Ultrafast time-resolved X-ray diffraction

K. Sokolowski-Tinten1. C. Blome1, J. Blums1, A. Cavalleri2, C. Dietrich1,A. Tarasevitch1, D. von der Linde1

1 Institute for Laser- and Plasmaphysics, University of Essen, 45117 Essen, Germany2Material Science Division, Lawrence Berekely Nat. Lab., Berkeley, CA 94720, USA

Abstract. Femtosecond laser-generated plasmas emit ultrashort X-ray pulses in the multi-keVrange, which allow the extension of X-ray spectroscopy into the litrafast time-domain. Wereport here on the generation of such short X-ray pulses and their application for time-resolveddiffraction as a means to directly study ultrafast structural dynamics in laser-excited solids.

INTRODUCTION

Most of our knowledge on the atomic structure of matter is due to X-rayspectroscopies, because the short wavelength of X-rays gives direct access to thespatial scale of atomic arrangements. However, standard X-ray experiments (using forexample synchrotron radiation) provide essentially a static picture because the timeresolution of such experiments is rather limited. Most fundamental processes in nature,such as chemical reactions and phase transitions involve dynamic changes of theatomic arrangements, which occur typically on time-scales comparable with thenatural oscillation periods of atoms and molecules, that is femtoseconds topicoseconds. Therefore, great efforts have been made during the past few years toextend X-ray spectroscopy into the ultrafast time-domain.

One of the most successful approaches has been made possible by the recentprogress in ultrafast laser technology, namely chirped pulse amplification. Multi-Terawatt, ultrafast table-top scale laser systems are now available in an increasingnumber of laboratories all around the world. When focused onto solid targets, suchintense light pulses generate high-density plasmas emitting short bursts of hard X-raysup to the MeV region [1]. This new kind of high brightness ultrashort pulsed X-raysources allow an extension of the established time-resolved techniques of ultrafastoptical spectroscopy into the hard X-ray range, thus providing simultaneously atomicscale temporal and spatial resolution.

This contribution discusses the generation of femtosecond, multi-keV X-ray pulsesand their application for time-resolved diffraction as a means to directly study ultrafaststructural dynamics in laser-excited solids.

CP634, Science of Superstrong Field Inter actions, edited by K. Nakajima and M. Deguchi© 2002 American Institute of Physics 0-7354-0089-X/02/$ 19.00

11

SHORT X-RAY PULSES FROM LASER-PRODUCED PLASMAS

As has been first demonstrated by Ktihlke et al. [2], the high-density micro-plasmas, generated through the interaction of very intense light pulses with solidtargets, represent an efficient source of hard X-rays. Particularly interesting is thestrong characteristic line emission (for example Ka-emission) of these plasmas. Thisline radiation is believed to result from the interaction of energetic electrons with thenon-excited target material underneath the thin plasma layer [3], a process quitesimilar to an ordinary X-ray tube. Because those energetic electrons are produced bydirect acceleration in the strong laser field, a very short duration of the X-ray pulses,of the order of the laser pulse duration, can be expected.

Femtosecond laser-plasma-driven X-ray sources are quite attractive, in particularfor small, university-scale laboratories, because they combine simplicity with lowcost, as compared to other, accelerator-based approaches [4]. A convenientimplementation of such a source uses a thin metallic wire, which is continuouslymoved through the focus of the laser, as a target [5]. The left part of Fig. 1 shows aphotograph of the wire-target assembly used at our set-up for time-resolveddiffraction.

4 6 8Photon Energy [keV]

2.746 2.748 2.75 2.752 2.754

FIGURE 1. Left: photograph of the Titanium wire-target assembly. Right: X-ray emission spectra ofthe laser excited Titanium target (top: overview spectrum obtained by photon counting/pulse heightanalysis with a X-ray CCD; bottom: higher resolution spectrum of the spin-orbit split K^-lines obtainedwith a crystal spectrometer).

It should be kept in mind that due strong laser-induced ablation the target has to bemoved between two consecutive laser pulses in order to provide a fresh surface areafor each individual pulse. As a consequence the wire-target offers two mainadvantages compared to other target schemes: (i) it allows a very compact design,which simplifies shielding issues, and (ii) virtually infinite measurement times arepossible simply by providing a sufficiently long spool of wire. Our source, which runsat 10 Hz repetition rate, can operate for nearly 70 h with a 500 m long spool of wireand a pulling velocity of typically 200 urn per pulse!

