Application of a picosecond soft x-ray source to time-resolvedplasma dynamics

J. Workman,a) M. Nantel, A. Maksimchuk, and D. UmstadterCenter for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109-2099

~Received 27 June 1996; accepted for publication 12 November 1996!

We demonstrate the application of an ultrashort x-ray source as an external probe to measure plasmadynamics. The plasma is generated by a 100-fs Ti:sapphire laser focused onto thin metallic films.Time-resolved spectroscopy of the gold x-ray probe transmission through a perturbed 1000 Åaluminum film reveals redshifts of theL-shell photoabsorption edge. We show that the dynamicbehavior of this shift is consistent with the relaxation of the aluminum following the compressiongenerated by a shock wave traveling through the film. An analytic plasma model, with comparisonto a numerical hydrodynamics model, indicates compression up to 1.4 times solid density. ©1997American Institute of Physics.@S0003-6951~97!00403-8#

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A new class of x-ray sources has recently been induced with the aid of high-intensity subpicosecond lapulses, generated using the chirped pulse amplificatechnique.1,2 Virtually no hydrodynamic expansion occuduring a subpicosecond laser interaction with a solid dentarget: what remains is a solid density plasma with up to kelectron temperatures.2–4 These conditions allow x-ray emission from high-lying ionization states that are short livedue to the extremely large collision rates. Such x-ray sourare useful as diagnostic probes because of their high briness, made possible by their short time duration and hdensities.2,5 Interest in these sources is wide spread owingtheir potential applications, which include microlithographtime-resolved microscopy, diffraction, and spectroscostudies of transient physical and chemical phenomena.6,7

Development and characterization of such x-ray sourhave been ongoing. X-ray pulse duration measurementthe 1000 eV spectral region have been accomplished uvarious target materials as well as various measuremtechniques.2,5,8The importance of laser intensity and contrahas been demonstrated for the generation of the shopulses as well as the broadest bandwidth x-ray source4,5

Important applications of these sources, so far, have contrated on the atomic physics of the plasmas from which thrays originate.2,3

We present in this work what we believe to be the fiuse of such a source as a time-resolving probe in an apcation relevant to atomic, plasma, and high-energy-denmatter physics. Using the x-ray source as a backlight,adopt a pump–probe geometry to investigate the dynachanges in electronic structure of a thin metallic film as itperturbed by an ultrashort-laser pulse. Because the laseposits its energy in a skin depth of about 100 Å beforepansion occurs, very large amplitude shock waves cangenerated without material preheat commonly found in lopulse interactions.9 Under these novel conditions, predictionindicate the generation of up to gigabar pressure shwaves lasting picoseconds in duration.10,11 This raises thepossibility of studying-high-energy density matter, relevato inertial confinement fusion as well as astrophysical staof matter in small-scale laboratory experiments. In the p

a!Electronic mail: workman@lanl.gov

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time-resolved measurements ofK-edge shifts in plasmasdriven by nanosecond pulses have been used to infer cotions in highly compressed materials.12,13 In this study, weuse 100-fs laser pulses to impulsively drive shocks intosample, measuringL-edge shifts, which are more accessibto soft x-ray sources. The 10-Hz repetition rate allows statical averaging as well as clean signal-to-noise data. To gan understanding of the experimental results, we use anlytic plasma model to describe the relationship betweL-edge position and the associated plasma parameters.erence to hydrodynamics simulation corroborates our unstanding of the shock and relaxation dynamics.

The pump–probe measurements were made usin100-fs 10-Hz Ti:sapphire laser system operating at a walength of 790 nm. The 50 mJ laser pulses were sent tbeam splitter where 90% of the energy was reflected ontof/5 off-axis parabola, used to focus the light to an intensity231017 W/cm2 onto a solid gold target. The broadbanultrashort-pulse x-ray point source generated from this inaction was refocused to a sample at the opposite end oexperimental chamber using a section of a grazing incideellipsoidal optic; a central beam block eliminated any pertbations to the sample that might arise from hot electronshard x rays. The characteristics of the x-ray probe are shoin Fig. 1. Broadband emission has previously been measfrom 20 to 200 Å,4,5 however, here we focus on the emissioin the range of 100–200 Å. The decrease in spectral intenin Fig. 1 is due to the spectral response of the spectromgrating and detectors. The pulse duration was limited tops due to intensity contrast limitations of the 790-nm lalight. While the pulse duration was measured with a jittefree streak camera,14 the time-integrated spectrum in Fig.was obtained with the microchannel plate~MCP! used forthe pump–probe measurements. A calibrated x-ray diowith filters transmitting photon energies at 100650 Å, mea-sured integrated soft x-ray energies of 10–20mJ at thesource. Only a few tenths of a percent of this energy wcollected by the ellipsoid, giving a total x-ray energy of aproximately 50 nJ at the sample plane. The remaining 5of laser light was sent into an optical delay line and thfocused with anf /10 lens onto the thin metallic sample to aintensity of 131014 W/cm2. The sample plane was coincdent with the slit of a variable spaced-grating grazing in

