PROSPECTS FOR X-RAY LASER APPLICATIONS

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PROSPECTS FOR X-RAY LASER APPLICATIONS

J. Trebes

To cite this version:

J. Trebes. PROSPECTS FOR X-RAY LASER APPLICATIONS. Journal de Physique Colloques, 1986, 47 (C6), pp.C6-309-C6-319. �10.1051/jphyscol:1986638�. �jpa-00225881�

JOURNAL DE PHYSIQUE,

Colloque C6, supplkment au n o 10, Tome 47, octobre 1986

J. TREBES

Lawrence Livermore National Laboratory, University of California, Livermore, CA 94550, U.S.A.

Abstract- The prospects for the near term application of current x-ray lasers are examined. Four areas of application are discussed: grating production, photoionization laser physics, contact microscopy, and holography. Evolutionary rather than revolutionary progress is shown to be possible in grating production and contact microscopy. The

photoionization experiment discussed is not possible without an x-ray laser. Dramatic progress is potentially possible in x-ray holography but numerous difficulties make this a long term problem requiring

significantly improved x-ray lasers.

Now that soft x-ray lasers with large gains have been

demonstrated1 y 2 s 3 j 4 it is appropriate to examine the prospects for near term applications of the soft x-ray lasers which are likely to be available over the next few years. There have been three previous reviews of applications of x-ray

laser^^,^,^

but these reviews were of a more general nature. This article will focus on four specific applications with the Ne-like collisional-excitation laser. These applications are grating production, photoionization laser physics, microscopy, and holography. The use of Ne-like lasers is a result of the author's familiarity with them and does not imply that other types of lasers could not be used or even might be more appropriate.

GRATING P R O D U r n Transmission Gratings

Two types of grating production techniques are potentially possible with x-ray lasers: spatial period division and holography. In spatial period division the near field diffration pattern of a parent transmission grating is used to produce a fringe pattern with a spatial period at harmonics of the parent grating period. The illuminating light source does not need to be

( l ) ~ h i s work was performed under t h e auspicesof t h e U.S. Department o f Energy by Lawrence Livermore National Laboratory under contract n' W-7405-ENG-48

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986638

C6-310 JOURNAL DE PHYSIQUE

coherent. It is only necessary to be monochromatic and intense. Current x-ray lasers meet these requirements. The process of microfabrication by spatial period division is fully discussed by Andy Hawryluk in reference 8.

The following discussion is based on this reference.

Fig. 1 shows a schematic arrangement for spatial period division. A photoresist is placed behind the parent grating and exposed by the diffraction pattern.

Figure 1. Schematic for spatial period division.

e-, dz

The distance z where the m th order diffration pattern from a grating of period P occurs is given by:

-

X-Ray Laser

The depth of field dZ over which this occurs is given by:

This figure clearly shows the highest spatial frequency occurring closest to the parent grating over a narrow depth of field. As one moves further away from the parent grating the spatial frequency decreases and the depth of field increases. The line to space ratio R needed to optimize the diffraction pattern is given by:

Parent Grating Photoresist

-

A possible arrangement for producing transmission grating using a soft x-ray laser is given in fig. 2. Here a 10 mJ, 155

A

yttrium laser is used.

The output is focused by a spherical multi-layer mirror down to a 1 mm diameter spot. A reflectivity of 25% is assumed9. Polymethylmethacrylate (PMMA) substrate is used. The transmission of the rating is assumed to be 10%. With an absorbtion length at 155

A

of 3000

AP0,

this results in an

4

z

^b

exposure on the PMMA of 1 0 0 0 ~ / c m ~ . This is quite adequate for exposing the PMMA. With a

~ O O O A

parent grating both 1 000A and 667A gratings should be possible. Less laser energy would be required for smaller gratings.

Y t t r i u m X-Ray Laser

Mu1 t i - l a y e r M i r r o r R= 25% @ 155

8

P a r e n t G r a t i n g

Figure 2. Schematic arrangement for using the yttrium X-ray laser in spatial period division for grating production.

