing Incidence Spectrometer

The instrumentation was changed to the GIS to obtain better resolution of the

observed spectrdm, In order to reduce the absorption of emitted radiation along

the capillary tube axis, a 40 mm long capillary with a 0.75 mm bore was used

Since no new spectral lines were seen, the region around the third harrronic was

selected for a more detailed study. Figure 16 illustrates the results, The laser

energy was -200 mJ or greater, The target gas density was changed by using a

capacitance manometer to monitor the pulsed valve backing pressure, Accuraie

reproducable low backing pressures to the pulsed valve were achleveable by this

wiethod. The time delay between the laser fire signal and the pulsed valve open

signal was held constant, It was selected so that the gas flow had reached the end

of the capillary bore, but not yet reached a maximum value. Considerable structure

was found In the region of the third harmonic as the target gas density was

changed. None of the lines could be Identified with the known spectrum of argon,

fashion. Note the presence of the 87-nm feature as seen in Figure 12 and that

the third harmonic has decreased significantly while lines at lower wavelength have

appeared to replace it. Changing the timing was a more accurate and reproducible

method of changing the target density than changing the pulsed valve backing

pressure on the present setup.

When a 9 cm long capillary tube with a 0.75 mm bore was used with Ar as the

target gas, the basic spectrum was not changed. However, the variation with target

density did become more pronounced. Figure 15 illustrates the variation in the

third harmonic region and the 87-nm region as the target density was changed.

Note the sharp growth of the signal in the 87-nm region and its near equal

inten3ity with the third harmonic region once it is established. More about this

variation with target density will be discussed later in association with the observed

change in the N2 spectrum.

B. Grazing Incidence Spectrometer

The Instrumentation was changed to the GIS to obtain better resolution of the

observed spectrum. In order to reduce the absorption of emitted radiation along

the capillary tube axis, a 40 mm long capillary with a 0.75 mm bore was used.

Since no new spectral lines were seen, the region around the third harmonic was

selected for a more detailed study. Figure 16 illustrates the results. The laser

energy was =200 rnJ Or greater. The target gas density was changed by using a

capacitance manometer to monitor the pulsed valve backing pressure. Accurate

reproducable low backing pressures to the pulsed valve were achleveable by this

method, The time delay between the laser fire signal and the pulsed valve open

signal was held constant. It was selected so that the gas flow had reached the end

of the capillary bore, but not yet reached a maximum value. Considerable structure

was found in the region of the third harmonic as the target gas density was

changed. None of the lines could be identified with the known spectrum of argon.

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Figure 15, Using a 9 cm long capillary tube with a 750 )jm bore, the region of Ar spectrum around the third harmonic exhibits simular structure as Illustrated 0n Figure 14. (a). 250 psec. (b). 500 psec, (c). 750 psec, (d). 1000 sec. (e). 1500 psec. (f), 2000 psec.

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Figure 16. Using a 4 cm long capillary tube with a 750 Prn bore, a more detailed Ar spectrum around the third harmonic Is obtained with the GIS The target dei~sity was now changed by varying the pulsed valve backing pressure. (a). 200 torr. (b). 325 torr. (c). 550 tort. (d). 760 tort

(e). 1975 torr. (t). 2475 torr.

The capiiilry tube was changed to one which was 30 mm long and had a bore

diameter of 0.25 mm. The results for the region around the third harmonic are

illustrated In Figure 17. Again the structure around the third harmonic is distinct

as the target gas pressure is Increased. With the smaller bore capillary tube, the

structure is more quickly achieved as the target gas density is increased and the

structure extends over a greater wavelength range. Once the extended structure Is

established, the observed lines exhibit a strong dependence upon laser energy.

Rarely do all of the lines appear together during a single laser shot, rather they

iump lo and out depending upon the laser energy. This is evident in Figures 17e

and 17f (ai whiTh the target gas density changes little, yet the spectrum changed

noticeably.

The most unuiv. feature found in the Ar spectrum Involves the 63-nm line,

see Figurju 10. This •i~e was found to exhibit amplificationl While the amplification

ýs smal!, only 20A.4 cm-', it did seem to lend creedance to the idea that this

capillary targek Vonetry could be longitudinally multiphoton pumped to produce

lasing. TNot only did this line exhibit gain, but it also exhibited line narrowing.

Figure 1- illustrates the increase in signal and the decrease in line width as a

function of pulse valve backing pressure.

The difficulty in understanding the 63-nm line in Ar is manifested by the

discovery that many of the lines observed around the third harmonic and 87 nm

could fit into a parametric process involving sly wave mixing. For details, see

Appendix C. Figure 19 illustrates the various six wave mixing processes Identified.

That six wave mixing had never been reported In Ar was not the problem. The 63

nm line playing a prominent role In the six wave mixing process does present a

problem. If the 63-nm line Is the result of six wave mixing, it can exhibit gain,

However, it Is not clear that such a parametric process would exhibit line

narrowing, The measurement was repeated with the same results, a rapid Increase

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Figure 17. Argon spectrum around the third harmonic using a 3 cm long capillary tube with a 250 pm bore. Note the increase in structure compared to Figure 16. The pulse valve backing pressure Is: (a) 50 torr; (b) 290 torr; (c) 620 torr; (d) 1040 torr; (e) 1950 tort; (f) 2000 torr.

*(x 20) Ar 630 and 1067 (A)

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in signal to a saturation point as the target gas density is increased and a

corresponding linewidth decrease. These aspects of the behavior remain to be

completely understood.

Nitrogen was an important target gas to consider for these capillary tube

target experiments because of the results obtained from multlphoton Ionization

experiments as well as the radiation experiment results obtained from using a gas

jet target.1 1- 13 Previous measurements on molecules suggested that molecular

species would exhibit Interesting properties in regard to the production of excited

states. Moreover, the previous N2 work implied that more complicated molecular

species containing high Z material symmetrically surrounded by lighter material

could be better candidates for target gases.1 4

When N2 was used as the target gas in a 23 cm long capillary tube with a 0.75

mm bore, qualitatively similar results were obtained. The spectrum Illustrated in

Figure 20 shows a strong third harmonic, a weak fifth harmonic and a few weak

unidentified lines. Regions of the spectrum not illustrated were devoid of spectral

lines, Figure 21a-b and 21c-e illustrate the rapid change in line Intensity as the

target density is changed by increasing the delay between the laser fire signal and

the pulsed valve open signal. The results are the same as observed in Ar,

especially as pertains to the region around the third harmonic.

Later a 9 cm long capillary tube with a 0.75 mm bore was used as the target

geometry. The typical N2 spectrum obtained Is illustrated in Figure 22. Basically

there is no change In the spectrum from using the much longer 23 cm capillary tube

except for the few additional longer wavelengths. Using the shorter capillary tube

did result In a significant change in the observed spectrum when the target density

was changed by altering the time delay between the laser fire command and the

pulse valve open command. Figures 23 and 24 illustrate the point. Figures 23a-b

show a dramatic change in the third harmonic with it being completely replaced by

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Flgure 20. Nitrogen spectrum obtained using a 23 cm long capillary tube with

a 750 pm bore.

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Figure 21. Nitrogen spectrum obtained using a 23 cm long capillary tube with a 750 pm bore. Note the rapid change in the line structure as the target density is changed by altering the time delay between the laser fire signal and the pulsed valve open signal. (a). 500 IJSec. (b). 1500 IpSec. (c). 500 pusec. (d). 750 pusec. (e). 1000 psec.

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Figure 22. Nitrogen spectrum obtained using a 9 cm long capillary tube with a 750 pm bore. Note the longer wavelengths.

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Figure 23. Changing the N2 target density by varying the time delay sIgnificatly altered the spectrum. (a). 500 psec. (b). 1500 pAsec. (c). 750 psec. (d). 1000 psec.

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Figure 24. Changing the N2 target density by varying tfie time delay significantly altered the spectrum. (a). 500 psec. (b). 1000 psec. (c). 1500 psec. (d). 2000 psec. (e). 3000 psec. (f). 4000 psec,

100

a new line of equal strength while Figures 23c-d show an equally dramatic variation

concerning the line at =95 nm. The variation of the spectral line intensities

observed In Figure 24 is difficult to explain on the basis of only a changing target

density. The pulsed valve is turned on for approximately 250 psec. During this

time gas is flowing into the small reservoir seperating the pulsed valve body and

the capillary bore. The pulsed valve closed before the gas reached the end of the

capillary bore at approximately 500 psec. Changes in the spectrum are expected to

occur as the target gas density increases and the capillary tube is filling with gas.

This change is clearly evident in Figures 24a-b. When the capillary tube was filled

with gas and the gas density was maximized throughout this volume, the spectrum

should exhibit little change for some range of timing change. This is clearly

shown In Figures 24b-c. Beyond this time delay the target gas density is

decreasing. As the time delay is increased, the resulting spectrum should

reproduce the spectrum seen with a small time delay. Figures 24d-f show that this

Is clearly not happening. What are the differences in the target between Figures

24a and 24f which could account for this variation in observed spectrum? They are

three. First the target gas density really is different. In Figure 24f the target gas

density is the result of residual gas remaining In the capillary tube. In Figure 24a

the gas is beginning to flow into the capillary tube but has not yet reached the end

of the tube. Second, the resulting lengths of the target gas are different for the

reason stated above. The target gas density integrated along the interaction length

should be much less in Figure 24f than in Figure 24a, because of the much lower

gas density. Third, the target gas density gradient in Figure 24f should be small,

since the gas flow has ceased whereas in Figure 24a It should be large since the

gas flow is just beginning, see Figure 5. This target gas density gradient Is

probably the dominating factor in the observed spectrum because the Increased

density gradiant would have a strong effect upon the propagation of the laser

101

beam through the capillary tube.

Gas mixtures of N4 2 and Ne were also tried. Neon was added because Ne is

the best harmonic generator found to date for KrF*.5 Mixtures ranging from 10%

Ne to 90% Ne were used. The resulting spectrum was either the same or the

spectrum was weaker depending upon the spectral region. Variation in the shot to

shot laser energy could have had an effect, but It was difficult to accurately

monitor such behavior. The spectrum changed with each laser shot, but generally

the time averaged spectrum exhibited little variation.

When the GIS was employed for greater resolution, emphasis was place upon

the third harmonic region. Using a 30 mm long capillary tube with a 0.25 mm bore

produced a spectrum strongly dependent upon target density, confocal parameter and

shot to shot variation in laser energy. The results are summarized in Figure 25.

Figures 25a-b illustrate the effect produced by increasing the confocal parameter

and, hence, reducing the laser energy. Figure 25c-d illustrate the large variation In

signal strength even when 10 laser shot averages were considered. Increasing the

confocal parameter by aperturing down the laser beam had the effect of producing

slightly shorter wavelengths when the target gas density was increased as Illustrated

in Figure 26.

A quick look was taken at the fifth harmonic. Similar structure was observed

which varied over the accumalated 10 shot averages as illustrated In Figure 27.

The other target gases used, H2, He, Ne, Kr, produced similar modulation of

the third and fifth harmonic as a function of target gas density and laser energy.

These results are summarized In Figures 28-31. A partial spectrum of Xe is

Included In Figure 32. Little was done with either Kr or Xe because of the ease in

Ionizing these gases. Ionization effectively prevents the laser beam propagation In

the capillary tube as shown in Figure 9. The lines which were appearing In all of

the gases around the harmonics were related to the fundamental or the harmonics

102

- 0

711

N p.O

7711.9 799.3 819.6 840.0 860.4 880.8 778.9 799.3 "819.6 340,0 860.4 "--oom Wavelength (Ang) Wavelength (Ang) ... . p

-o

(AI

778.0 799.3 819.6 840.0 960.4 880.8 778.9 799.3 819.6 840.0 8f0,4 ago$ Wavelength (Ang) Wavelength (Ang)

Figure 25. Nitrogen spectrum near the third harmonic using a 3 cm long capillary tube with a 250 pm bore. The spectrum changes with target density, confocal parameter as well as varied from shot to shot. (a). Pulsed valve backing pressure ý 20 torr. Beam aperatured to V" diameter. (b). Pulsed valve backing pressure - 50 tort. Full beam aperature. (c). Pulsed valve backing pressure = 100 torr. Full beam, (d). Pulsed valve backing pressure = 100 torr. Full beam.

103

778.9 799.3 819.6 840.0 860.4 880.8 Wavelength (Ang)

-c

L 2

71d.9 799.3 619,6 a 40.0 860.4 880,

!.-Wvelength (Ang)

Figure 26. Nitrogen spectrum as in Figure 25 with the laser beamn apertured to lo. The pulsed valve backing pressure was changed. (a). 20 torr. (b). 465 tort. (c). 700 torr.

104

0 (AA 0 0

458.5 474.2 490.0 505.7 521.5 537.3

Wavelength (Ang)

42 Ii .4#

4 15 8 5 474.2 490.o 505.7 521.5 537.3

Wavolength (Ang)

Figure 27. Nitrogen spectrum near the fifth harmonic showing that even here the spectrum changed as the target density was changed and that there varatlons over 10 shot averages. (a). 20 torr. (b). 50 torr. (c). 50 torr.

105

a' 'A

778.9 79 9..3 819.6 840.0 860.4 a go 's 77 4.9 79 .3 ,819.6 8 40.0 560 ,4 8 80.8 Wavelength (Ang) Wavelength (Ang)

s. 790.3 B D.6 I4 9.0 86 0.4 ado.$ 77 a., 9 9 .3 8 .1 $ 40.0 8 o.4 ado.& Wavelongith (Ang) Wavelength (Ang)

"ta

778.9 79g.3 819.6 840.0 860,4 880,8 Wavelength (Ang)

Figure 28. Hydrogen spectrum near the third harmonic showing the modulation obtained by varying the target density. Each spectrum is a ton shot average. (a). 30 torr. (b). 65 tort. (c). 105 tofr. (d•). 117ý, tort. (e). 250 tort.

'10 1'

aýc

L)/

N.,

77B 7g.3 8196 b40.0 860.4 880.8 7178.9 799.3 8g, "Wavelength (Ang) 196 40.0 860.,4 8M8. Wavelength tAng)

.0 .0

-i N

778,9 79i,-3 819.6 840.0 660.4 B8O.8 Wavelength (An g,

VCU

Waval'ngt* (Ang) 471,4 487.3 503,3 519.T 5.2 5512 Wavdl•ngth (Ang)

FigUre 29. Helium spectrum ilear the third and fifth harmonlic. Note the strong variation in th3 single shot plot$. (a), 58 torr. 10 shots, (b). 750 torr, 10 shots. (c), 1400 torr. 1 shot. (d), 850 tort. I shot. (e). 850 torr, 1 shot.