12

We have chosen titanium as target material. The Ti-Ka-emission (4.51 keV) of oursource allows Bragg diffraction on a wide variety of materials and overcomes theinherent limitations with respect to lattice parameters and/or diffraction orders ofsources operating at longer wavelengths. The left part of Fig. 1 displays spectra of oursource in the keV-range. The top viewgraph represents an overview spectrum obtainedby operating an X-ray CCD-detector (thinned, back-illuminated) in the single photoncounting mode with pulse height analysis (to assure single-photon detection we had toreduce the laser energy). The dominant features are the titanium Ka- and Kp-lines at4.51 keV and 4.93 keV, respectively. The bottom viewgraph shows a high resolutionspectrum of the spin-orbit split Kai and Ka2-lines, which was obtained with a crystalspectrometer.

As has been mentioned above, the X-ray-tube-like Ka-emission from laser-produced plasmas is caused by fast electrons accelerated in the intense laser field. Theefficiency of this process depends on the energy of the electrons and is thereforestrongly influenced by the details of the laser-plasma interaction. For example, it iscommon experience [6,7] and also supported by theoretical calculations [8] that forgiven laser- and material parameters the highest laser intensities not necessarily lead tohighest Ka-yield. This is demonstrated by the data shown in the left part of Fig. 2,where we measured the relative yield of our titanium Ka-source (dots) as a function ofthe position of the focusing optics (f = 150 mm) relative to the target.

-2.0 15-1.5 -1.0 -0.5 0.0 0.5 -5 0 5 10Lens Position [mm] Delay Time [ps]

FIGURE 2. Left: normalized X-ray signal as a function of the lens position (relative to the focus; dots:Ti- Ka-emission; squares: hard background). Right: Ti-Ka-yield in the two-pulse excitation scheme as afunction of the delay time between the plasma generating pre-pulse and the main pulse.

The Ka-signal has been normalized to the yield with the titanium target exactly inthe focal plane of the optics (zero position), corresponding to the highest laserintensity. The highest Ka-flux is observed 0.5 mm away from the focal point, whichcorresponds to approximately two times the Rayleigh length. At the same time thebackground signal detected by the CCD (squares; arbitrarily normalized to fit into theplot window of Fig. 2), which is due to hard X-rays and secondary radiation, issignificantly reduced. Therefore, optimizing the focusing conditions does not onlyincrease the Ka-flux, but allows at the same time a substantial improvement of thesignal-to-noise ratio.

A second way to tailor the plasma properties in order generate exactly thoseelectrons which have the highest efficiency in the production of K-shell holes is theuse of a double-pulse excitation scheme. This scheme separates the steps of plasma

13

production and electron acceleration. A first, medium intensity laser pulse is used forplasma generation. After a certain delay, the main high intensity pulse interacts withthe pre-formed plasma to accelerate the electrons. The right part of Fig. 2 shows theKa-emission as a function of time-delay between the pre-pulse (I « 1015 W/cm2) andthe main pulse (I« 5xl016 W/cm2), which produces the fast electrons.

In agreement with results obtained at a silicon Ka-source at 1.8 keV [9] we observean enhancement of almost an order of magnitude at a delay time of just a fewpicoseconds. In [9] this enhancement has been attributed to resonance absorption inthe slightly expanded plasma (scale length « K), which leads to a very efficientproduction of electrons in the optimum energy range (note that the cross section forionization of K-shell electrons peaks for most materials at approximately 3 - 4 timesthe Ka-energy).

TIME-RESOLVED X-RAY DIFFRACTION

It is an important advantage of laser-plasma driven X-ray sources that the laserdriving X-ray generation provides at the same time an absolutely synchronous sourcefor optical excitation. Therefore, the basic experimental concept of ultrafast time-resolved measurements, the so-called pump-probe scheme, which is well establishedin the optical domain, can be directly extended to the hard X-ray regime: an opticalpump-pulse is used for excitation while an ultrashort X-ray pulse serves as a probe tomonitor the transient dynamics induced by the pump.