Reflection Gratings

It is also possible to consider using the x-ray laser to produce reflection gratings holographically. Here the x-ray beam is split into two beams using an x-ray beamsplitter9. They are then recombined at the photoresist producing an interference pattern. This is shown in fig. 3.

Fig. 3 Schematic arrangement for reflection grating production.

The beams intersect with an angle 29

.

This results in a path difference dy=ysine where y is the grating width. Since this path difference should not exceed the longitudinal coherence length of the laser the grating period that can be produced is restricted to:

C6-312 JOURNAL DE PHYSIQUE

For the yttrium laser

.am - 2 X 1

o - ~ s o for a

1

mm grating the minimum period that can be produced is 1 0 0 0 ~ . However for a high quality grating the fringe contrast must not vary by more than

5%

across the grating. This restriction can be expressed as1

:

This means that

a m

must be

less

than

5 X 1

o - ~ ⁱⁿ order to make reflection gratings holographically. The longitudinal coherence of the current soft x-ray lasers must

be

improved by two orders of magnitude.

PHOTOlONlZATlON PHYSICS

A

second application

is

in the field of laser physics.

It has

been proposed to use the

Se

neon-like laser as a pump for a laser kinectics experiment12. In this experiment the intense

206A

and

209A

lines from the Se laser are used to photoionize and excite the

2p63s

ground state of neutral Na to the 2$3s excited state of the Na+1 ion. With strong pumping a se!f-terminating laser a t

372,4

results on the 3s-2p trasition. This

type of laser has been dernostrated zt longer wavelengths13. The

photo-absorbtion cross-section peaks around

200A

making the S e laser a good pump candidate.

Fig.

4

shows the caicul~ted gsin length product for an assumed

175

ps

Se

laser pulse. Gain for a

1.0

mJ

^Se

laser input is shown. Gain-lengths of up to

15

for

50

p s

sre

possible. For a

1 mJ

input a 1.0 microjoule output results. The linewidth is narowed by a factor of

200.

TIME (psec)

Fig. 4

Time behsvior of the gain length product at

^372A

and the

^209A

input

Se

X-Ray laser (from ref.

12).

Such an experiment is planned. Verificstion of the laser kinectics with a rnonochrornatic pump will lead to the use of broadband pumps such as laser plasmas. Initial use of a broadband pump makes diagnosing the Na laser difficult since multiple levels will be pumped. This type of

experiment is a good example of the possible uses of current x-rzy lasers.

It takes advantage of the monochromatic, intense short pulse output. It is not dependent on the coherence properties. Appiicstions such as this utilizing the high brightness of x-ray lasers will be predominate until the coherence properties of x-ray lasers improve from multimode to single mode.

Contact microscopy offers the possibility of near term biological applications of x-ray lasers. In contact microscopy a sample to be analyzed is placed on a photoresist and exposed to incident x-rays. The resist is then developed and an opacity map of the sample is obtained. This opacity map on the photoresist is then scanned by an electron microscope.

Resist is viewed with electron microscope

m""

Photoresist

m

Developed Resist Fig. 5. Contact Microscopy

Interesting results have already been obtained by R. Fader and co-workers using plasma sources with 44A emission14. Resolutions of approximateiy 1 OOA have been acheived using PMMA as a resist1 5. Flash ima es have been obtained of live blood platelets using pulsed plasma sources18 ^AISO

quasi-three dimensional information is possible by using stereo pairs14.

Current x-ray sources such as laser plasmas and z-pinches are sufficiently bright for this type of microscopy and there is no need for coherence. The use of an x-ray laser for contact microscopy has only limited advantages over existing sources. The x-ray laser offers d

monochromatic output. By exposing a doped or stained sample to a single wavelength the distribution of the highly absorbing stain can be mapped yielding structural information. Such elemental mapping has been discussed previously for plasma and synchrotron sources 17,18,19

In addition to elemental mapping, the use of an x-ray laser would allow the maximum dosage required to destroy the cell's structure to be

C6-3 14 JOURNAL DE PHYSIQUE

determined with well characterized radiation. This information is vital if x-ray holography of living cells is to be achieved. The current damage limit is thought to be about 1 0 0 0 ~ / c m ~ (ref 20).