107

778.9 799.3 819.6 840.0 860.4 880.8 Wavelength (Ang)

CCo

0C(N ,,4

7,78.9 799.3 819.6 840.0 860.4 880.8 Wavelength (Ang)

Figure 30. Neon spectrum near the third harmonic using 10 shot averages. (a). 105 torr. (b), 2050 torr.

108

(A1

7 78. 91 7 t)9., 019 . 6 040 .0 8 6 0.4 e 8 ,8. o-! . .,s Wavelength tAng) . 79.3 619., 34o.0 ISSO. 660A Wavelength (Ang)

C!C

F. LI

S" V

778.9 709.3 819,O 840.0 660.4 s8. " Waeeghtn )776,9 799o,3 819,6 840.0 460,4 800.8 eWavelength tAng)

Figure 31. Krypton spectrum near the third harmonic. Single shot spectra. (a). 85 torr. L~aser energy = 22:3 mJ. (b). 85 tort, Laser energy - 252 mJ. 0 75 torr, Lser energy 260 mJ. (d). 375 tort.

Laser energy 19l 3 mJ,

109

440

CI4

cJr

604.7 622.7 640.6 658.5 676.5 694.4 Wavelength (Ang)

oi..,.

0 V) V

1010.5 103J3.9 10O5-P7. ' 1080.7 1104.1 1127. 5 Wavelength (Ang)

Figure 32. Partial xenon spectrum showing the s•trongest observed li'nes,

(a). 297 torr. Laser energy - 99 rnJ. (b). 628 torr. Laser energy =77 mJ.

.10

In some unknown way. It is not clear at present if they are due to a wave mixing

process as discussed for the case of Ar, although a six-wave mixing scheme does

exit In Xe.

IV. Photographic Study of Propagation in Static Gases

In addition to the longitudinal pumping studies using the capillary tube to

obtain spectra, beam propagation was investigated by photographically recording the

focal region in H., He and Air for different gas densities. Initially an f/10 lens

system was used to focus =150 millijoules of energy In air. Figure 33 shows the

typical results. The convergence of the Incoming laser beam is observable as a blue

glow due to excitation of the air. Once the laser intensity becomes sufficient to

Ionize the air, the observed emission becomes white. The resulting bright fireball

at the focal zone of the lens system Is the result of the explosion of the ionized

gas. At this point the laser intensity is sufficient to significantly ionize N. and 02

producing many free electrons. These electrons should change the refractive index

of the surrounding space. Hence, the resulting plasma should have either ab3orbed

the laser beam, deflected it or caused it to diverge. What the photograph clearly

shows leaving the breakdown region Is a beam with a cone angle less than that of

the incoming laser beam. Either this emerging beam is the result of third harmonic

excitation of the surrounding air, or some other process allowed the KrF* beam to

propagate through the breakdown region and emerge with a smaller divergence.

The results of focusing the laser beam in air lead to a more careful study of

the laser beam propagation through static gas targets of various density. Now the

KrF* beam was focused using a 30 cm focal length aspheric mirror. With up to 200

mJ of energy available from the Prometheus amplifier, the maximum focal intensity

Is expected to reach 1019 W/cm 2. It is assumed that the laser beam was focusible to a spot size of 2 micrometers. 7

Figures 34-38 illustrate some of the salient features of the laser beam

lii

Figure 33. Laser beam focused In air using f/10 optics. In (a) note that the beam leaving the focal breakdown region has a smaller divergence than the Incoming beam. In (b) note that the focus region has two lobes, the smaller one appearing just before the main breakdown.

112

Figure 34. Laser beam focused Into Xe at 0.59 torr pressure, Photo Is single shot. Note the center dark region. This could be caused by either a lack of radiating target material In the focal volume or by the radiation In the focal volume being of higher energy.

113

Figure 35. Laser beam focused into air at 0.15 torr, 0.35 torr, 1.23 torr and 3 torr. Photos are of 10 to 20 shots. The photos are generally of two colors, a blue or white center with long red tal!s along the laser beam direction. Note that as the target density increases the dark central region vanishes.

114

Figure 36. Laser beam focused into He at 0.4 torr, 1.2 torr, 3.3 torr' and 10 torr. Photos are of 10 to 30 shots. Photos appear simular to those of air except at the lowest target density where now a bright central region appears.

).15

Figure 37. Laser beam focused Into He at 100 torr pressure. Photos are single shots. (a). Basically a bright breakdown with small red tails. (b). Filter is used to block out the red. The focusing lobes are now clearly visible.

I L6

Figure 38. Laser beam focused into H2 at 0.36 torr, 1.1 torr, 3 torr, 10 torr, 30 torr and 100 torr. Photos range from 40 shots at low pressure to 1 shot at high pressure, Note the formation of the bright focusing lobes In (a) to (e) and the collision of these lobes in (f) as evidenced by the small jets perpendicular to the laser beam direction.

117

propagation through the focal region observed In Xe, air, He, and H2. All of them

show basically the same phenomena, a breakdown symmetrical about the point of

focus. This is in contrast to the asymmetric breakdown illustrated In Figure 34. In

all cases, the laser beam intensity became sufficient to produce breakdown quite

some distance before the focal region, The figures show that the width of the

breakdown region Increases as the target gas density increases.

Using He, the spatial extent of the observed blue and red radiation was

mapped. The focal region was mapped onto the entrance slit of a 0.3 meter

spectrometer using a Nikon 105 mm lens. The imaging experienced a reduction of

approximately 1.7. Ion lines were seen to originate from the center of the focal

region where the laser intensity would be greatest. The neutral lines are spread

out over the entire breakdown region and experience a dip at the center where

neutrals would tend to be more scarce.

There are perhaps many factors necessary to explain the propagation of a laser

beam through the gaseous targets illustrated in Figures 33-38. But in each case it

is clear that a large portion of the laser energy is deposited into the region leading

up to the focal area as evidenced by the observed breakdown. This must have some

effect upon beam propagation because the abundance of free electrons changes the

refractive index of the medium. Figure 33 clearly indicates that breakdown can

become so catasrophic that the laser beam propagation is strongly affected. The

results of Figures 6-9 give an Indication when this occurs for the different gases.

V. Conclusions

Three main conclusions are drawn from the experiments described above.

First, the laser beam propagation Is strongly af;ected by Ionization of the target

gas and by any density gradient in the target gas that the laser beam has to pass

through In order to reach the focal point. Second, as evidenced by the spectrum

obtained from the various target gases, the lengths of the capillary tubes used in

118

these studies were probably too long for longitudinal pumping of the target gases

because a) once breakdown occurs in the capillary tube the capillary tube wall

absorbs the laser energy, presumably in a nonlinear process, rather than reflecting

it back Into the target gas, and b) the resulting laser intensity along the capillary

tube was too low to produce sufficient ionization for the surrounding gas to be

transparent to the short wavelength radiation produced. A somewhat greater

intensity is required. Third, longitudinal pumping of the target gas in the capillary

tuue generates a very strong third harmonic at 82.8 nm. This signal could have

enough power to be experimentally quite useful. For example, using Ne as the

target gas could yield a third harmonic with as much as 1% energy conversion.

This would be a sufficient conversion to serve many important application, such as

holography.

119

Vt. RferenUes

1 Johidale C. Solem, Ting Shan Luk, Keith Boyer and Charles Kirkham Rhodes, IEEE Journal of Quantum Electronics, V25, December 1989, pp 2423-2430.

2. A. B. Borisov, A. V. Borovskiy, V. V. Korobkin, A. M. Prokhorov, C. K. Rhodes, 0. B. Shiryaev, submitted to Physical Review Letters.

3. T. S. Luk, U. Johann, H Egger, H. Pummer, and C. K. Rhodes, Physical Review A, V32, July 1985, pp 214-224.

4. Charles K. Rhodes, Science, V229, 27 September 1985, pp 1345-1351.

5. A. McPherson, G. Gibson, H. Jara, U. Johann, T. S. Luk, I. A. McIntyre, K. Boyer, and C. K. Rhodes, Journal of Optical Society Society of America B, V4, April 1987, pp 595-601.

6. W. T. Siifvast, L. H. Szeto, and 0. R. Wood, 1I, Applied Physics Letters, V36, 1 April 1980, pp 500-502.

7. T. S. Luk, A. McPherson, G. Gibson, K. Boyer, and C. K. Rho"ýx, Optics Letters, V14, 15 October 1989, pp 1113-1115.

8. John Shannon and Robert Hunter, Applied Physics Letters, V24, 15 May 1974, pp 488--490.

9. W. --G. Wrobel, H. Rolr, K. -H. Stever, Applied Physics Letters, V36, 15 January 1980, pp 113-115.

10. T. Efthlmlopoulos, 8. P. Stoicheff, and R. I. Thompson, Optics Letters, V14 15 June 1989, pp 624-626.

11. K. Boycr, T. S. Luk, J. C. Solem, and C. K. Rhodes, Physical Review A, V39, 1 February 1989, pp 1186-1192.

12. T. S. Luk, A, McPherson, G. Gibson, K. Boyer, and C. K. Rhodes, Nuclear Irnsiruments and Methods In Physics Research, V43, 1989, pp 468-470.

13. G. GibA;)n, T. S. Luk, 4,. McPherson, K. Boye,, and C. K. Rhodes, Physical Review A, V40, 1 September 1989, pp 2378-2384.

14, G. Gibson, T. S. " uk, A. McPherson, K. Boyer, and C. K. Rhodes, Submitted to Applied Physics B.

.20

VII. Appendices

Appendix A

Measurement of the gas target pressure in the capillary tube Is determined

u,.ng a HeNe laser interferometer. The experimental setup is illustrated In Figure

Al. The index of air at a pressure of 760 torr is given by

n - 1 = 0.000293. Al

Since the optical path in the capillary tube is given by

Op = (n - 1) d P/760 A2

where d is the length in the capillary tube and P Is the averago gas pressure in

torr Inside the capillary tube, the number of fringes for the HeNe laser Is given by

f ý (n - 1) d P/760X = 0.00609 A3

where f is in units of fringes per cm-torr. The average pressure in the capillary

tube is given by combining Equations A2 and A3.

As an example, the flow through a 10 cm long capillary tube with a 150 Pm

bore was measured by placing it inside a vacuum chamber as illustrated in Figure

Alb. Port B was opened to vacuum and the entire chamber was pulled to less than

1 mTorr pressure. Using I atm backing pressure at port A, gas flow was

established through the capillary tube with port B open to vacuum. When port A

was closed, 29 fringes were counted, Likewise, when port A was reopened, 29

fringes were counted, When the capillary tube was not in position, so that air

leaked Into the vacuum chamber through port A, the fringe shift was approximately

1/2. From equations A2 and A3 this gives the average pressure In the capillary

tube as

Pa = 28.5 frlnges-cm-torr/(10 cm x 0.00609 fringes)

Pa = 468 torr.

The rate of flow through the capillary tube can be found by closing port B and

opening port A to atmosphere. When this Ih done, 32 fringes per 30 seconds Is

121"

-..... " "_ _/

ScreeN

_______C (o 1/y %

Figure Al. The target gas pressure Is determined by interferometric means as Illustratod In (a). The capillary tube was positioned Inside a vacuum chamber as Illustrated in (b) and gas flowed In and out of ports A and B producing a change in the fringe pattern observed on the screen.

122

recorded. Since the volume of the vacuum chamber is 1.7 liters, the flow rate Is

found to be

0 = 32 fringes x 760 torr x 1.7 liters 30 sec 296 frng

Q = 4.65 torr-Ilter/sec.

123

APpoendix B

Assume viscous flow through a capillary tube of length I and bore radius a.

The conductance, 0, through the capillary tube at each point x along the tube Is

given by

Q = ira" P dP S1 81"i dx

where P is the pressure along the tube and q is the viscosity of the gas. The gas

flow through the capillary is subject to several assumptions. First, the flow speed

is equal to the speed of sound, or the molecular speed, vm. Second, steady state

flow Is assumed so that

0 = 1a 2P'vm = constant B2

where P' is the pressure at the output end of the capillary tube. The boundary

condition is

PJx=I = P'. B3

Putting these conditions together gives

P2 - -16TP'vmx/a8 + P' B4

where P. is the pressure at the Input end of the capillary tube. Applying the

boundary condition again results In a quadratic equation for the output pressure In

terms of the input pressure as

2P' = -- + (o2 + 4P )0/2 B5

where

X 169lvmL/a 2. 136

Both equations B5 and 66 are in units of pbars. The average pressure in the

capillary tube can be obtained by integrating Equation B4 to give

ra -(1/i) fl[Po - P'cxx/i]'/2dx

(2/3P'oi)[PO - P"] 37

124

where t is the length from port A to the end of the capillary tube.

Consider Figure B1. The basis structure of the gas flow path is laid out with

the respective length and dimensions indicated. For a given Input pressure of P. =

760 torr, what is the output pressure P2 when the gas Is argon? The molecular

velocity is given by

Vm = (2kT/3m)'/2 88

where k in Bcitzman's constant, T is the gas temperature and m Is the molecular

mass. For argon at room temperature. 300 OK, the molecular velocity is 3.4 x 10*

cm/sec. The viscosity is 1.8 x 10- dyne-sec/cm2 . For an effective length between

P. and P, of 1.55 cm and between P, and P2 of 5 cm, equations 85 and B6 give

aq =74.5 torr

P= 723.7 torr

% = 6840.3 torr

P2 - 75.7 torr.

The average pressure In the capillary tube can be calculated from Equation B7. It

is 487.4 torr. This is to be compalred to the experimental value found In Appendix

A of 468 torr. From the difference of only 4%, it would appear that the viscous

flow model of Appendix A is sufficient to describe the average flow through the

capillary tube. With this average output pressure in a 0.15 mm bore capillary tube,

the average target density would be 1.7 x 10"9 cm-3.

125

1PI

I o. I• c tube assemb

Figure Bi. Typical schmatic of the gas flow path in the pulsed vaI~e

capillary tube assembly.