/97/70(3)/312/3/$10.00 © 1997 American Institute of Physicsct¬to¬AIP¬license¬or¬copyright,¬see¬http://ojps.aip.org/aplo/aplcr.jsp

FIG. 1. Soft x-ray source generated by a 100-fs laser interaction with a solid gold target.~a! Temporally and spectrally resolved images of the source.~b!Temporal lineout near 170 Å.~c! The spectrum was measured separately with a microchannel plate.

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dence imaging soft x-ray spectrometer. The MCP couplethe end of the soft x-ray spectrometer enabled us to recspectrally and spatially resolved x-ray absorption throughperturbed material at various delay times with respect tolaser pump. The sample consisted of 1000 Å of alumindeposited onto 2000 Å of parylene-n (C–Hn) as a rear-sidesupporting structure. Comparison of absorption spethrough cold aluminum to that of the perturbed aluminuwas possible for each data point because the laser focu125mm was smaller than the 300–400mm x-ray probe fo-cus.

The absorption feature used for analysis of the aluminsample was theLII ,III photoabsorption edge at 72.78 e~170.04 Å!.15 The position of the edge gives a direct measof the ionization potential and, thus, the electronic structof the probed sample. The energy of the edge will changa function of material density, temperature, and ionizatstate.12,13The measured shift in theL II,III edge is recorded aa function of probe delay in Fig. 2 for a pump–laser intensof 131014 W/cm2.The early times~t,0! correspond toprobing the unperturbed sample with the soft x-ray soubefore the laser arrives. We then see a maximum redshi1.660.4 Å, which decreases at later times as the samdecompresses. The decompression occurs within appmately 50 ps, consistent with sound speed time scthrough a cold 1000 Å aluminum sample. Error bars indicthe time resolution of the 20-ps x-ray probe on the time aand the averaging of several data points on the shift aEach data point consisted of 10–20 laser shots.

Hydrodynamic calculations of the 100-fs laser generashock waves indicate that the shock will be generatedpicoseconds and will traverse the 1000 Å material on a tscale of the order 5–10 ps.10,16,17This implies that our timeresolution is ideal for measuring the dynamics of the relation of the material after the shock, but not yet sufficientresolve the dynamics of the shock wave itself.

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Analysis of the edge shift must address three physprocesses in a purely plasma state model. These includeization blueshifts, redshifts from continuum lowering,well as edge shifts due to changes in the chemical poten~and, thus, the Fermi energy!. Ionization blueshifts,DEion ,are calculated using Hartree–Fock theory while the StewaPyatt formalism is used to calculate redshifting due to ctinuum lowering,DEcl .

18 The effects of electron degeneracon the chemical potential,DEdeg,are determined by an expansion around the Fermi energy.19 These three contributionsum together to give a total shift,DEtot5DEion1DEcl

1DEdeg.12 Of course, in our geometry an absorption me

surement integrates the contributions from each region ofsample, which can be roughly divided into a higtemperature low-density blow-off region, a high-density lotemperature compressed region, and the undisturbedmaterial. High-temperature regions contribute ionizati

FIG. 2. Redshift of the aluminumI II,III edge as a function of x-ray probedelay time.

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blueshifts that are masked by redshifts. We, therefore, prthe higher-density regions at any given time. In fact, wepect the measured shift to be a weighted average, resufrom contributions from different positions in the highdensity region.

Using the above plasma model, with Saha-based scafor ionization as a function of temperature, we can plotlocus of points representing the maximum observed eshift as a function of temperature and ion density. Thisrameter space is indicted in Fig. 3. The experimental errothe shift measurement is included in the width of the curThis plot demonstrates the need for a measurement oftemperature to accurately determine the density, and hethe compression in our sample. Without such a complimtary measurement, detailed hydrodynamic informationnecessary to isolate a single set of parameters describinplasma. With the use of the detailed 1.5-D hydrocodeCHI-

VAS, we find a temperature and maximum compressionfall within our experimental boundaries.17 The numerical re-sults show consistency with our analytic approach and incate a maximum compression of 1.4 times solid densitytemperature of 3.9 eV. Analysis is ongoing.