A possible arrangement for performing contact microscopy using a yttrium laser is shown in figure 6. A 0.1 mJ 155A yttrium laser is

focused by a multi-layer mirror with reflectivity 25% to a spot size of 200 microns. The mirror would not reflect any high energy background from the plasma of the x-ray laser. A boron nitride window is used to protect the biological specimen from the vacuum. The transmission of 300A of boron nitride at 155A is about 70%. The sample is placed behind the vacuum window on a PMMA resist. The exposure would be about 2000J/cm3. With a 5 mrad divergence laser and a 1 micron thick sample the resolution would be about 1000-2000A. Since the transmission of water and protein at 155A is extremely low the sample would have to be dehydrated and only the edges would be resolved with any contrast.

Yttrium X-Ray Laser

focus to 200 boron nitride window

Mu1 ti-layer Mirror

R= 25% ^@ 155 8,

micron spot

Figure 6. Contact microscopy with the yttrium laser.

X-Ray Holography

No paper on x-ray laser applications would be complete without a section on x-ray holography. This is the one application which most justifies the high cost and complexities of x-ray lasers. Standard biological techniques for obtaining information on the the structure of a cell involve the removal of water, staining, and thin slicing. These processes can significantly alter the the structure of the cell. An x-ray hologram could in principle reveal three dimensional information about the structure without resorting to slicing and if appropriate wavelengths are used the cell could be in water. The short pulse lengths of x-ray lasers would also provide time resolution allowing the dynamics of cell processes to be studied.

X-ray holograms have already been obtained. The earliest known x-ray hologram was obtained by Gunnar Kellstrom in 1 9 3 2 ~ ~ . Since then significant progress has been achieved both in experiments 22,23,24,25

and in theory26~27~28. However the sources used to date have had poor coherence. The number of photons per spatial mode is small and long exposure times have been required. The resolutions achieved have been about 1 micron. A high brightness single mode x-ray laser would eliminate the problems of vibration and alignment drift. The design of such x-ray lasers has been ~ o n s i d e r e d ~ ~ .

No matter what type of x-ray source is used there are some

fundamental limitations on the maximum resolution which can be achieved.

As pointed out by E.

i ill er^^

diffraction limited resolution is not

possible for three dimensional holograms. Consider an x-ray hologram of a cube. The number of spatial modes emerging from one face of the cube is equal to the number of diffraction limited resolvabie spots on this face.

The imaging of additional planes within the cube with diffraction limited resolution would require more spatial modes which are not available within the available surface area. As Spiller points out the imaging of

N

planes would require a reduction in resolution by

N~/*.

During the reconstruction process an additional problem occurrs. When viewing one reconstructed plane the fringe patterns from the other planes will appear as laser speckle in the plane being observed30. The correlation distance of this speckle is equal to the diffraction limited spot size and the contrast is equal to one. In order to reduce the contrast the resolution must be compromised by averaging over a large number of diffraction limited spot sizes. in order to avoid both mode and speckle limitations the object of the hologram should either have a low information content or lower resolution must be used.

The least difficult holography scheme is Gabor holography. In this holograph scheme the object is placed between the source and the detector3

Y

. Light diffracts from the object and interfers at the detector with light which misses the object and serves as the reference beam.

Figure 7 shows a schematic of Gabor holography. The source to detector distance is Zs. The object to detector distance is Zo and the object radius is ro.

Source

Object

Detector Figure 7. Gabor holography

C6-3 16 JOURNAL DE PHYSIQUE

The intensity on the detector can be shown to be given by:

l= A2/zS2

+

B ~ / z ~ ~

+

2AB/ZoZs cos@ where

G = Function(object coordinates, source coordinates, detector coordinates, wavelength)

e2= Ratio of scattering cross section to geometric cross section e = Source divergence

No= Number of photons into e

A = Wavelength.

E equals one for opaque structures and (,~2)* for semi-transparent objects. CCL is the opacity of the object.

It is useful to consider what holographic capabilites existing x-ray lasers have. Toward that end consider a design for holography of opaque objects (€=l) with a typical dimension of 5-1 0 microns. For design

purposes an yttrium laser at 155A will be used. In this wavelength regime the detector will have to be Kodak 101 film with 5-1 0 micron resolution.