126

APPENDIX G IEEE JOURNAL OF QUANTUM ELECTRONICS. VOL. 25. NO. 12. DECEMBER 1989

2423

Prospects for X-Ray Amplification with ChargeDisplacement Self-Channeling

JOHNDALE C. SOLEM, TING SHAN LUK, KEITH BOYER, AND CHARLES KIRKHAM RHODES, F.LLOW, Iti.

(In vited Paper)

Absura-c-We develop an analytic theory orcharge-displacement self- which leads to the following qualitative behavior. The channelingi: a mechanism Mhat can dynamically irap a short intense conversion of electrons from bound to free states by the pulse of light. We focus our attenlion on the case or most interest for X-ray ampliflatslon: the strongly saturated channel, for which all free ionization induces a strong reduction in the refractive reelectrons are expelled from the channel core and the channel wails are sponse of the atomic material. The resulting plasma exuverdnse. S'mme curious results are: 1) the intensity at the channel hibits a further decrease in index from the free-electron walls is independent or the total lawer power, 2) the radius of the chan- component collaterally produced. If the ambient electron nel increases very slowly with loser power, asymptotically as the fourth density no is such that the plasma frequency w, root, and 3) the power In the channel wall is a constant. The channel is energetically stable in the sense that a bifurcation will cause a net VN/mrn is below the frequency of the radiation w, an increase in the electrostatic potential energy, but is only marginally interesting mode of channeled propagation could develop. stable against relativistic lilamentation in the walls. We also fnd that For a sufficiently short pulse ( - 100 fs), the ions remain the channel is an effective wavegulde (or all secondary radiation. Scal- spatially fixed while the relatively mobile electrons are ing studies show a substantial advantage exists by using the highest frequency driving laser available. Since very large energty-dep•sit ion expelled by the ponderomotive pote.ntial fromi the highrates from multiphoton coupling are expected, the channel i% ideal for intensity region of the beam. A state of equilibrium can generating coherent short-wavelength radiation, perhaps well into the then be established between the ponderomotive and the X-ray range. electrostatic force densities owing to the net charge displacement. Since the electrons, which embody a negative I. INTRODUCTION contribution to the index, are expelled, -in on-axis region o f relatively high refractive index is formed which canA MPLIFICATION in the X-ray region requires pro- support channeling of the laser beam. dZoigious energy deposition rates I II spatially orga- For the optimum condition conducive to laser action.nized in a high-aspect-ratio volume of material. We will we would like the density of excited states to be as highshow that the use of extremely intense, short-pulse radia- as possible, and consequently, a high atom density is de-tion may be able to produce these conditions by combin- sired. However, we may not have w < w, or the lasering the energy deposition [2J, 131 arising from high-order beam may not initially propagate to form the channel. multiphoton processes with a mode of channeled propa- While this constraint may be somewhat mitigated in the gation involving the complete expulsion of free electrons relativistic regime, in general it will be most propitious to from the central region of the channel. It is significant that set the atom density such that at the anticipated ionization the conditions needed for the strong multiphoton coupling level, the plasma frequency is just below the laser hrcare identical to those found required for the confined quency. propagation. Previous work [2) discusses the energy dep- In this paper, 1) we review self-channeling in a lowosition rates. Recent work suggests a paradigm for state- density plasma and present some simple physical models selective excitation by the observation of inner-orbital for calculating the critical intensity for channeling, the molecular transitions produced by multiphoton coupling channel radius, and the power at which all electrons are 141. The present discussion concentrates on the propaga- expelled; 2) we discuss the ionization state achieved in tion. initially cold material, and show that it is a good approx-When a very intense pulse of radiation enters a cold Imation to assume a common ionization state for all atomsmedium, multiphoton ionization produces a local plasma, involved in the channel; 3) we show that the index of refraction is dominated by the free-electron component; 4)Manuscript received June 27. 1919. revised July 25, 1989. Thi% tkirk we develop an analytic solution for the strongly saturatedwas suppirted by the t)epartment ot |inergy. the Air V'rve Oflic' of Ski channel with overdcnse walls, which is the case of greal-enlitic Research, the Ottic ol Naval Research, and the SDIO. J. C. Solern is with the Theorctical Division, Lis Alanuos Nalional Lab- cst interest for X-ray amplification; 5) we show ihat the owiiory. Los Alaiio,. NM 87545. motionless-ion approximation is valid in the regime of inT. S. l.uk, K. Boyer, ind C. K. Rhdes are wilth the Laboratory hir terest; 6) we demonstrate energetic stability of the chanAtomic, Molecular, and Raditdion Physics. Department of Physi'.. Lini verwiy of Illinois, Chicago. IL 60680. nel, although it is only marginally stable against relac•ivIEEE Lug Number 89.31013 istic lfilamentation; 7) we discuss channeling of secondary

0018-9197/89/1200-2423$01.00 -) I 989 ILEE

127

2424 MEEE JOURNAL OF QUANTUM ELECTRONICS, VOL.. 25, NO. 1Q. DECEMBER I989

radiation and energy deposition rates; and 8) we derive Here is a very simple and intuitive model. We estimate scaling relationships that show a substantial advantage of the radius r., of the saturated region In (r < r.) = 0 1j with the use of high-frequency driving lasers. by treating the channel as a dielectric waveguide. We assume that the expelled electrons form a cylindrical sheath II. REVIEW OF SELF-CHANNELING IN Low-DENSITY around the saturated region with a thickness comparable PLASMA: SIMPLE ANALYTIC MODELS to r, so that n(r,. < r < 2r.,) = . n1 and n(r > 2r,,) Some aspects of the self-channeling behavior have been no. Consequently, the charge density n(r) changes from calculated numerically in the regime of low-plasma den- no to 4 no at the interface. The cutoff frequency w,. of the sity 15). To get our arms around the problem, we review dielectric waveguide 18) can be written as those results and show how they can be obtained from simple analytic models. We then proceed to the case of X.o 0 c m( an initially cold material subjected to exceedingly high 'w - r (4) intensities. r~c 41rn(r > r..) - n(r < r,71' The steady-state force balance between the radially out- where xoI = 2.405 is the first root of the Bessel function ward ponderomotive force and the oppositely directed Jo(x), i.e., Jo(xo 0 ) = 0. We ignore the effect of the reelectron-ion attraction for a completely ionized tenuous gion r > 2r.. The threshold for propagation occurs at wo (w >> w,) plasma is given by = ta, for which we find r.. = (xoc1"%64 )/w,, =2/ 1 ,. This agrees with numerical calculations 151. which also 2we, [no - n( r' (I ) show r,, = 2c/ow, for the unsaturated case and that (2) is -- ) V1( F) =e..... (I m'C- a good description of the charge distribution with r, = r. The power P, for the onset of saturation can be found For a cylindrically symmetric Gaussian intensity distri- from the condition ](0) -moawcnor2, which gives bution 1(r), the electron density n(r) is _0 PI = - - - 1.74 X 10W n(r) = no - -21(0)----- )I] exp J .1 (2) r, W mw-cr6~[ t r0 } L o rJJ ,\~ which is also in good agreement with numerical calculain which ro is the Gaussian radius. We can estimate the tions 15]. condition necessary for self-focusing by describing the charge displacement by two regions such that n(r < 111. IONIZATION STATE AND REFRACTIVE PROPERTIIES roV2) = n(0) and n(r • ro 0%2) = n(ruv/2 ) and equat- The state of ionization and its radial profile are imporing the angle of total internal reflection to the angle cor- tant aspects of the analysis. For these estimates, we use responding to the first minimum of diffraction. This yields the formulation of Keldysh 191 corresponding to the situa critical intensity ation in which tunneling dominates I101, 1111. and have included a correction for the effect of the Coulomb field mrn•w c W-2 in the final state I I I l. The influence of atomic shell struc64( I + e) r, 43X10 ture is accounted for through the use of computed ion(3) ization potentials for multiply-charged ions 1121. An example involving holmium illustrates the outcome of this where r, = e 2/mc 2 denotes the classical electron radius. procedure. It is found that for a pulse of 100 fs duration. Significantly, this intensity is independent of the ambient nickel-like holmium (Ho' 9 ) will be produced for intenelectron density no and contrasts with the power threshold sities spanning the 1.3-4.8 x 1021 W cm 2 range. 171 normally arising from induced index changes in trans- Therefore, if the spatial beam profile were a Gaussian with parent dielectrics. We note that if the critical intensity is a radius ro and a peak intensity 4•, = 4.8 x 102 W not achieved, ionization will generally result in a refrac- cm-2, the nickel state would exist from r = 0 to r tive index profile that will defocus the beam 171. The abil- 1.3ro, nearly the entire region of interest. ity to focus and form a self-trapped beam will also depend The radial dependence of the refractive index Nr r) on the initial intensity distribution and the rise time. governs the condition for channeling. Since the ions. arc If the intensity exceeds I4. the beam will tend to form inertially confined for the short time ( - 100 Is) conrida channel, but the intensity range over which (2) is a good cred, the expulsion of the free electrons from the channel description of the electron distribution is quite narrow, results in a reduced plasma frequency or, equivalently. an "The more common and more interesting situation is de- increase of the refractive index in the central region. scribed by /(0) > 1 r•nw'n or2. Undersuch conditions. all The contribution to the index from the ions under the free electrons are expelled from the core of the chivincl, strong-field condition involves two opposing effects. lhey which is said to be saturated or cavitating. The radius of are: I) the nonlinear contribution proportional to N.. the channel will not necessarily be the same as the laser which is enhanced at more elevated field strengiths. and focal spot, and will, in general, depend on the clectron 2) the tendency to produce high charge states. korredensity. sponding to low polirizabilitics and depressed vaducs of

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N2.The resulting ionic component of the index, however, where w,, is the relativistically corrected [151 plasma freis overwhelmed by the free-electron contribution. This quency, fact is demonstrated by another example involving Ho at an intensity of 3.7 x 1019 W . cm- 2, the value at which opwo (8) Ho becomes krypton-like (Ho 3' + ). The refractive index, e2E2 through the first nonlinear term, is N = I + No + N2E2 +I 2 + 2 where No = 21rN~ao and N2 = (r/3) N.0 2, with N, rep- m o resenting the ion density. We can relate these suscepti- and wp0o = ,4rnoe 2/m is the nonrelativistic plasma frebilities for Ho31' to known values [13] for neutral Kr, quency in the channel wall. We assume the same ionizanamely, cf0 = 24.8 x 10-25 esu and c2 = 137.7 x 10-38 tion state throughout the channel wall as was justified in esu. Since Ho31 + has a radius of -0.032 that of Kr, hy- Section 111. The amplitude of the high-frequency field for drogenic scaling yields *0 = 10-28 esu and Cf2 = 10-46 r > r. is given by esu for Ho3" +. At the given intensity, the ionic contribu- (9) tion then is (N - I),, = 10-2Na. I-or a collisionless (Hoe3+) plasma with an electron density of 3 INa, we oh- Assuming that all the electrons from the saturated region tain the free-electron contribution to the index ot 248 nm are moved to a thin cylindrical shell with inner radius r. as (N - I ), = - (4/21W) = -8.5 x 10-"2 N0 so that and outer radius r, + 6 where 6 << r,, because n,• »>> (N - I ),/(N - I ),t = - 107. The free-electron contri- no, by using (7) and (8) we can approximate bution is far larger than the ionic contribution. Therefore, -(r.) the ionic contribution can be safely neglected. 6 2 (10)

IV. THE STRONGLY SATURATED CHANNEL WITH where 3 - 41rrl/mw 2c.

OVERDiENSE WALLS The solution to (6), which represents the saturated reWe now turn to the case of greatest interest for X-ray gion r < r,,, is amplification: the strongly saturated channel. In Section 0(r) = 0(0) Jo(qr), (11) I1, we reviewed the case of a tenuous plasma (w >:> w. Here, no such restriction applies, but we assume a density an expression we want to join to (9) at r = r. We note that allows enough initial propagation for the channel to that this treatment is substantially different from the treatform. We are seeking the steady-state conditions of prop- ment used by Barnes et al. 1161 which assumed a Gaussagation: the electrons are completely expelled out to a ra- ian solution near the beam axis. dius r. and the channel is bounded by overdense walls The boundary conditions are continuity of 0 and dl/dr, with a very rapid rise in electron density. Our objective which give is to obtain an analytic approximation. 0(0) Jo(qr.) 0(r) (12) To this end, we make the following assumptions: I) all electrons are expelled from the center of the channel out 2 rnor, r. to a radius r.; 2) beyond r., the electron density rises to -q (0) JJ,(qr..) 1- /+2 (r (r,,), (13) a overdense value no, in a radial distance that is very small compared to r.; 3) the density remains at n0,. for a radial or distance 6 representing one e folding of the high-fre- q J0(qr) 2-m-nor. quency field and then drops to the ambient value no << (14) _or__ 2__ r, no,; and 4) the relativistic shift of the plasma frequency r, J1(qr.) ]/ + 2I3/(r,) (141 is determined by the high-frequency field at r,. We now use local force balance to obtain the wall raA. Analytic Solution diis. The radial ponderomotive force on an electron it r. is Within the cylinder r < r, the fields for circularly polarized light are described by the equation [6] F,(r.) = 2 ._r - ý'(r.) MW-c dr

I d/2(O)qJo(qr.)JI(qrJ). ( 15) where 0i is defined so the intensity I = and q is an flwC eigenvalue established by the boundary conditions. The Equation (15) is nonrelativistic. As the intensity apamplitude of the high-frequency field will drop off expo- preaches the relativistic regime, the quiver motion of the nentially [14] in the overdense channel wall with a pen- electrons changes from a linear displacement in the ¢1cc etration depth given by tric-tield direction to afigure 8 pattern [171 extending in the directon of propagation. Under extremely relatvitsn, S= =. ,(7) condIltons, the ponderomotive force will be mainly in the W propagation direction, although the radial gradient of in

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tensity will still contribute. We use (15) as an approxi- 2.0 mation to the force and we shall show that the intensity at r, is independent of the total laser power. Thus, the use 1. of (15) is equally good for all laser powers. From Gauss' law, we obtain the electrostatic force 1.04w r£ F,(r.) = e2n- pdp rw o0 .5

=2e 2norw. (16) ,o 20 40 so,1oo Balancing the forces F(r,,) -F(r) givesp2() Fig. 1. The Bessel function parameter at the channel wall as a function of 7-= 2 2(0) Jo(qrw) J, (qr.). (17) central intCnsity. q nomw

Combining (14) and (17), we obtain directly ___

J00) JO(qr.) = o(qr.) (18) 3

and, using (12), we find

V2(r) 11 + 2,60'(r.). '19)

The solution to (19) is then simply l+.