We have successfully demonstrated the utility of theultrashort-pulse x-ray sources in an application that encpasses atomic physics, plasma physics, and high-energysity matter physics. Our preliminary analysis is consistwith an analytic model as well as with numerical simulatio

FIG. 3. Temperature and density parameter space spanned by the maxobserved edge shift~solid line! and the result of a 1.5-D hydrodynamisimulation~square box!. Solid density for aluminum is 631022 cm23.

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Further analysis should consider solid-state effects due tolow temperatures and high densities. Also, it is possiblereduce the time duration of the source to below 5 ps, whwould allow us to resolve clearly the propagation of tshock wave in the sample. We expect that these short-px-ray probes will have a significant contribution to the uderstanding of ultrafast dynamics of this and other physand chemical processes.

This work was supported by NSF Grant No. STC PH8920108, LLNL DOE Contract No. W-7405-ENG-48, anby the FCAR fund. The authors would like to thank GMourou, J.-C. Kieffer, and J.-C. Gauthier for useful discusions and A. Decoster for the use of the codeCHIVAS, andGabor Keiesar for integrated x-ray energy measuremeThe authors also thank Spectrogon for the compressor gings.

1G. Mourou and D. Umstadter, Phys. Fluids B4, 2315~1992!.2J. C. Kieffer, M. Chaker, J. P. Matte, H. Pepin, C. Y. Cote, Y. BeaudoT. W. Johnston, C. Y. Chien, S. Coe, G. Mourou, and O. Peyrusse, PFluids B5, 2676~1993!.

3P. Audebert, J. P. Geindre, A. Rouse, F. Fallies, J. C. Gauthier, A. Myrowicz, G. Grillon, and A. Antonetti, J. Phys. B27, 3303~1994!.

4M. M. Murnane, H. C. Kapteyn, M. D. Rosen, and R. W. Falcone, Scie251, 531 ~1991!.

5J. Workman, A. Maksimchuk, X. Liu, U. Ellenberger, J. S. Coe, C.Chien, and D. Umstadter, Phys. Rev. Lett.75, 2324~1995!.

6OSA Proceedings on Short Wavelength Coherent Radiation: Generaand Applications, 1991, edited by P. Bucksbaum and N. Ceglio~OpticalSociety of America, Washington, DC, 1991!.

7C. P. Barty, F. Raksi, C. Rose-Petruck, K. J. Schafer, K. R. Wilson, V.Yakovlev, K. Yamakawa, Z. Jiang, A. Ikhlef, C. Y. Cote, and J. C. Kiefer, in Proceeding of the SPIE-The International Society for Optical Egineering2521, 246 ~1995!.

8M. H. Sher, U. Mohideen, H. W. K. Tom, O. R. Wood II, G. D. AumillerR. R. Freeman, and T. J. McIlrath, Opt. Lett.18, 646 ~1993!.

9Ya. B. Zel’dovich and Yu. P. Raizer,Physics of Shock Waves and HighTemperature Hydrodynamic Phenomena~Academic, New York, 1967!.

10V. E. Gusev, Phys. Vibr.57, 1 ~1993!.11A. Ng, A. Forsman, and P. Celliers, Phys. Rev. E51, R5208~1995!.12D. K. Bradley, J. Kilkenny, S. Rose, and J. D. Hares, Phys. Rev. Lett.59,2995 ~1987!.

13L. DaSilva, A. Ng, B. K. Godwal, G. Chiu, and F. Cottet, Phys. Rev. Le62, 1623~1989!.

14A. Maksimchuk, M. Kim, J. Workman, G. Korn, J. Squire, D. Du, DUmstadter, G. Mourou, and M. Bouvier, Rev. Sci. Instrum.67, 697~1996!.

15B. L. Henke, P. Lee, T. J. Tanaka, R. L. Shimabukuro, and B.Fujikawa, J. Opt. Soc. Am. B3, 1540~1986!.

16J. C. Gauthier~private communication!.17S. Jacquemot and A. Decoster~private communication!.18J. C. Stewart and K. D. Pyatt, Jr., Apl. J.144, 1203~1966!.19E. Kiess, Am. J. Phys.55, 1006~1987!.

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