Using the restrictions on resolution discussed in ref. 26 and requiring that the f~inge visibility be at least 0.3 results in the following dimensions:

%=l 0 mm

G=

5 mm.

For 5 micron resolution the hologram size will be about 120 microns in diameter. With a film sensitivity of 1 photon/micron2 (ref 32) 1

o4 -

¹

o5

coherent photons with a 6 mrad divergence from an 8 micron diameter source are required for a reasonable exposure. This would give

approximately 100-1 000 detected scattered photons for each 5 micron resolution element. To achieve a signal to noise ratio of 5 will require about 200 detected photons33. Current yttrium lasers have an output of approximately 1

o1

photons into about 5000 spatial modes. By spatial filtering it would be relatively easy to achieve 1

o5

coherent photons. With current x-ray lasers it should be possible to demonstrate the feasability of x-ray laser holography of simple opaque objects.

Extending Gabor holography to regimes of biological interest appears to be very difficult. in order to have maximum contrast between water and protein the laser waveleqgth needs to be between the carbon edge(44~) and the oxygen edge(23A). Laboratory lasers at these wavelenghts have not been demonstrated yet. The high opacity of protein ( 3fmicron) and the numerous complex structures within the cell means that multiple

scattering and a high photoabsorption loss will occurr within the cell. This will result in a loss of contrast between structures at different depths within the cell.

The mechanics of holography at 100-500

A

resolution will be quite difficult. Fringe visibility drops as the square of the resolution. This forces the object and source distance to go from 5-1 0 mm to 10-20 microns. In addition for a sphere of protein 500A in diameter c2 = 0.005.

The fringe visibility will drop by another factor of 10 due to the object

not being completely opaque. Alternative holography geometries need to be considered.

Damage limits of cells may prove-to be a severe limitation to holography. in order to detect a 500A diameter sphere at least 200 scattered photons must be detected. A protein sphere of this size has a scaitering efficiency of 5 X 1

o - ~ .

If all of the scattered photons are detected with 100% efficiency then 4 X 1

o4

photons are needed in the incoming beam to be scattered. This corresponds to about 6 X 108 photons/micron3. The structural damage limit of 1 0 0 0 ~ / c m ~ is equal to 2

X 1

o7

photons/micron3 for 40A photons. Damage limits due to short pulse x-ray exposure need to be determined perhaps in contact microscopy experiments in order to accurately quantify the damage issues. If damage proves to be a limitation, x-ray laser pulses with durations of 2-5 ps may be required27. In this case the hologram is made before the structure has time to fail. Techniques for shortening x-ray laser pulses have been

~ o n s i d e r e d ~ ~ .

The problems of holography of cells can be simplified with some of the developed techniques of biology35. Specific structures within the cell can have there opacity enhanced by attaching Fe or other materials to

anti-bodies which will attach themselves to specific structures. This

enhanced contrast would allow the wavelength of the laser to be increased up to about 60a. In addition the cell can be emptied of all but a few

specific structures significantly reducing the information content of the object cell, reducing multiple scattering effects, and simplifing the hologram analysis.

Conclusions

Current x-ray lasers offer the possibilities of evolutionary rather than revolutionary progress in a variety of applications fields. In

microfabrication sub-l

OOOA

transmission gratings appear to be feasable.

With substantially improved longitudinal coherence reflection gratings should also be possible. The use of x-ray lasers as high brightness monochromatic sources for physics experiments such as photoionization appears to be very promising. The photoionization experiment in sodium is the type of experiment that current x-ray lasers are best suited.

Coherence is not important and the current power levels (1 MW)

may

be sufficient. In the field of contact microscopy, x-ray lasers will probably not be able to compete with z-pinch and laser plasma sources except where monochromatic output is important as in elemental mapping.

C6-3 18 JOURNAL DE PHYSIQUE

However as the cost and ease of use of x-ray lasers improve this could change. What is important is that critical issues for holography such as damage and specimen handling can be developed in contact microscopy experiments. X-ray holography promises to be the most exciting application of x-ray lasers. For the first time it may be possible to image live cells and have biologically interesting resolution. Before this becomes a reality many critical issues must be solved such as damage, development of single mode lasers, and appropriate holography geometries.