2(r•) = (20) 0-0 40 ý5 '

The intensity at the channel wall is independent of laser Fig. 2. The channel radius as a function of central intensity. power. Furthermore,

lion of a laser beam through a charge-displaced channel. p, + j -- 0.414 O'~pwo, (21) The channel walls are overdense. The treatment is relativistic and appropriate for very high intensity. The radial always holds, From (12) and (20), we have dependence of tC laser intensity is

qr_,_ + f(O)Jw(qr), r < r( qr• J (o) 4 (.2) (r) ([(l + ,42-)/13 exp [2(r, - r)/16, r > r..

where J"'o[J 0 (x)j = JO[J'o"(x)] = x and we have re- (24) stricted the functions to principal values. Fig. 1 shows qr,. and the radial dependence of the electron density is as a function of the channel center intensity 1(0), as given by (22). Notice that qr,, - 0 as 1(0) - (1 + v• )/0 and (0, r . r; qr. -- x01 a.. 1(0) -. co, From (17) and (22), we have the n(r) norw/26, rw < r < r, + 6; 1251 result 0 .no, r ýt r, + 65;

2 _ + _nnr I,() + N where 6a (1 + Vr )/(2wnorr,,), as can be seen by %uh21rnor, (0) -,0)v- stituting (20) into (10). Fig. 3 shows the intensity i% function of radius as given by (24) for some repreeesna+ qv'2- tive values of the intensity. Fig. 4 shows electron dcn.ii, tIk (0 ,)jJ (23) distributions as given by (25) together with the cotre sponding intensity distributions. Fig. 2 shows the channel wall radius r. as a function of C Asymptic Forms the channel center intensity 1(0), as given by (23). Notice A Taylor expansion at x0I gives 10(x) =J1 t, Oolthat r,, "- 0 as 1(0) -,(1 + Nf2 )/0, as expected, and r., ATyo xaso txjgvsJ~)--J .,)••, thtre 0ases inef l wh (1 ), asxpete r - x), which can be used to find the asymptotic bchat ,ir increases indefinitely with 1(0), of(22) as /(0) co.

B. Summary 1 3 To summarize results so far, we have derived a solution qr., -- xo - IO + 12 = 3 to an approximate model for the z-independent propaga- J,(X 0 , 1 -J7 ,

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100

606

31 4 40

0 1 2 0 4 2(0) 600 am 1000

Fig. 3. The radial distribution of intensity for various values of the central Fig. 5. Comparison of the asymptotic solution obtained by Taylor expanintensity. sion, (27), to the exact solution. (23), for the channel radius as a function of central intensity. Dashed curve is the asymptotic solution.

limit of very high power, the channel radius increases as the fourth root of intensity at the channel center or about as the fourth root of total laser power.

S10- V. VALIDITY OF THE MOTIONLESS-ION APPROXIMATION

The motionless-ion approximation is quite good over the intensity and pulse-length regime of interest. The ponderomotive force on an ion is proportional to its charge 1 0 2 - 4 5 and inversely proportional to its mass M. For Ho at 1o (s) 4.8 x 100 W . cm-2, the ponderomotive force moves an ion a distance -4 x 10-'° times that of the electron, 100 which is completely negligible. A far larger force on the s \ions results from the charge displacement itself since the ions' mutual electrostatic repulsion will expel them from 6, sthe beam. However, the ion motion due to this force is also small for a sufficiently short pulse. Assume that the "40 saturated charinel has been formed at I = 0. With the electrostatic force on an ion at radius r < rw given as F = 2ir ( re)2n. by Gauss' law, an ion with initial position r0 has its motion described by r = r0 exp ( y2) where 0 1 2 3 4 5 6

(b) 1exp (y2) dy' tc (28) Fig. 4. The electron density distribution superimposed on the density distribution for various values of central intensity. (a) 0, 2(0) - 20. (b) which leads to the approximate result for small displace#,2(O) , 100. menits;

A Taylor expansion also gives Jo(x) J, (x) = J2(xo 0 ) (x0o r - 'o = eM 2

- x), which can be combined with (26) to find the asymp- 1020 W CM2 , totic behavior of (23), specifically, At lo = 4.9 X 100W•c-,the ion migration gives a relative change in its position of about 9 percent after IOU

x01J, (x0 1) %/( l + v'2 ) 0(0) - 1 - 12. S2 rnor, VI, ENI4RC3Y Loss MECHANISMS

The two principal mechanisms leading to energy loss 1.94,ffl__(0) - 2.41 (27) in the channel are: I) ionization, and 2) the field energy 2ino r, associated with the ch,'rge displacement.

Fig. 5 compares the asymptotic solution given in (27) to A. Ionization the exact solution given in (23). Remarkably, the asymp- The energy lost to ionization can be found by summing totic solution agrees with the exact solution within a few the ionization potentials [12) of electrons successively percent over the entire domain of definition: it is an ex- stripped from the atom up to the ionization level produced cellent approximation. Equation (27) shows that in the by the field. For the Ho 39+ case, with n5 = 2.3 X 10"

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2421 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 25, NO. 12, DECEMBER t989

cm and a channel radius r, = 2c/w, = 1.1 jro, ion- while in the region outside ization produces a loss of -4.3 mJ. cm-. 2 (I + B. Charge Displacement P = exp [2(rw - r)/6 r dr The radially directed electric field is given by 2i( + ./2) [r 62]

r, r S r,;

-+ C2 21 1rn + r.(r - r)/6, mac(l + F2)U + v2 + 4wnorr.)E(r) 2 w'eno (30) 3 2 'r r r, < r < r, + 5; ( r w 0, r > r, + 6 m 2c + N)2 (34) 8--8rnor, ).(4

where we have corrected for the approximation in (25). By direct integration, the charge-displacement field en- Combining (31) and (33) and taking the limit 1(0) -- oo ergy per unit length of the channel is given by we find further d Ir (3r,+ + ) -2 ) 4ro /mrc2noP / E(r) rdr=1 4dx 4 0 12 = , ) 2224 a result which shows that the electrostatic energy c,. per _2e4n4r 1 ' mc unit length I scales as (laser power) 21 3. Therefore, this 4 16r, energy would be increased by splitting into two channels, "] [2. each of which contained half the laser power. Further inS[X0 1J1 (x0) V 7 I +-f2ý1(O) - I - spection of (3 1) and (33) shows that an even more stable (31) situation exists for powers less than the asymptotic limit. For the example above, this would be -44 J0 cm- 1 . B. Stability Against Relativistic Filamentation The total power propagating in the channel walls is VII. STABILITY OF THE CHANNEL given by (34). The power threshold for relativistic selfTwo questions concerning the stability of such channels focusing 1 19], [201 is have been examined. The first involves energetic stability Kma associated with the splitting of a single channel into two P, (23h) or more channels, each having a fraction of the total electrostatic energy. The second considers stability against re- in which K = 1.5 X 10") W, so that the condition P, > lativistic self-focusing in the walls of the channel, a mode P1 barring relativistic self-focusing in the walls is observed in numerical simulations [18]. A. Energetic Stability K> -cr (1 + vf2)' = 2.6 X 1010 W. (37) For appraisal of the case based on energetics, using the approximation Although this stability criterion is not met, it is only t 23 2 slightly surpassed. The assumption that the relatIVINt1C ) I - ]) shift of the plasma frequency is given by the intensity it 10j2(') X, dX X01 1(x/ I 0 the channel wall slightly overestimates the total pfwcr flowing in the walls. Thus, we can only conclude that the (32) stability against relativistic filamentation is marginal Furthermore, although the power in the wall of the which is accurate to within 10 percent over the 0 < x < stronl atted c hel is the wame ( heii xol ang, fr atotl pwer = 0 +P1,we indtha strongly saturated channel is always the same, (he time x 0 1 range, for a total power P = P0 + P1, we find that (or distance) for the instability to develop fully depends inside the saturated region on the geometry of the channel. A thick annulus will III"r. ament more quickly than a thin annulus. The more pfower P0 = 2r](0) J0J'(qr) r dr flowing down the channel, the longer it will take for (he instability to develop. The question of relativistIc 1i.a. mentation can be addressed by detailed numerical 'milu11(xo,) B fl(O) _+, _ lations, but ultimately only by experiment [211. Both -ire (2nor,[xoJ,(xo,) 0(-) %[ +I beyond the scope of this paper. V+I 2.3 2 C. Linear StabilityI . ... If the channel is to be used as the excited volumc or SxiJ o (, N i) ' (33) an X-ray laser, it is desirable to have the channel lin,pi

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gate in a straight line, Linearity of the channel can be D. Scaling with Driving-Laser Frequency ensured by laying down a track of hieher refractive index The deposition rate has a strong dependence on primary that the pulse will follow. For a gas, this might be done laser frequency, which gives a substantial advantage to with a laser prepulse. In a solid, microfabrication tech- the use of higher frequencies. niques might be used. When a tenuous-plasma channel is unsaturated or only slightly saturated, its radius is - 2c/owp. The power propVIII. TOWARD X-RAY AMPLIFICATION agating down the channel is roughly proportional to its Several additional considerations pertain to the use of a central intensity times its square of the radius 151. To charge-displaced channel for obtaining X-ray amplifica- compare the effectiveness of different driving lasers for tion. creating channels that might become lasers themselves, it is appropriate to compare systems with the same values A. Channeling of Secondary Radiation of ai/p. So for a given central intensity, the driving-laser Using the dielectric waveguide as a model for the chan- power scales as W-2. nel reveals an interesting aspect of this mode of propa- For the strongly saturated channel, which is the main gation. We see that if the frequency w exceeds the cutoff subject of this paper, driving-laser power scales even more in (4), a condition essential for the channel to form, then rapidly with frequency. From (33), we obtain all higher frequencies will also be guided. This occurs because the index is dominated by the free electrons or, J+4(xoD) I/3g3(0) (I + v') equivalently, the angle of total internal reflection is pro- P Po 2n0 r=x 0 ponional to the angle of the first minimum of diffraction. Since all of the radiation which falls within the acceptance V i I + angle of the waveguide will propagate as a guided mode, (x01) -_) (___ + ._) (3 these channels can serve naturally as bNighc directed xoiwm sources of energetic radiation. We note that linear stability is only a practical require- in the high-power limit. For cnannels with the same w/w,. ment: we need to know which way the laser is pointing. the driving-laser power scales as W-3. Furthermore, highei The channel acts as an optical fiber for all secondary ra- frequencies generally allow shorter transform-limited diation. pulse lengths, enabling the ions to remain motionless ,kt higher driving-laser power. The ion migration scales apB. Deposition Rates proximately as the square of the pulse length. If pulse Energy deposition rates associated with such channels length then scales as w-1, the migration sca:es as Ld 2. are expected to be extremely high. Extrapolation (21 if finally, the charge-displacement energy given in (31) beexperimental energy-transfer rates suggests that the mul- comes tiphoton absorption cross section will limit to a fre- d(_ =xJ(x 0 1 ) (1 + 42) Bl(O)mc2 quency-independent value of - 10-20 cmz for heavy ele- 16r ments at intensities above 1019 W .cr- 2, a value near the Compton intensity in the ultraviolet. For the holmium lrxo4JI(Xo1 ) ( I + ,/ ) 0/1(O}e c

1019 cal-3. the rate is in excess of 1019 W ' cm 3, a value far above that needed for amplification of radiation in the in the high-power limit. So the charge-displacement enkilovolt regicn I I]. ergy scales as w 2, while the corresponding ionization energy goes as C. Population of Active Atoms IX. CONCLUSIONS From (27), we find that the number of atoms per uni, length in the saturated core of the channel is The analysis we have presented shows th•,t nunlinear absorption anc channeled propagation combine cooperatively in producing conditions favcrable to the use ol inX0 1(_1 + f2-) 0I(0) I( tense short-pulse high-frequency radiation for the produc21~ (38) tior~ of X-ray amplit-itation. Numericai calculations %hill be re.uired to obtain a detailed description of the dynam. where " is the degree of ionization. Using 248 nm radia- ics of channel formation and stauility: lWowever, the pre%tion at 4.8 x 1020 W . cmu-. we find n=,, = 7 x I0'" ent technology 0: clcse to being able to answer nijf1, cm or, recalling the example ofholmium, n,,, - 1.8 x questions in the laboratory 1211. The ability to channel 10l cm-1. This is a rather small Iin"ar density, altheugh both the energy deposition and resulting emission ,:.in volume density is quite high. It v ill be necessary to use enormously increase the high-frequency g.in and pro, ide the atoms very efficiently. Perhaps inner-shell multipho- a natural mechanism for generating low-divergence high ton ionization is the test metchanism. brightness X-ray scaurces.