Because laser-plasma driven X-ray sources are essentially monochromatic, they areparticularly suited for time-resolved Bragg-diffraction experiments. A schematic ofsuch an experiment is shown in Fig. 3.

laser pulse to generate a plasma

sample

Bragg diffracted X-rays —^s^ variable delay

rocking curve -

X-ray CCD camera

FIGURE 3. Schematic of an optical pump X-ray probe experiment for time-resolved diffraction.

In our set-up near-infrared laser pulses from a 10 Hz amplified Ti:sapphire lasersystem with pulse energies of about 150 mJ and a pulse duration of 120 fs are focusedonto the surface of the moving titanium wire. A small fraction is split off the mainlaser pulse and (after passing through an optical delay line) is used for sampleexcitation.

14

The Ka-radiation from the plasma is emitted incoherently into the full solid angle.Efficient use of the produced X-rays requires, therefore, re-collection and focusing ofthe X-rays onto the surface of the sample under investigation with a spot sizecomparable or smaller than the area excited by the optical pump. Focusing of the Ka-radiation is accomplished with the help of a toroidally bent crystal. As has beendiscussed in detail by Misalla et al. [10], monochromatic point-to-point imaging of theplasma source can be achieved in this way. The experimental geometry is shown in theleft part of Fig. 4.

toroidally bent crystal:Si (311), 5x15mm2

Rowland circle

topography

FIGURE 4. Focusing of the keV-plasma emission. Left: experimental geometry. Right top:topography of the bent crystal mirror; Right bottom: 1:1 image of the plasma source (FWHM: 85 |nn).

For a given crystallographic orientation of the mirror the horizontal and verticalbending radii are determined by the imaging geometry on the Rowland-circle and therequirement that the Bragg-condition has to be fulfilled for the chosen wavelength.

The results of an imaging experiment at our Ti-Ka-source using a bent Siliconcrystal with (311) surface orientation are shown in Fig. 4. In the lower right the spatialdistribution of the focused X-rays, as detected with our X-ray CCD (pixel size 27 um),is displayed. The spot is nearly circular and exhibits a FWHM of approximately 85jLim. This focus typically contains 30.000 detected Ka-photom per pulse.

The upper right part of Fig. 4 shows the local reflection characteristic of the mirror,which is obtained by placing the CCD-detector away from the image plane close to themirror. This reflection characteristic provides direct information on the surfacetopography of the mirror because of its very narrow rocking curve (angulardependence of the diffraction efficiency for a fixed X-ray wavelength). Any spatialdeformation of the mirror surface leads to a deviation of the local incidence anglefrom the Bragg-angle and thus to a reduction of the diffraction efficiency. The Silicon-crystal used in our set-up shows an excellent topography with a peak-to-peak non-uniformity of the reflectivity of only 10 %.

In the diffraction experiment the crystalline sample is placed in the image planeunder the appropriate Bragg-angle. The optical pump spot and the X-ray focus mustoverlap spatially on the surface of the sample, and the diffracted X-rays are recordedby an X-ray sensitive area detector. Please note, that the X-rays, incident from the X-ray mirror onto the sample, cover an angular range much larger than the width of the

15

rocking curve^ which can be, therefore, recorded in a single exposure without anyangle-scanning of the sample.

A major problem in optical pump, X-ray probe diffraction experiments results fromthe different penetration depths of optical radiation and multi-keV X-rays. Insemiconductors and metals the optical absorption depth is usually below onemicrometer, becoming even shorter at high levels of excitation due to strong nonlinearcontributions to the overall absorption. Therefore, only a very thin layer of the order of100 nm near the surface can be optically excited. X-rays, on the other hand, havetypically a penetration depth of a few microns or more.

To overcome this mismatch between pump- and probe depths we used thincrystalline films grown on silicon substrates. Using surfactant mediated growthtechniques [11] highly perfect single-crystalline films can be obtained on large sizesubstrate (4" wafers). Most important, these films are stress free and grow with theirnatural lattice constant. Therefore, it is easily possible in a diffraction experiment toseparate the contributions of the thin film from the bulk substrate if the difference inthe lattice constants and the corresponding difference in the Bragg-angles issufficiently large.