Acknowlegdements

The author wishes to thank D. Matthews, M. Rosen, A. Szoke, M. Howells, E. Spiller, R. Fader, T. Barbee, A. Hawryluk and W. Silfvast for many useful discussions.

References

1) D. Matthews, these proceedings 2) S. Suckewer, these proceedings 3) P. Jaegle, these proceedings 4) M. Key, these proceedings

5) S. Jorna, X-Ray Laser Applications Study, Report PD-LJ-77-159, Physical Dynamics,lnc., La Jolla, Ca. 1977

6) D. Nagel, Potential Characteristics and Applications of X-Ray Lasers, NRL Memo 4465, Naval Research Lab, Wsshington, D.C.

7) Proceedings of the First Symposium on the Applications of Laboratory X-Ray Lasers, N. Ceglio ed., LLNL CONF-850293-ABSTS, LLNL, 1 985 8) A. Hawryluk, Transmission Diffraction Gratings for Soft X-Ray

Spectroscopy and Spatial Period Division, Ph.D. thesis,M.I.T. 1981 9) N. Ceglio, these proceedings

10) D.Nagel, Comparison of X-Ray Sources of Photoresists, in Ultrasoft X-Ray Microscopy: Its Application to Biological and Physical Sciences, Ann. N.Y. Acad. of Sci.,D. Parsons ed., vol342, 1980

11) G. Schmahl and D. Rudolph, Holographic Diffraction Gratings, in Progress in Optics, vol XIV, E. Wolf ed.,N. Holland pub. 1976

12) W. Silfvast et. al., in Short Wavelength Coherent Radiation: Generation and Applications, D. Attwood and J. Bokor eds. AIP, N. Y. 1986

13) R. Lacy, et. al. in Short Wavelength Coherent Radiation: Generation and Applications, D. Attwood and J. Bokor eds. AIP N.Y. 1.986

14) R. Fader and D. Sayre, Recent Developrrients in X-Ray Contact Microscopy, in Ultrasoft X-Ray Microscopy: Its Application to

Biological And Physical Sciences, D. Parsons ed. Annals N.Y. Acad. Sci., v01 342, N. Y. Acad. Sci. N.Y. 1980

15) J. McGowan et. al.,J. Cell Biology,80,732, March, 1979 1 G) R. Fader in ref. 7

17) E. Bigler et. al., Nuclear Instr. and Methods,208,387, 1983 18) l. Weinberg and A. Fisher, Appl. Phys. Lett.47,1116,1985 19) J. Kirz,

in

ref 14.

20) D. Sayre et. al. Science,196,1339, June 17, 1977 21) G. Kellstrom, Nova Acta Soc. Sci. Upsal. 8,61(1932) 22) S. Aoki and S. Kikuta, Jap. J. of Appl. Phys. 13,1385,1974 23) S. Kikuta et. al.,Optics Comm., 5,86,1972

24) B. Reuter and H. Mahr, J. Phys.,E9,746,1976

25) M. Howells et. al. , Experiments in X-Ray Holographic Microscopy Using Synchrotron Radiation, LBL Report LBL-21364, April 1986

26) A. Baez, J. Optical Soc.,12,756,1952

27) J. Solem, Los Alamos National Lab. Report LA-9508-MS1982 28) J. Solem and G. Chapline, Optical Eng. 23.1 94,1984

29) M. Rosen et. al., A Strategy For Achieving Spatially Coherent Output From Lab. X-ray Lasers, UCRL-94697,June 16, 1986, submitted to Comments in Plasma Physics

30) E. Spiller in ref. 7

31) D. Gabor,Proc. Royal Soc. of London,197,454,1949 32) B. Henke et. al.,J. Optical Soc. Am. B,1,829,1984

33) J. Goodman,Statistical Optics, John Wiley and Sons, New York,I 985 34) J. Trebes and D. Matthews, LLNL internal memo,1986

35) G . Albrecht-Buehler, Dept. of Cell Biology, Northwestern U.,personal communication