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'2430 IEEE JOURNAL OF QUANTUM EL-ECTRONICS. VOL. S*J NO. 12, DECEMBER 1999

ACKN4OWLEDGME~NT J.haall C. Saietns was born in Chicago, IL, in Thc ulhrs ratfuly aknoledg th vauabe ds- . r - ~ 1941. He received the B.S.. M.S. M.Phrl., and The uthrs ratfuly akno lede te vluale is-Ph.D. degrees all from Yale Univcrsisy. New H-acussiuns with G. Gibson and A. McPherson. 7,~ yon, CT. :a 1963, 1"Y6S. 1967, and 1968, respec'A4 tively. -. After a brief tenure as a Research As~sociate at

REFEZEN~aS1969. His research tactivitics have included boith REFEl~NC~5 .theory and experimtent. Sone career highig±ht% in. III A. V. Vinogradov and 1. 1. Subelrnan, "The problem of laser radia- dlud, the artas of inagrieti-sm, mi-n'.porl iticory. fion sources in the far ultraviwa.et and X-ray rcejions, - Sow. Phr. - plasma physics, nuclear physics. equations ofmsaie, artilicial intelligence JETP, v-31. 36, p. 1115, 1977. .and rohote'., computational science, X-ray microholography. and l~aser 2JK. Boe.H. ;w7,T. S. fuk. 1. A. McIntyre, A. McPherson. R. theory, particularly as it applies to concirptual designs f-or the -f-ra) las.er Ro( ui2Jn C .Ro s-Limi~ing cross sections I' r multiphoton ! addition to his research, he has held several management post%. Grouap couplingps," Rev. Phys. A4fpl.. vol. 22. p. 1793. 1987. Leader of Thermonuclear Weapons Physics, Group Leader ot Neutron 131 J. Cobble. G Xyrala. A. H-auer. A. Taylor. C. Corrz, N. Delame- Physics. Group Leader of H-igh-linergy-Dvnsitiy Physics, Deputy D~i vision ter. and 0. Schappen, " ilovoli; X-ray spectro~copy of a subpictt. Leaderof Physics, and Associate Divition Leader ofThcory. He ha'. 'rseied secoaid laser c:-cit-d siurce.- L.A-UR-88& 1 18 1. on the Air Force Scientific /.dvixory Board and numerous Departmecnt ,I 4J0. Gitsi-i, T. S. Luk. A. McP~aerton, K. Boyer, ano C. K. Rhtudes. Defense and Department of Energy panels%. His currews researchi interest'. "Ob-.cirvatity. of A htew tn111ter-o0bial moleo.ular transition at 55.8 rim include: X-ray lasers. quantum mechanics, and nucl,ýarlleei'. in nitteiils .;n N"' produced by muiti'hoton coup-ing." Phyi. Re".,4, A. obe publisbed. Ting Shatc; Luk waa born' iro Hung Kong it 1953 151 G. Z.. _fun. L. Ott. Y. C. Lee, and P. Guzdar, "Self-focusing of shot lie received the B.Sc. degree in -)hysi,'s Irointhelk intense pulses in pLsmas.' Ph.'s. Flui4's. vol. 203, p. 526. 1947. University of Hawaii, Manoa, and the Ph 1) Lie t6j P. L. Kelley. "Scif-focusing of' optical becams.' Phyv. Res. Lvi't.. re loi hscfo h tt ate'i~o Vol. IS. p. i005, lv'l5. Nirew alou it ptoysis frook ine 1St5ate ri'iv i ro17 1 P. B. Corkuns anu C. Rolland, "Atomnic and tnolccuar Irocessci Yvith Npew toi a tnyBokln17y.u19I shon', intenst ýaser pulsef, " in NATO ASI Serirw 3: Phy~ics, vol. I Atprestientl e;y.esac sosa~Poe. A.D. Bandrauk, 'Ed. New Yor!k: Plenum, KoV,7 p. 157. sor and R:search Assistant Director 01 dii Ljb~p181 i. D). Jatikson. f'lasi-e1Elet-rorwyhmcr'it. Neitw York: Wiley, Fi 2, ratory for At~,mic. Molecular, and k..diaoiin 19 . V.K2 yh Inuto ntefedofasm5e9tomgei -~-. Physics at the University oaf Illiwicis at Chic.,g4 191 . V Kc~ysh 'I~nizlio inthe iel ofa stontcluctronag,",The. reseinch projects being conducted tuvolse i-) wave,- S,-)Y. PIays -JETP. vol 20. p, 1307, 19t5. rain disciplines wPorking towaray the development of a laboratory ,:ale 1101 S . Stev.Ailm. Lamir-inducrci mtultiphotr.n transicions." Crntirrp. X-'ray la,,er. These two areas are. ; tKe developmnent of higl-powecr liscr %iou re tnays. Vol. 20,a Pyic. 7', 1979; 1-. R. frroolouts AdanJ.Peesoin. Aiu in thL terawatt range. and with intensity grca~cr than 10"' W/cn:. arid 2 I wNc- York.la Acadeics o. 192. . D. R. B.te DnJlon . Pce~sn Ed %lt invut'gation of some basic as-,wcts of atu-mic rind molecular inieraL...On it-iwo Yofl.Aaei,17.p b-6"- H. B. orinc it iy?-io oa.To with inicnisc laser field vxceeding typical atomic field strength- a,'el. i m .io ofat..i. St;. .s-s, vul. IA,. p. 161) 975 T.P laser plasmA interaction. ltge.Plasnma and Lue~r L~ii, L~ondon: Adam-Hilger. 1975. p. l4f; C. Grey Morguai 'Laser-induced breakdown of gases,' Rfep. Prog. Phy~s., vol. 38. p. 621, 1n'75. Keith Boyer was born in 1915. Hie rccixisd irc lit) K. Li. H-. Baklwin ai~d F.. W. Borcham, "Investigation of tun iel'ng B-13.S. degree in cecctrical engineering trii 1iw process in latser-induc,-1~ ioniitiiacni of argoý..' J. AppI. Phi',s. vol. .University of Utah., Sall Like Ciiy. and the l1h I) 52, p, 2627. 198 1. deg%-ei in physics trowi the Massachu'.cis tn'.ijiuii 1121 'r, A. Carlson. C. %~. Nestor. sr,. N. 1% asserman, and 1. P). W O i''chnology, Catmb~idge. in 1942 ,and 1949) Dowell, 'C:'lculated ionitt-wion p-itcntials fit-nur~~ charge,, on~s. speetively. Atom.;c Data, Yo,. 2. p. 63.1. 970~. . ~ ~ At present he holds the ra'k of Rc'.ejrdt P"1131 M P. Rogiari and 0. J. Orr. "'Electric dipol rou .i.a i~iicOf al les'or with the Dvpartment of l'l)'.cs.L ,i,. omis and nwleeuL-'s," In,. Reit. SO-.; Phys. C'hem. Serl. 2. Vol). 2. ofiltlinois at Chicago, is a Seniorfcivnli't .it %it W A. 0. hucktnigham. Zd, LonCs>.n: Butterworths. 1975, p. 14Y? Technology Corporation, and actively v.r~c-.a iiI il 1). U.Jackson. Clasical 0evro~~lynatric. New York: Wiley, l1%?. consultant i 'he Los Alamnos National L..a-'. F, 227. tory. Hi'% past reiponsibilniies includid the origina; leadersltim ot ihc I ,,, 1151 .ý. Kaw and J. Da'i4son, 'Relativistic nonlinear r't~opagatioý, of liser Division at the Los Alamci. Nationil Laboratory and the psti..in1 -I A-, beams in ccld ovirr'iensc pi. smas,'' PAys. Fluids, vol. 13. 'J. 474, tiani Director fut, Advenced Tecitnologies. Hi.i professiornal puhlikat'a.. 1970. nettiber appruxiroately 105S and covtr an extraordinary range oi . 1161 D. (7, Wimnes. T. Kurki-Suonio, and T. Tajima. lREF Trani. P'latin," trea%, includin- short wavelengith getteration. From his% long perto o, SH.. vol. PS-!S p. 154, 19$47, vice it Li-s Alamos. in a number oi i'iporlantl posts. he ha'. e~lwt'is,. 1171 F. S. Sarachik and 0. Sothappen. "Clrssicat thr4L-y of scalte-irig itt pertettee ii. kdefensc' and energy i'.sucs. ..siensc lh&sC radiAition i'y tree'elvr'rons.' Phiiy. Rev. 1] , vol~. 1. p Dr. 134.iycr wus ithe recipicnt of an Atomsic Energy ('omtttt's.iin , aij..n 2738. 197). in 17, I ~lj W. 71. Mori, C. Iushi, J. M. Dawson. f). W. Fors!tnd. :and J Is' Ki- dcl. ''lvolutsoui tiself'fitcusiiug of i.'tense eleetnrt;,agrwn.tt waves in~ ýastrrA" ft~ ý, 9ev. "it. , vol. 60, p. 1298,. 191'8. -Charles Kirkham Rhodes (SM"79-f- sij 1191 H. I-Iota. -l". rtiF.eau-confiring and decionfining fo.-Ws assoctateil with ceived his undergraduate training at U',i.nc:il oiiitra..tscn 1 lai.r radiatiior with pahi''Physr. Fluids, vol 1-1, p. vcrstty, Ithaca. NY, and %h,: Ph.D) degfLee f) 1842, 1"9;t -, P~ysnti.n if Lase, Li-ven P1;i'rmas. Ne 4 Yuork *..physics from the Mvassachusetts lnsI'!uie olt I. Wil-:y, 19141. pp. 72-.153. ogyCabien19. 1201 C. E~. uIrax, J. ArueŽý. and A. B. L~angc~on. 'Stif-modulafioai ,nd sell - Helgy Campridse.tl teAbn . hin V9-9 i.KuL'ng ofeJ-,ctrýraniignctic nwaves in plasmies.'' Phys. Rey, /qit., vol. feastir of Physics at the Univetsity of 111.-. ~.p. 20. 197-". P. Sprarkqlc. C. M. Tin 6 , and E. h!;.r,-y. "'kela- Chticago. ta'.aaic wlf-fxus~ing of s.'on pulse radiatitit, beamns in plirsmas,l. -.- .'h' Dr. Rhode.- is a Fellow of the Ameri iti F'r) irrans. PLtsma Sci., vcl. PS-I1S. p. 2. 19N7. ic la: Society. the Optical Society o1 Aniciri, .i 1211 '. S. 'iLuk, A. MePherso.ný G Cjihsucn, ;,. llnyer, -nd C. X. Rhictes,, the American Association for the Adsujik,i. " Ultrahigh intensity K.F* laser systecm." O pt. Lert., to be publishned Science.

J. 34

APENDIX H

VOLUME 65, NUMBER 14 PHYSICAL REVIEW LETTERS I OCroBE 1990

Stabilizatimo. f Relativistic Self-Focasing of lateme Subpicosecoind Ultraviolet Palme i. Ptasuma A. B. Elorisov,(') A. V. Boiovskiy,(2 V. V. Korobkin,(2) A. M. Prokhorov,t2 ) C. K, RhodesW and 0. B. Shiryaev 0) S')Laboratoryjbfr Comput ciSimulation. Research Computer Center, Moscow State University, Moscow 119899. U.S.S.R. t3 "Coherent and Non -insear Optics Dekpartment, General Physics Institute, Academy of Science. U.S.S.R.. Moscow 117942. U.S.S.R. 13'Deprtment of Physics, University of Illinois at Chicago. Chicago, Illinois 6(4580 (Received 21 March 1990)

The characteristics of relativistic propagation in plasmas of subpicoeecond ultraviolet (248 nm) radiation are studied for both spatially homogeneous plasmas and plasma columns. It is established for the first time that the defocusing properties of the interaction can represent a dynamical mechanism stabilizing the mode of propagation against radial oscillations. The dependence of both quasistabilized modes and pulsing wavcguide regimes on the initial transverse intensity distribution is examined and, for the latter, the locus of the first rocus produced in a homogeneous plasma is calculated.

PACS numbers: 52.40.Db, 42.10.--s.42.6S.Jx The dynamics of propagation of extremely intense value in the central high-intensity region. In addition, as subpicoecnd pulses of radiation in self-generated plas- shown below, the process of defocusing can give rise to a mas is a rapidly developing area of study. In this Letter previously unknown mechanism of stabilized propagacertain aspects of this question are treated theoretically tion. (or the following ranges of physical parameters: peak in- For subpicosecond pulses and the radial dimensions tensity 12o 1011-1021 W/CM 2, pvlse duration saO.5-1.0 considered, grows plasma motion involving the ions as ps, initial radial aperture roam 1-3 pm, wavelength X negligible.3 Similarly, for full ionization. noninertial -248 nm, and plasma electron density -l0*- 1021 Kerr self-focusing' is insignificant. In the present case, cm 3. A key finding of these calculations is insight into both the relativistic increase in the elecron mass arising the physical processes enabling the formation at a from induced oscillations in the intense fields.6 and the quasistable mode of propagation of such high-intcnsity charge displacement resulting from expulsion of frec radiation in plasmas. electrons from the ionized column by the pondcromoueve For pulses having a peak intensity in the l0isl0t2l. force 1-1- can p, oduce strong self-focusing action. W/cni 2 range, rapid (A rm I fs) processes of multiphoton We now describe the specific equations governing the ionization'- occurring on the leading edge (I?- W06 propagation and present calculated solutions illustrating W/cm') of the wave form will remove several (-6-10) the development of self-focusing and channeled propaga. atomic electrons. Thereby, for a focused beam with a tion. The origin of the stability exhibited by the propagiven radial aperlure rC, a plasmA column is rapidly gating radiation is of primary impcrtance. In considerformed in which the central temporal zone of the pulse ing the central temporal region of the pulse, we neglect propagates. The spatial and temporal dynamics govern- the energy losses associated with further oruizttion ind ihg the resulting propagation are determined by 6ce corn- spatial broadeninig of the picums %-I;s. s.cr 14Ma petitive interaction of diffraction, defocusing, and self- e#tT%- ican be quite smxl'L Wjr. , condition, the focusing. We note that in the early portion of the Pulse, sv.t'M be1'a,:`2r ;)A'he s,.LJzo,; acicd in the column. some defocusing is expectod to occur while the ionization including relativistic-cherge-displaoement self-focusing. is commencing, since the contribution to the refractive diffraction, and dcfocusing, is governed by the nonlinear index from the free electrons tends to locally reduce its Schrd'dinger equation

_LL±+±)E+-LA.E+-Lo-,;,tE+--ko-(E-0 Icia 821ez 2ko 2 28&. where EVi,z,r) is the slowly varying (r,z) complex amplitude of the wtive, r is the transverse coordinate. A4-. (18/r*3)r 8/Or, koin2xA, c, -ccR is the group velocity in the. plasma, and cis the vacuum speod of light. The noal~iner term 6&UR(r, IEEI2) dcocribing the combined relativistic-charge-displacement iclf-focuxing and defoaming of the pulse is expressed as

4j~IU f~z 2N~tI/[m(l +~ w2,-44aw2 N,,o/m,, I,-3m.' 2C3/4Xe2 , 1(c/2x)E11.