This is the case for the thin Germanium films grown on Silicon, which we used inthe diffraction experiment discussed in the next chapter. An example of the diffractionpattern of such a Ge/Si-heterostructure is shown in Fig. 5.

170 nm Ge on Si

•2 0.10

m(O

0.05

0.0024 25 26

Diffraction Angle [°]FIGURE 5. Bragg-diffraction profile (rocking curve, Ill-reflection) from a 170 nm thick, single-crystalline Germanium film grown on Silicon using the laser-driven Ti-Ka X-ray source. Insert: imagedirectly recorded on the X-ray CCD-detector.

The data shown in Fig. 5 represent the angular diffraction profile (rocking curve) ofthe 111-Bragg reflection of a 170 nm thick Germanium film on Silicon obtained froma two-minutes integration on the CCD (the insert shows the CCD-image). The Bragg-peaks of the Germanium film (QB = 24.88°) and the Silicon substrate (0B = 26°) arewell separated. Note, that the significantly larger width of the Germanium-peak isrelated to the small thickness of the overlayer and not to structural imperfections.

X-RAY PULSE DURATION

As in an ordinary, all-optical pump/probe experiment the temporal resolution of anoptical pump/X-ray probe experiment is determined by the duration of the probe pulse.

16

While there are reliable and very sophisticated methods to accurately measure thepulse shape of optical pulses (for example FROG), comparable methods for the multi-keV X-ray range are not yet available.

Nevertheless some information on the X-ray pulse duration can be obtained from atime-resolved diffraction experiment. In the most general case the observed transientchanges of the diffraction signal represent the convolution of the actual materialresponse with the X-ray probe pulse. If the material response is sufficiently fast themeasured transients directly provide the X-ray pulse duration.

A process, which should give such a very fast material response, is femtosecondlaser-induced melting of semiconductors. It has been investigated for nearly 20 yearswith ultrafast optical techniques [12-14] and there is strong evidence that a transitionfrom the ordered solid phase to the disordered liquid state is possible within just a fewhundred femtoseconds. Therefore, this process has become some kind of a test-casefor ultrafast X-ray techniques [15-19], in particular diffraction, and a number of recentstudies [18-20] have clearly demonstrated sub-picosecond X-ray response.

Results from our own work [19] are depicted in the left part of Fig. 6. It shows theangular integrated X-ray reflectivity of the Ill-reflection of a 170 nm Germaniumfilm as function of pump-probe time delay for two different pump fluences.

1.0>> 0.9

0.8oi(T

I0.6

0.2J/cnTo 0.4 J/cm2

0 1Delay Time [ps]

-0.4 0.0 0.4Delay Time [ps]

FIGURE 6. Left: angular integrated X-ray reflectivity of a 170 nm Germanium film (111-reflection) asa function of the time delay between the optical pump pulse and the X-ray probe for two different pumpfluences. Right: fits of the measured diffraction data (0.2 J/cm2) assuming a step-like material responseand Gaussian-shaped X-ray pulses.

The most prominent feature is the rapid initial drop of the integrated diffractionefficiency within a few hundred femtoseconds, a clear indication for a very fast loss oforder over a depth of about 40 nm. Here we do not discuss further the details ofultrafast melting and the interesting material behavior observed on longer time-scales(for this the reader is referred to [19]), but want to focus on the X-ray pulse duration.

To get an estimate on the pulse duration we assume the limiting case of acompletely instantaneous, step-like material response (which obviously over-estimatesthe X-ray pulse duration!). In the right part of Fig. 6 the initial decrease of thediffraction signal measured for a pump fluence of 0.2 J/cm2 has been fitted by aconvolution of a step-like material response with Gaussian-shaped X-ray pulses ofdifferent duration. Satisfactory fits are obtained with X-ray pulse widths between 250fs and 350 fs. As a result we can give an upper limit for the X-ray pulse duration ofabout (300 ± 50) fs. To our knowledge, these are the shortest X-ray pulses in themulti-keV range reported so far!

17

ACKNOWLEDGMENTS

The authors are indebted to I. Uschmann and E. Forster for providing the X-ray-mirror, and to M. Kammler and M. Horn-von-Hoegen for preparation of theheterostructure samples. Financial support by the Deutsche Forschungsgemeinschaft,the European Community, the German Academic Exchange Service, and the NationalScience Foundation is gratefully acknowledged.