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VOLUME 65, NUMBER 14 PHYSICAL REVIEW LETTERS 1 OcrOBER 1990,

In these expressions, o), is the rslativistically shifted showed, for the range of parameters studied, that I plasma frequency, w is the laser angular frequency, v is > 501. In this connection, we note that the spatial limithe electrop-lo.n collisional frequency, I, is the relativistic tations on the minimal dimensions of the focal zones, and intcnsityj' 5 and N,1I1 is the electron density arising consequently the bounds; on the intensities developed in frein the quasistatic balance of the ponderomotive and those regions, arise from saturation of the nonlincarity the electrostatic forces densities.' We assume that expressed by 8apt (r, IE I'). N,(r,z) inNi.of(r,z) is the static ion density, in which The dynamics of propagation vith relativistic selfNt.o is a constant denoting the maximal ion density and focusing have been examined usiný rnodel pulses / (:,r,i f(r,z) describes the spatial distribution of the ion&. having spatial and temporal Gaussa,,t~ or hyper-Gaussian Hence, f(r,z) a: 0, maxf(r) - I V(f,z) a I for homo- incident beam profiles given by geneous plasma). For quauineutrality, this gives N.,of(,,z) as the irnitivil electron density with N,,o J(0,r,,)mi@(r,:)-ineip1(:/r)N,-(r/ro)NvI , (4) -ZNI~o and Z the ionic charge. N, a2, N2 a:2 The expression for N,1f1, which describes the pertur- /C2 bation of the plasma from local neutrality by both the for II,#1 O~W/mthe latter being the ponderomnotive and rclativistic mechanisri, is given by threshold intensity above which rapid multiphoton ionIr aI ization occurs to the particle& in the medium. FurtherNEmJNn0axj~j(r~z)+srdiv I rd i1,(2) more, we have concentrated on the central tempor~al zone 1(1+3II)IjJ of the pulse, It I _- to, where f o satisfies the expression where I1000) "'Io(O,io) -1.~ expfI - Go/l) N~j )1 I" . (5)

K - (2cap 2 mN,.o) ~,(3) The transierse profile of the plasma column created' 6y and m, denotes the electron mass. In deriving Eq. (2), the front of the pulse is simulated by the hyper-Gaussiian the electronic pressure has been neglected, since it Is small for the beam and plasma parameters examined f(r)-mexp - (i-/i,) N 1 , N, r 2, (6) here. The expression max to,- i simply provides for N, 111-- . with the aperture of the plasma column Ps defined by Calculations of the propagation have been performed 4d(r,:o) 0(o()eXpI _ (r/lra)Nji corresponding to the following parameters for the radiation field and the plasma- 1 -248 nm, 1, - 1.34 x 1020 For a Gaussian transverse intensity distribution (N., - 2i. W/cm', lo - jI, a3.0 x10'9 W/cm2 , ro-3 pum, N1,o an aperture defined With 10-1 0 16 W/cm 2, and takinit -7.5x'C0 19cm), andZ-lO. Io(to)(OMlIM, we find ,'.a2.68ro. Consequently. itie Normalization of r and I by the initial beam radius ro homogeneous plasma approximation f(,) 0 1 15 valid and the maximal value of the initial intensity lo, respec- The beam profiles of the solutioni exhibit severAI tively, gives ti ~x~o ri- Il .4 x 10- 3. the dimensionless salient features. Among them are the production o rul saanalog of the coefficient or defined by Eq. (3). Therefore, tiple foci and, under appropriate conditions, the devetup-. for this set of conditions, the charge displacement'3 is cx- ment of a quasistable confined mode of propagatio !j % pected to be relatively small so that its influence is ing the generation of sharply focused regions. lairm* confined to the structure of the focal regions which arise tantly for the latter, the calculations reveal the nawurr '4 from the relativistic mechanism. Because of this, we will the, mechanism leading to the stabilization, The resuit utilize the approximation that a-mO in the current work, of computations presented in this work are given in the It is important to note, however, that other choices of the frame (s,z,r), sin,-(zlci), connected with the *esparameters can lead to a generally strong role for the front of the beam. The data in the figures bekr* -itu% charge-displacemnent process and future study is being tratc the propagation of the radiation along the Z A ii ,directed to that case. 1 icnsOit. The collisional' absorption of light in plasma is given Figure I(a) illustrates the propagation that devvh.,c% by el - - (wp/oj) v- -Mu /ko, where pu - is the absorp- for the physical parameters stated above correspigulng tion coefficient. Estimatess of p -, for the Values of the to a pulse with an entrance intensity Io(t) - Ih ha~virgi parameters used in this work, show that the collisional a transverse distribution giveni by N2 8. In accorwd eii % absorption is extremely weak and does not influence the an earlier study,' as the nonlincarity 68jr(IE12t is %anumerically obtained solutions. Therefore, the results turated at a sufficiently low level, the behavior of pulw' presented below have been calculated withj --0in. propagating in a homogencous plasma without astuxlyThe nonlinear Schrbdinger equation stated in Eq. (1) tion can generate a mode similar to a puaisng is only applicable to the description of the dynamics of waveguide.'' 0 The result shown in Fig. ](a) illtntrite, the field in a small focal zone, if its longitudinal scale I the formation of such a regime. Interestingly, (hr pu i satisfies the condition I)>X. The numerical experiments ing waveguide represents an a'lternation of ring ,iiu,

I 7~4136

VOLUME65, NUMBER 14 PHYSICAL REVIEW LETTERS I OCroBER 1990

t (10-'3 S)

(loolldr V/=?)

2.4 44 1.6

1. 22

3 4 1 6 7 8 9 10"Lz 00 1o =1-(2/3)1 1/m F(W) '4

(b) 50 FIG. 2. Te locus (6,) of the first focal zone for a pulse xlooll W/ws) zowith a Gaussian initial spatial and temporal intensity distribulion for the parameters defined in the text. The solution per0.6 tains to relativistic self-focusing in a spatially homogeneous 0.slim) fully stripped plasma.

-4 2 0diffraction. In this way, the dynamics of the interaction 2 otend toward the formation of a spatially stable mode of propagation. In addition, as evident from Fig. l(b), a FIG. I. The (O,z) distributions for relativistic self-focusing substantial fraction, nominally 25% of total initial beam of the propagating intensity for a peak initial intensity Is- 11, power, can be confined in such a mode. Importantly, the having a transverse distribution given by N,-8 in Eq. (4) for further study of such cases has also indicated that such the beam and plasma parameters defined in the text. (a) Spa- stable patternm can be maintained for a length of at least tialy homogeneous fully stripped plasma. (b) Axially sym- twenty normal diffraction lengths. metric pluma column with re -re whose transverse profile The solutions of Eq. (1), with the initial conditions corresponds to N) -8 in Eq. (6). given by Eq. (4), satisfy important similarity laws. Specifically, the results shown in Figs. I (a) and I (b) are tures and focal regions along the axis of propagation, valid for any other set of physical parameters &stisfyinj Significantly, the power trapped in it coresponds to ap- the relationships alim[(koro)2/aRo]m2./w'2-2.486x 10 proximately 90% of the total initial power of the beam. and 4 2010o/I, '2/9. Furthermore, if full account of both From Eq. (7), it follows that, for plat&ulikc initial in, the relativistic and charge-displacement mechanism. tensity disttibutiorns, the aperture r, of the playma (KI 00110) is mde, these similarity statements are precolumn becomes wonparable with the beam apcrturv o t. icrvcl7 anrd ri "014aj. For example, in the case treated above, we have Following earliet studia'°"'a we have anaiyzed the rs ft 1.28ro. As the aprtures of the laser beam and the ewiotion of plane waves having small perturbations in plasma column tend toward coincidence, however, de- order to determift the depenoence of the self-focusing focusing becomes relatively more signiicant. Tberefore. length on the initial pulse pararmeter. The results of this this aspect of the propagation must be carefully taken analysis showed that the Largest growth rate of the perinto account when the evolution of pulses having plateau- turbations and, consequently, the minimum self-focusing like initial intensity distributions was studied. length occur at an entrance intew;ty on the beam axis of Figure I (b) shows the propagation of a pulse in a plas. i(:0) - I,. Specifically, the computations have shown ma column for N2 -8 corresponding to the same physi- that this inference remains true for the relativistic selfcal parameters as those pertaining to the illustration in focusing of pulses with Gaussian initial intensity dittriFig. 1 (a). In this case, the transverse profile of the ele- butions. tron density in the plasma column is described byf(r) in The dynamic motion of the foci illustrated in Fig. 2 Eq. (6) for N, -8 and r. -ro. The comparison of Figs. exhibits important characteristics. Fin example, the calI (a) and 1 (b) demonstrates the strong eofect of defocus- culations show that the locus of the first focus in the ins on the spatial dynamics of propagation when the (6,z) plane.' for the initial condition determined by Eq. column has a radial dimension close to that of the beam. (4) with N '-2, N2 -2, !,-612a•8,04xl02° W/vlnmA, Defocusing causes a fraction of the beam to spread away ir-50(j fs, and to-3 pm, reaches the minimal z for from the column while the remaining energy adjusts to 10(t) j I,. If the extreme intensity on the beam axis is balance the relativistic self-focusing, defocusing, and 4, > I,, this locus has three reversal points. Two of

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! 37

VOLUME 65, NumBFEa14 PHYSICAL REVIEW LETTERS I OcrorF 1990

them, corresponding to the same z, are due to computational results using the procedures described lo(t)- 1 I,. The third and central one, corresponding to above are available.' 2 a greater z, occurs at lo(t) -,I. In principle, this One of the authors (C.K.R.) acknowledges fruitful feature establishes a clear diatnostic signature for rela- conversations with J. C. Solem, T. S. Luk, K. Boycr, and tivistic self-focusing that enables it to be distinguished A. McPherson. Support for this research was partially from the Kerr nonlinearity4 which produces a locus with provided under Contracts No. AFOSR-89-Ol59, No. a single point of reversal. N00014-87-K-0538, and No. N00014-86.C-2354. The spatial character of propagation of sufficiently intense subpiosecond ultraviolet pulses in self-generated plamas is significantly influenced by a relativistic- 'L V. Keldyoh. Zh. Eksp. Teor. Fiz. 47, 1945 (1964) (Soy. charge-displacement mechanism and two important Phys. JETP 2. 1307 (1965)]. classes of propagation have been distinguished. In a spa- 2T. S. Luk. U. Johann, H. Eger, H. Pummer, and C. K. tinily homogeneous plasma, the interaction can produce Rhodes, Phys. Rev. A 32, 214 (1985). self-focusing in the form of a pulsing waveguide involv- 3J. C. Solem, T. S. Luk, K. Boyer. and C. K. Rhodes, IEEE ing a structure of alternating rings and focal zones. The J. Quantum Electron. 2S, 2423 (1989). location, formation, and nature of the focal regions, in- 4V. 1. Ludgovoi and A. M. Prokhorov, Usp. Fiz. Nauk III, cluding the maximal intensities occurring in them, de- 203 (1973) [So0. Phys. Usp. 16, 658 (1974)A. pend essentially on the properties of the initial transverse 3H. Hors, Physics of Las Driven Plaas (Wiley, New intensity distribution. The locus of the first focus in the York. a981). (t,z) plane for pulses with Gaussian initial intensity dis- 6P pageE.ssy ndATi&hs RvLt.6, 2011 (1990); Phys. Rev. A 41. 4463 (1990); P. Sprangle, C. tributions and maximum intensities I. Z i, has three re- M. Tang, and E. Esarey, IEEE Trans. Plasma Sci. 15, 145 veral points. For this class of pulses, the relativistic (1987). self-focusing length is minimal, if the intensity of the ini- 7G..Z. Sun. E. Ott, Y. C. Lee., and P. Guzdar, Phys. Fluids tial pulse on the axis equals • ,. 20, 526 (1987). A mechanism leading to a spatially stabilized mode of IT. Kurki-Suonin, P. J. Morrison, and T. Tajima, Phys. Rev. propagation is also revealed. In this case, the defocusing A 40, 3230 (1989). that occurs when the plasma column has a radius close 9V. E. Zakharov, V. V. Soboley, and V. S. Synskh, Zh. Eksp. to that of the beam is particularly significant. The Teor. Fiz. 4. 136 (1971) (Soy. Phys. JETP 33,77 (1970)1. effectie taction of the defocusin, pa icha r vs s fan. Te syV, 1. Bespalov and V. 1. Talanov, Pis'ma Zh. Eksp. Twor, effective action of the defocusing, which serves as a spa- Fix. 8, 471 (1966) [Soy. Phys. JETP 3, 26 (1966)). tially distributed nonuniform energy loss, causes the rap- IT, T . Benjamin and J. E. Feir, J. Fluid Mech. 27, 417 id formation of intensity profiles that become quasistabi- (1967). lized along the beam axis. Therefore, it is accurate to 1 2A. B. Borisov el al., Institute of General Physics, U.S.S.R. state that the process of defocusing contributes directly Academy of Science, Moscow, Report No. 4, 1990 (unpubto the dynamical development of stabilization. Further lished).

138

1756

APPENDIX I

STABLE CHANNELED PROPAGATION OF INTENSE RADIATION IN PLASMAS ARISING FROM RELATIVISTIC AND CHARGE-DISPLACEMENT MECHANISMS

A. B. Borisovt, A. V. Borovskiy, V. V. Korobkin*, A. M. Prokhorov*, 0. B. Shiryaevt, J. S. Solem , K. Boyertt, and C. K. Rhodestt

t Laboratory for Computer Simulation, Research Computer Center, Moscow State

University, Moscow, 119899, USSR

*' General Physics Institute, Academy of Sciences USSR, Moscow, 117942, USSR.

tt Department of Physics, University of Illinois at Chicago, Chicago, IL 60680,

USA.

Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

ABSTRACT

Calculations of the dynamics of propagation in plasma of intense axisymmetric

laser pulses Incorporating both relativistic and charge-displacement mechanisms

are presented. It is shown that the combined action of these two processes can

lead to stabllizzd conflnea modes of propagation whose asymptotic properties

correspond well to the lowest z-independent eigenmodes of the governing nonlinear

Schr•idinger equation. The effect of the charge-displacement is large and cavitation

of the electron distribution Is a general feature of the solutions for the range of

parameters studied. The results of a specific example show that approximately

one half of the incident power (- 4 TW) can be trapped in the channels which,

for a wavelength of 248 nm, an electronic plasma density of - 7.5 x 1020 cm- ,

and an initial beam radius of - 3 pim, develop propagating intensities of - 102, W/cm 2.

139

The dynamics of propagation of intense subpicosecond laser pulses in plasmas

in the strongly relativistic regime is currently undergoing vigorous theoretical

analysis 1"6. Of primary significance is the possible formation of quasistable selfchanneled modes of propagation. Separately, the analyses of both relativistic6-9

and charge-displacement 3 mechanisms give indications that confined modes of

propagation can exist. In this Letter we examine theoretically the consequences of

the combined action of these two mechanisms with particular attention to the

development and character of highly stable confined modes of propagation. For

radiation with a wavelength of 248 nm, the range of intensity of interest is I >

10" W/cm2, since both relativistic6 and charge--displacement 3 mechanisms are

significant in that region.

The basic formalism and procedures describing these calculations have been

described previously 6 . Assuming collisionless and lossless propagation8 (Ip = V = \o

= 0), as in the earlier analysis, the complex field amplitude E(t,z,r) is governed

by the nonlinear Schr6dinger equation

+ I a 9 E + - 6ERE = 0, (1) 11. 2k ./ 2ER_

where r Is the transverse coordinate, L., = ý 2/3r2 + r" 1 )/!r, ko = 2T/XAC,c = CER/2

is the group velocity in the plasma, and c is the vacuum speed of light. The

nonlinear term 6R(r, E 12) of key significance, which describes the combined

action of the relativistic and charge-displacement mechanisms, is given by 6ER Po 1 - (1 + 3 1/1r) -1/2max[O, f(r) + (c 2/Wo)A2.1 + 31/2r)1/]

(2) in which Ir 3m22 c0/41re 2 is the relativistic intensity3,8, W is the plasma frequency,

43 is the laser angular frequency, and f(r) denotes the spatial distribution of the

Initially unperturbed electron density. The last term in the inner brackets of Eq.

(2), (c2/o)A,.(1 + 31/1r) 1/ 2, describes the charge-displacement process and was

140

neglected In the earlier studies 6 of purely relativistic propagation. The form of

Eq. (2) assumes that the propagating radiation is circularly polarized, since the

relativistic 'Y-factor can be written as (1 + 31/Ir)1/2 in that case. Futhermore,

the structure of the terms in Eq. (2) clearly reveals that the relativistic and

charge-displacement processes are fundamentally connected and cannot be considered

as truly separate and Independent mechanisms.

In consideration of the charge-displacement, the motion of the heavy ions is

neglected, In accord with previous treatments 3 , since the time scale of the laser

pulse is assumed to be sufficiently short (- 100 fs). In this picture, it is known

that the electronic component of the plasma can be substantially perturbed by the

ponderomotive potential generated by the propagating radiation 1,3. For the present

calculation, the electron distribution is determined by the force balance between

the radially outward ponderomotive force and the oppositely directed electron-ion

attraction3 . This ponderomotIvely driven process is capable of completely expelling

electrons from certain spatial regions, a condition described as "electronic cavitation"1 .

The calculations show that the charge-displacement plays a strong role in determining

the spatial character of ti.a propagating energy and, particularly, in stabilizing the

confined high-intensity modes.

The essential finding of these calculations is insight into the formation and

stabilization of spatially confined modes of propagation arising from the coiperative

action of the relativistic and charge-displacement processes. The numerical

simulations were performed with model pulses I(z,r,t) having spatial and temporal

Gaussian or hyper-Gaussian incident beam profiles and initial homogeneous or

hyper-Gausslan unperturbed electron densities, in accord with the procedures

described previously 6 . Using that earlier notation6, the relevant paramelers are X

- 248 nm, Ir = 1.34 x 1020 W/cm 2, 1, = 3 x 101! W/cm 2 , r, = 3 irJ, Ne,o - 7.5 x

141

1020 cm"=, N2 2, and f(r) * 1 (spatially uniform plasma). Moreover, the results

described below are valid 6 for any other set of physical parameters satisfying the

relationships a1 ) = 2.486 x 102 and a2 " Io/T r = 2/9. The results of

computations presented In this work are given in the frame (q,z,r), q = t -(z/c,),

connected with the wave front of the beam. The graphical data represented below

illustrate the propagation of the radiation along the z axis for q = constant.

The solutions for the spatial profiles of the electromagnetic energy and the

corresponding electron density exhibit several important characteristics which are

shown In Fig. (1) and Fig. (2), respectively. Among them are (1) the generation

of Intense focal regions, (2) the development of a stable confined channel of

propagation, (3) the trapping of a substantial fraction of the incident power in

the channel, and (4) strong cavitation in the electron density,

Figure (la) illustrates the overall behavior of the normalized intensity distribution

I(r,z)/1o and Fig. (1b) magnifies the region between the first two peaks appearing

in Fig. (1a), The corresponding normalized electron density N(r,z)/No is shown in

Fig, (2a). Initially, although a small amount of energy can be seen diffracting

away, a large fraction of the incident radiation collapses to form both an axial

focal zone (-45%) and a high-intensity annular region. This early stage of evolution

involves very little charge-displacement and resembles rather closely the dynamics

seen in the purely relativistic case 6 . However, in the present situation, the

development of these high-intensity zones leads rapidly to strong perturbations in

the electron distribution and the formation of two regions of cavitation, This

modified charge distribution, the cross section of which is shown In Fig. (2b) for

the axial position z = 95,4 pm, clearly illustrates simultaneous cavitation along the

axis (r Z 0.25 pm) and in a relatively narrow annular region located at r - 1.7 pm.

Subsequently, as energy is exchanged between these two structures, the annulus

transfers some of its energy to the axial region and a smooth stable confined

142

mode of propagation on the z-axis containing - 46% of the incident power develops

(z W 600 pm). The electron density evolves in a corresponding manner and a

single cavitated zone on the axis is formed (z 5 600 pm). The peak intensity

associated with the channeled propagation is exceptionally high, reaching values of

- 1021 W/cm2 in this example.

The relativistic influence on the electron mass and the ponderomotively

driven displacement of charge both encourage the formation of channeled propagation.

On the basis of the dynamical picture revealed above, the combined effect of

these two mechanisms can be, in reasonable approximation, summarized in the

following simple way. The relativistic effect leads to the initial concentration of

the radiation and the resulting displacement of electronic charge reinforces this

tendency and stabilizes the confinement. The cooperative nature of this action

appears to lead to highly stable conditions of propagation.

The behavior of the computed solutions for large z can be compared with

the z-independent eigenmodes1 ,10 of the nonlinear Schr6dinger equation (1). In

this comparison, it has been shown that the field amplitude tends asymptotically

to the lowest elganmode Vs(r) of this equation. Vs(r) is a real-valued positive

monotonic function of r, carries a real dimensionless index s, vanishes as r ,

and is related to the electric field amplitude by the relation

E(r,z) = Vs(r) exp[-iz(1 - s)W2 /2koc 2J. (3)

The computations have shown, for the case studied, that the field distribution

for large z tends to the amplitude Vs(r) for the index s Z 0.554. The appropriate

intensity distribution is then given by Is(f) = Vs2(r). Figure (2c) exhibits the

normalized asymptotic field amplitude (15(r)/Io)1/2 and the corresponding asymptotic

electron density distrihution N,(r)/N0. Significantly, these asymptotic prolileu,

when compared to the corresponding distributions arising from the numerical dynamical

143

calculation of the propagation for z 900 p~m, differ by less than one percent.

Therefore, the limiting behavior of the dynamically derived propagating solutions

seems well described by the simpler z-independent analysis.

Earlier work3 concerning the charge-displacement mechanism with a steady

state picture showed that the electrostatic energy per unit length associated with

a channel arising from the charge-displacement scaled as p2/3' with P denoting the

laser power. Consequently, for a fixed total power, this electrostatic energy will

necessarily be a minimum for a single channel. Therefore, the collapse of the annular

structure shown in Fig. (1) Is consistent with the system dynamically seeking to

minimize this electrostatic energy. Indeed, for the electron density distributions

shown in Fig. (2), direct calculation shows that the electrostatic energy per unit

length of the large z solution (asymptotic) is approximately one half that associated

with the region containing the cavitated annulus [Fig. (2b)]. In addition, it

should be noted that the electrostatic energy per unit length in this example is far

less (< 0.2%) than the corresponding energy per unit length in the radiation field.

The results of the dynamical calculations shown in Fig. (1) and Fig. (2) can

be directly compared In three ways to the simple analytical picture previously

used to estimate the properties of the propagation arising solely from the mechanism

of charge-displacement 3 . (1) The analytical treatment found that the intensity at

the wall 1w was independent of laser power and equal to Iw = (1 + /2)mw2 c/47rre S1.1 x 1020 W/cm2 . The dynamical result presently calculated gives good agreement

with 1w - 1.2 x 1020 W/cm 2. (2) The peak intensity developed at the center of

the channel In the asymptotic regime is also found to agree within - 10%. (3)

Finally, the channel radius shown in Fig. (2c) is - 0.4 pm, while the analytic

treatment 3 gives a corresponding value of - 0.7 pm. This difference is at least

partially understood by the fact that the analytic theory3 did not take into

account the relativistic increase in electron mass in the equation for the force

144

balance. The use of a higher mass would naturally reduce the efiective ponderomotive

force, a correction that would translate into a reduced channel radius as indicated

by the relativistic calculation. Overall, however, we can state that the full

numerical treatment of the dynamics and the approximate analytical theory produce

results that are in rather close agreement for these important properties of the

asymptotic behavior.

Several other cases have also been considered. In particular, calculations

examining the dynamics of propagation have been studied for initial intensity

distributions other than Gaussian 6 (e.g. N2 = 8) for both homogeneous plasmas

[f(r) a 1] and simulated plasma columns [f(r) = exp [-(r/ro)']. The behavior was

found to be qualitatively the same as that shown in Fig. (1) with asymptotically

trapped fractions of the initial laser power ranging from 34% to 77%.

The dynamical behavior of channeled propagation arising from both relativistic

and charge-displacement mechanisms has been studied theoretically. Two principal

findings have been established. (1) These two processes can cooperatively reinforce

one another and lead to stable confined high-intensity modes of propagation in

plasmas which are capable of trapping a substantial fraction (- 50%) of the incident

power. (2) The spatial profiles of both the intensities and electron distributions

tend asymptotically to those derived as lowest eigenmodes of a z-independent

analysis.

The authors acknowledge fruitful conversations with A. R. Hinds, R. R, Goldstein,

T. S. Luk, and A. McPherson. Support for this research was partially provided

under Contracts No. AFOSR-89-0159, No. NO0014-87-K-0558, No. N00014-86--C-2354,

No. DE-FGO2-91ER12108, and N00014-89-C-2274.

145

REFERENCES

1. Gou-Zheng Sun, E. Ott, Y. C. Lee, and P. Guzdar, Phys. Fluids 30, 526 (1987).

2. P. Sprangle, E. Esarey, and A. Ting, Phys. Rev. Lett. 64, 2011 (1990); F.

Sprangle, C. M. Tang, and E, Esarey, Phys. Rev. A 41, 4663 (1990).

3. J. C. Solem, T. S. Luk, K. Boyer, and C. K. Rhodes, IEEE J. Quant. Electron.

25, 2423 (1989).

4. T. Kurki-Suonio, P. J. Morrison, and T. Tajima, Phys. Rev. A 40, 3230 (1989).

5. J. N. Dardsley, B. M. Penetrante, and M. A. Mittleman, Phys. Rev. A 40, 3823

(1989).

6. A. B. Borisov, A. V. Borovskiy, V. V. Korobkin, A. M. Prokhorov, C. K. Rhodes

and 0. B. Shiryaev, Phys. Rev. Lett. 65, 1753 (1990).

7. G. Schmidt and W. Horton, Comments Plasma Phys. Controlled Fusion 9, 85

(1985).

8. H. Hora, Physics of Laser-Driven Plasmas (Wiley, New York, 1981).

9. S. V. Bulanov, V. I. Kirsanov, A. S. Sakharov, Flzika Plasmi 16, 935 (1990).

10. A. 8. Borisov, A. V. Borovskiy, V. V. Korobkin, C. K. Rhodes, and 0. B. Shiryaev,

Trudi IOFAN 4 (1991), In press.

146

FIGURE CAPTIONS

Fig. (1): (a) The normalized intensity distribution l(r,z)/I, calculated for an

initially homogeneous plasma [f(r) w 1]. 10 = 3 .< 1019 W/cm 2, r. 3

pm, . = 248 nm, and N. 0 , = 7.5 x 1020 W/cm 2. (b) Enlargement of the

transition zone between the first two peaks shown in rig. (la).

Fig. (2): (a) The normalized electron density distribution N(r,z)/No calculated for

an initially hormogenecus plasma [f(r) a 11 and corresponding to the

data shown In Fig. (1), N, i Neo = 7.5 x 1020 cm"3. (b) Radial cross

section of the normalized electron density shown in Fig. (2a) for axial

distance z = 95.4 pm. Cavitation is present along the central axis (r 2

0.25 pm) and in an annulus located at r - 1.7 prm. (c) Radial dependence

of the asymptotic solutions for the normalized amplitude [is(r)/101 1/2

and the normalized electron density Ns(r)/tlo for the index s = 0.554.

147

FIGURE (1) 148

0.0

4.00

2.0 ()8.0 (C)

C)1.0 4.0

S ~~~2.0 NQ)N

0.0 nr -r 10.0 - T- 0.0 1.0 2.o 3.0 0.0 1.0 2.0

FIGtJRE (2) 149

APPFNDIX J

Observation of Relativistic/Charge-Displacement Self-Channeling of Intense Subpicosecond Ultraviolet (248 nm) Radiation in Plasmas

A. B. Borisovt, A. V. Borovskiy* V. V. Korobkin*, A. M. Prokhorov*, 0. B. Shiryaevt, X. M. Shit', T_ S. LukTI, A. McPherson"", J. C. Solem**, K. Boyertt, and C. K. Rhodestt

t Laboratory for Computer Simulation, Research Computer Center, Moscow State

University, Moscow, 119899, Russia

General Physics Institute, Academy of Sciences of Russia, Moscow, 117942, Russia.

t Department of Physics, University of Illinois at Chicago, Chicago, IL 60680,

USA,

w. Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

ABSTRACT

Experimental studies examining a new relativistic regime of high-intensity

short-pulse propagation in plasmas have been performed which present evidence for

the formation of a stable mode of spatially confined (channeled) propagation. For

an electron density of - 1.35 x 102' cm-' and a power of - 3 x 10"1 W, the

results indicate a channel radius < I pm and a peak intensity - 101w W/cm2 .

Comparison of these findings with a dynamical theory yield agreement for both

the longitudinal structure and the radial extent of the propagation observed.

130S

A fundamenta!ly new regime of electromagnetic propagation is expected to

arise in plasmas for short-pulse radiation at sufficiently high intensity. Calculations

of the propagation in plasmas, incorporating both relativistic 1 ,2 and charge3-7displacement mechanisms, indicate that the combined action of these processes

can lead to a new stable form of spatially confined (chanr.eled) propagation.