REFERENCES

1. Kmetec, ID., et al., Phys. Rev. Letters 68, 1527 (1992).2. Kiihlke, D., Herpers, U., and von der Linde, D., Appl. Phys. Letters 50, 1785 (1987).3. Rousse, A., Audebert, P., Geindre, J.P., Failles, F., Gauthier, J.C., Mysyrowicz, A., Grillon, A.G.,

and Antonetti, A, Phys. Rev. E 50, 2200 (1994).4. Schoenlein, R.W., Leemans, W.P., Chin, A.H., Volfbeyn, P., Glover, I.E., Balling, P., Zolotorev,

M., Kirn, K.-J., Chattopadhyay, S., and Shank, C.V., Science 274, 236 (1996); Schoenlein, R.W.,Chattopadhyay, S., Chong, H.H.W., Glover, I.E., Heimann, P.A., Shank, C.V., Zholents, A.A., andZolotorev, M., Science 287, 2237 (2000).

5. Rose-Petruck, C., Jimenez, R., Guo, T., Cavalleri, A., Siders, C.W., Raksi, F., Squier, J.A., Walker,B.C., Wilson, K.R., andBarty, C.P.J., Nature 398, 310 (1999).

6. Eder, D.C., Appl. Phys. A. 70, 211 (2000)7. Fill, E., Bayerl, J, and Tommasini, R., Rev. Sci. Instr. 73,2190 (2002).8. Reich, C., Gibbon, P., Uschmann, I., and Forster, E., Phys. Rev. Letters 84, 4846 (2000).9. Bastiani, S., Rousse, A., Geindre, J.P., Audebert, P., Quoix, C., Harminaux, G., Antonetti, A, and

Gauthier, J.C., Phys. Rev. E 56,7179 (1997).10. Missalla, T., Uschmann, L, Forster, E., Jenke, G., and von der Linde, D., Rev. Sci. Instr. 70, 1288

(2000).11. Horn-von-Hoegen, M.,AppL Phys. A 59, 503 (1994).12. Shank, C.V., Yen, R., and Hirlimann, C., Phys. Rev. Letters 50,454 (1993).13. Saeta, P., Wang, J.K., Siegal, Y.N., Bloembergen, N., and Mazur, E., Phys. Rev. Letters 67, 1023

(1991).14. Sokolowski-Tinten, K., Bialkowski, J., and von der Linde, D., Phys. Rev. B 51, 14186 (1995).15. Chin, A.H., Schoenlein, R.W., Glover, T.E., Balling, P., Leemans, W.P., and Shank, C.V., Phys.

Rev. Letters 83,336 (1999).16. Siders, C.W., Cavalleri, A., Sokolowski-Tinten, K., Toth, C., Guo, T., Kammler, M., Horn-von-

Hoegen, M., Wilson, K.R., von der Linde, D., and Barry, C.P. J., Science 286, 1340 (1999).17. Linderiberg, A.M., Kang, I., Johnson, S.L., Missalla, T., Heimann, P.A., Chnag, Z., Larsson, J.,

Bucksbaum, P.H., Kapteyn, H.C., Padmore, H.A., Lee, R.W., Wark, J.S., and Falcone, R.W., Phys.Rev. Letters 84, 111(2000).

18. Rousse, A., Rischel, C., Fourmaux, S., Uschmann, L, Sebban, S., Grillon, G., Balcou, P., Forster, E.,Geindre, J.P., Audebert, P., Gauthier, J.C., and Hulin, D., Nature 410, 65 (2001).

19. Sokolowski-Tinten, K., Blome, C., Dietrich, C., Tarasevitch, A., von der Linde, D., Horn-von-Hoegen, M., Cavalleri, A., Squier, J.A., and Kammler, M., Phys. Rev. Letters 87,225701 (2001).

20.Feurer, T., Morak, A., Uschmann, I., Ziener, C., Schwoerer, H., Reich, C., Gibbon, P., Forster, E.,Sauerbrey, R., Ortner, K., and Becker, C.R., Phys. Rev. E 65, 16412 (2002).

18