This Letter reports (1) the results of the first experimental study probing the physical

regime relevant to the observation of relativistic/charge-displacement self-channeling

and (2) presents the initial compar!son of these experimental findings with matching

theoretical calculations performed with the computational procedures described in

Ref. 7.

The experimental arrangement used in these studies is illustrated in Fig. 1(a).

The source of radiation was a subpicosecond KrF* (X = 248 nm) laser that has

been described elsewhere.8 It delivered a linearly polarized power of ~ 3 x 10"

W (- 150 mJ, pulse duration - 500 fs) in a beam with a diameter of ~ 42 mm.

When this radiation was focused into the chamber with lens L1 (f/7), a focal

radius ro - 3.5 pm was measured, giving a maximum intensity 10 - 8.6 x 10=

W/cm 2. The medium was provided by filling the chamber statically with gas [He,

Ne, Ar, Kr, Xe, N2 , CO2 , or a mixture of Xe (4%) and N, (96%)i up to a maximum

density of - 1.89 x 1020 cm 3 .

The diffracted 248 nm radiatic, n was measured as a function of the angle (9)

with respect to the direction of the incident radiation. The incident laser beam

was blocked by a metal disk on the output window of the chambor and lens L2

imaged the region near the focal zone on a fluorescent screen S. The diaphragm

D in front of lens L2 restricted the collection of the diffracted light to a solid

angle of -- 50 while simultaneously increasing the depth of field. The angle

between the axis of the lens L2 and the axis of the incoming laser radiation

151

could be readily varied up to a maximum angle of e - 15*. Two flat. mirrors coated

for high reflection (- 99%) at 248 nm, both having a spectral bandwidth of - 10

nm, served in reflection as spectral filters (F) for the diffracted laser radiation

so that only the scattered 248 nm radiation could illuminate the screen. An

attenuator A was employed to adjust the intensity on the screen and the images

formed were recorded through the visible fluorescence produced with a microscope

and a CCD camera.

The characteristic behavior observed is well Illustrated by the data recorded

with N2. The measured result, shown in Fig. (1b), corresponds to a density PN

1.35 x 10O° cm 3 . To the left, in the photographic inset, a relatively large cone

of light Rayleigh scattered from the plasma is visible at all angles as the energy

propagates toward the focal point of the lens, while, in the region to the right of

the conical apex, a narrow filament developed. The diameter of this filament is

not greater than 10 pm, the measured spatial resolution of the imaging system

The distribution of intensity observed along the filament exhibited several bright

features attributed to diffraction because they could not be seen for 9 > 200

Since the axis of the imaging lens corresponded to an angle e = 7.50, the scale

along the abscissa of the photographic data is reduced by - 8-fold, giving the

maximum length of the filament as - 1 mm. The graph in Fig. (1b) represents a

one-dimensional axial profile, taken along the direction of propagation (z), of the

observed intensity pattern (inset). Three peaks (ot, P, Y) are visible with a spatial

separation of 6 = 200 ± 20 pm. The normal Rayleigh range for the focal geometry

used was - 200 pm.

The diameter of the filamentary channel is an important dynamical variable

which we estimated by measuring the maximum angular deviation of the diffracted

light. The experimental value ý of this diffracted cone was • ~ 200, a magnitude

1 52

indicating a radius rO - 0.9 Ipm though the relation * = 1.22 X/ro. Filaments of

this general nature were observed at densities above - 1.35 x 1020 cM-n 3 in N2,

Ne, Ar, Kr, CO2 and a mixture of Xe (4%) arid N2 (96%), but not in He and Xe,

two materials discussed further below.

Two mechanisms exist that could modify the refractive index of the medium

and lead to the observed behavior. They are (1) the Kerr-effect stemming from

the ions and (2) the relativistic/charge-displacement process.7 Since the pulse

duration is very short (- 50C fs), the motion of the ions is negligible,3 and no

contribution can arise from expulsion of the plasma from the high-intensity zone.

An implication of the estimate of the channel radius (rO - 0.9 pm) is that the

observed propagation is associated with intensities in the 10"-10l" W/cm2 range.

Under tnese conditions, available experimental evidence9,10 on muitiphoton ionization

indicates that He should be fully ionized and the C, N, and 0 atoms constituting

the molecular materials would retain, at most, only is electrons.

Consider explicitly the case of N2, which has estimated9',10 threshold intensities

for the production of Ns*, N", and N"* of 1.6 x 10'6 W/cm2 , 6.4 x 10" W/cm'.

and 1.3 x 10' W/cm2, respectively. Hence, the volume of the channel would be

largely ionized to Ns', with certain localized high-intensity regions contributing

some N"*. Two consequences of this pattern of ionization follow, namely, (ca) the

Kerr-effect arising from the ions is small, since the polarizabilities of the remaining

is electrons are low, and (P) the electron density (ne) initially produced in the

focal region is nearly uniform. Therefore, ne = 1.35 x 1021 cm"3 for the data on

N2 shown in Fig. 1(b).

A critical power Pcr for self-channeling, arising from the relativistic/charge-

displacement mechanism, can be defined 1 1 as

153

2+ . 0/ 0r2r g 0( / p o 2n c r)Pcr =m•e•/e ) (r)rdr 1/,) x 10 _- W, (1) 0 nef

where me, e, and c have their customary identifications, w is the laser angular

frequency, wp,o is the plasma frequency for the uniform unperturbed plasma with

electron density ne, ncr is the critical electron density ( for X = 248 nm, ncr

1.82 x 1022 cm2 ). and go(r) is the Townes mode.12

The critical powers associated with the experimental conditions, for He and

N2 at a medium density p = 1.35 x 1020 cm" 3, are 108 x 1012 W and 2.19 x 10"

W, respectively. Therefore, since the incident power was P = 3 x 10'1 W, no

filament was expected in He, a prediction conforming with the observation of

none. Moreover, the diffracted cone of radiation was also at-sent with He. In

contrast, P/Pcr = 1.37 for N2, a condition that held generally (P/Pcr > 1) for all

materials which exhibited evidence for channel formation. We note, however, that

some contribution from the Kerr-effect may be present, even for the light materials

(Ne, N2, and C02), in the early stage of channel formation prior to the development

of a substantial level of ionization, and that the heavier gases, (Ar, Kr, and Xe)

may involve a more significant influence from the Kerr process. A specific

estimate of the nonlinear index change arising from both N5° and No* at an

intensity of - 1019 W/cm 2 indicates that their contribution is less than 10" that

of the free electrons, hence the ionic contribution can be neglected in N2 for the

conditions studied.

A direct comparison will now be made between the theoretical analysis, fully

described in Ref, (7), and the experimental findings for N2. This comparison can

be accomplished for both the longitudinal intens!ty profile and the radial extent

of the channel. Figure 2(a) illustrates the int-ansity profile l(r,z)/[o calculated

154

with physical parameters corresponding to those of the experiment (i.e. P 3 x

10" W, ro - 3.5 pm, ne = 1.35 x 10= cm"3, and P/Pcr - 1.37). Importantly, all

of these parameters are based on inde .ndenX measurements of (1) the laser pulse

involving determinations of the energy and power (P), (2) the focal radius (ro) of

the incident radiation, and (3) the characteristics of the multiphoton ionization10

generating the electron density (ne). Therefore, this comparison does not involve

a fit to a free parameter. The normalized electron density calculated is presented

in Fig. 2(b) from which it is seen that electronic cavitation occurs only near the

positions of the maxima in the intensity profile (Fig. 2(a)]. Curve (A) in Fig. 2(cj

reprf-.ants *he one-dimensional axial intensity profile I(0,z)/Io corresponding to

the calculated distribution shown in Fig. 2(a). The spacing (6) of the maxima is

seen to be 6 - 185 pm, a value in close agreement with the experimental figure (S

200 ± 20 pm) illustrated in Fig. 1(b). Furthermore, analysis has shown that the

spacing 6 is quite sensitive to the power PO arid electron density ne, particularly

in the region close to the threshold (see Eq. (1)]. With respect to the results

illustrated in Fig. 2(c), an increase in ne by less than 5 percent causes a reduction

in the spacing 6 by approximately 25 percent. Therefore, substantial changes in tne

physical parameters would grossly alter the comparison of the experimental an'J

theoretical results.

3-7 111 Theoretical studies indicate that the charge-displacement plays a ver,

important dynamical role. In order to test this hypothesis, identical calculations

were made for N2 for the purely relativistic case2 which explicitly neglects the charge

displacement term, namely, elimination of the term [cr/(up0 ro)] A,(1 + I/Ir)

in Eq. (24) of Ref. (7). Significantly, the resulting axial profile [Curve B in Fig

2(c)] exhibits only a single relatively weak maximum, for 0 ct z 4 600 pm, an outcome

sharply at variance with both the full theoretical analysis and the experimental

155

)

observation. Although the expected charge-displacement is highly localized [Fig. 2(b)], this comparison reveals the strong influence it has on the propagation. 3' 7',11

At a greater incident power (P/Pcr - 10), a continuous channel in the electron distribution is expected to develop. 7

The measurements indicated an approximate value of ri = 0.9 pm for the

radial extent of the channel, a result that can be compared with the corresponding

theoretical figure. Figure 2(d) illustrates five radial intensity profiles [(r,zi)/1o

of the distribution pictured in Fig. 2(a). Since the measurement of this angularly

scattered radiation did not correspond to a known longitudinal position, this comparison

can only be qualitative, but the radial distributions shown indicate that the expected

value lies in the interval 0.5 :ý r 4 1.0 pm, a range that comfortably includes the

e;,perimental value ri.

The results observed with Xe deserve additional discussion, since those

experiments did not give evidence for the formation of a channel. In significant

contrast to the case involving N 2, the electron density ne produced by the multiphoton

ionization10 in Xe is expected to be very nonuniform spatially. For intensities

spanning 1011 - 1018 W/cm 2, the corresponding density ne would vary by over a

tactor of two. Since this nonuniformity woula tend to reduce the refractive index

locally in the central high-intensity region, a significant defocusing action is

expected which could suppress the channel formation.

Finally, we note (1) that the intensity distribution is not expected to depena

13,14 strongly on the state of polarization and (2) that losses to the plasma may Le

significant, particularly at electron densities close to ncr.

The first experiments examining a new relativistic regime of high-intensit,

pulse propagation in plasmas have been performed and the findings indicate the

formation of a channeled mode of propagation over a length considerably greater

156

than the Rayleigh range. Specific comparisons of the experimental observations

with a dynamical theory, which explicitly includes both the influence of the

relativistic mass shift and the displacement of the electronic component of the

plasma, produce excellent agreement for both the longitudinal structure of the

intensity profile and the radial extent of the channel. While the present channel

contains several foci, a continuous channel is predicted to develop at higher

power. Finally, the intrinsically very high concentration of power associated with

this mechanism of channeled propagation provides an efficient and general method for the production of conditions necessary for x-ray amplification. 15

The authors acknowledge the expert technical assistance of J. Wright and P.

Noel in addition to fruitful conversations with A. R. Hinds, R. R. Goldstein, and B.

Bouma. Support for this research was partially provided under contracts AFOSR

89-0159, (ONR) N00014-91-J-1106, (SDI/NRL) N00014-91-K-2013, (ARO) DAAL 3-91

G-0174, (DoE) DE-FG02-91ER12108, and (NSF) PHY-9021265.

157

REFERENCES

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Rhodes, and 0. B. Shlryaev, Phys. Rev. Lett. f6, 1753 (1990).

3. J. C. Solem, T. S. Luk, K. Boyer, and C. K. Rhodes, IEEE J. Quantum Electron.

QE-2_, 2423 (1989).

4. P. Sprangle, E. Esarey, and A. Ting, Phys. Rev. Lett. _4, 2011 (1990); P.

Sprangle, C. M. Tang, and E. Esarey, Phys. Rev. A _1, 4463 (1990); A. Ting,

E. Esarey, and P. Sprangle, Phys. Fluids B 2, 1390 (1990).

5. G. Z. Sun, E. Ott, Y. C. Lee, and P. Guzdar, Phys. Fluids 2Q, 526 (1987).

6. T. Kurki-Suonio, P. J. Morrison, and T. Tajima, Phys. Phys. Rev. A 4Q, 3230

(1989).

7. A. B, Borlsov, A. V. Borovskly, 0. B. Shiryaev, V. V. Korobkin, A. M. Prokhorov,

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(in press, 15 April 1992).

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158

14. S. V. Bulanov, V. I. Kirsanov, and A. S. Sakharov, Fiz. Plasmy 1.., 935 (1990)

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159

Figure Captions

.". 1: (a) Experimental apparatus use in studies of propagation. See text for

description. (b) Data concerning the pattern of propagation observed

with a single pulse in N, at a density of - 1.35 x 1020 cm ". The maximum

intensity is half the detector (CCD) saturation. The radiation is incident

from the left. The inset shows the photographic data with a vertical

spatial resolution of - 10 pim. The graph illustrates the one-dimensional

axial profile taken along the direction of propagation (z) of the photographic

data (inset). A spacing of the maxima 6 = 200 ± 20 pJm is indicated.

Fig. 2: Calculations for N, with P = 3 x 10,' V, r = 3.5 pm, ne = 1 35 x 1071

cm", and 1, = 8.6 x 1017 W/cm2, (a) Normalized intensity l(r,z)/[,.

(b) Normalized electron density N(r,z)/N, for N. with N, = ne, (c)

Normalized one-dimensional axial intensity profiles I(0,z)/10 . (A) Full

theory for data in panel (a), 6 = 185 pin. (B) Calculation with charge

displacement term neglected. (d) Normalized radial intensity profiles

I(r,zi)/l 0 corresponding to panel (a). Longitudinal positions z, = 172

pm, z, = 245 pm, z3 = 358 Jm, z. = 441 pm, and z5 = 559 pm and r(=

0.9 Pm.

11 160

191

"I R

I L

C\; c

-j

0 0)

LL Iy)

1ih

z9 I

-Ht

CL 0 00 CL."

Il C44r

(a)

N I N

125 A () 100. 0.

_0 1.100

o 185 4i / / ~50.0 z 50-A

0 •00 200 300 400 1 O', 0 05 1.0 1 5 2.02f

Axial Distance z (dmt

Radial Distance

Figure 2

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28 Jan 2020

"High Brightness X-Ray Source for Directed Energy and Holographic Imaging Applications"

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