X-ray sources based on subpicosecond-laser-produced plasmas

JOURNAL OF X-RAYSCIENCEAND TECHNOLOGY 4, 312-322 (1994)

X-Ray Sources Based on Subpicosecond-Laser-Produced Plasmas J. C. KIEFFERAND M. CHAKER INRS-Energie et MatOriaux, C.P. 1020, 1650 Mont& Ste-Julie, Varennes, QuObec, Canada J3X 1S2 Received December 22, 1993 The production of an efficient user friendly ultrafast x-ray source requires an understanding of the role of the various factors which aflbct the x-ray emission. Here we examine several issues which control the source brightness and the pulse duration. Picosecond timeresolved, high spectral resolution spectroscopy is used to study plasmas produced by a subpicosecond laser pulse with intensity between 1016 W/cm 2 and 5 × 10 TM W/cm 2. © 1994AcademicPress,Inc. 1. INTRODUCTION

During the past 10 years, nanosecond-laser-produced plasmas have been extensively studied for, among other aspects, their potential use as efficient x-ray sources (1, 2). The very high conversion efficiencies observed in the keV and in the sub-keV range (up to a few %/sr) in several laboratories have made compact laser plasma sources an attractive alternative to costly electron storage rings for x-ray lithography either in proximity printing (3-5) or projection approaches (6-8). The recent advent of compact high intensity ultrashort (< 1 ps) lasers has opened up new horizons in laser-matter interaction studies (9, 10) and offers the unique opportunity to develop high brightness ultrafast x-ray sources (11). The availability of such sources may have significant impacts in many different areas such as time-resolved x-ray diffraction, probing of the dynamics of molecular reactions, and pumping of x-ray laser plasmas. The production of an efficient user-friendly source requires an understanding of the role of the various factors, as the laser irradiance, the pulse duration, the prepulse level, the laser polarization, and the target material which affect the x-ray emission (12). Recent work on ultrashort x-ray sources have dealt mostly with continuum emission (13), line emission from the hot ultrashort plasma (14-18) and K~ radiation generated by hot electrons (16, 19). The production of an efficient ultrashort x-ray source is a very difficult task, and the improvement of the conversion efficiency has been of particular concern. Relatively interesting values of the conversion efficiency were reported for the case of a short pulse interacting with a preformed plasma created by a prepulse or by a substantial level of amplified spontaneous emission (ASE). Hard x-ray radiation (20 keV to 1 MeV) has been recently generated by focusing a 120-fs 0.8-#rn laser (with a peak-toprepulse intensity ratio of 10 6) a t very high intensity on solid Ta targets (13). Continuum spectral distribution was observed to fall as 1/hv and a maximum conversion efficiency of 0.3% was measured in radiation above 20 keV. Line emissions from various ion0895-3996/94 $6.00 Copyright© 1994by AcademicPress,Inc. All rightsof reproductionin any formreserved.

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ization states generated in the hot plasma have been also extensively studied. X-ray emission in the keV range having a brightness of 102° photons/s mm 2 eV into 27r steradian was measured with a 1-ps 0.58-#m laser incident at moderate intensity on a low temperature preformed plasma (14). The scaling of A1 He-like and H-like emissions with ASE prelase irradiance and main pulse intensity has been studied with subpicosecond short-wavelength lasers (20, 21) and a conversion efficiency up to 0.4% was measured in a single line (L~ line) at 5 × 1018 W/cm 2 with a high prepulse level (21). Furthermore, time-integrated conversion efficiencies of 0.5% in 27r sr and 5% in 2~r sr were measured for respectively the keV (0.75-2 keV) and the sub-keV (0.1-0.75 keV) ranges with a 1-ps 1-~m laser pulse at 1015W/cm 2 surimposed on a 30-ps prepulse at 1012 W/cm 2 (15). In addition, the presence of short prepulse dramatically changes the soft x-ray line spectra, and lines at shorter wavelength become much stronger when the intensity of the prepulse is increased or when the time separation between main pulse (500 fs) and prepulse is increased (22). However, picosecond x-ray streak camera measurements (11, 18, 23) have shown that the x-ray pulse duration is much longer when there is a preplasma than in the case of a high contrast laser-matter interaction (prepulse free). Recent work with a 1ps laser without prepulse suggests that K~ emission may be an interesting short x-ray source (19), this radiation being produced by hot electrons generated during the laser pulse. In these experiments the conversion efficiencies of laser energy into Si K~ and A1 H% were measured to be 3 N 10 -4 and 8 N 10 -4 % / s r , respectively, giving around 108 photons/sr with emission duration of a few ps. Further investigations are required in order to explain the observed duration of these emissions (19, 24). We performed several sets of experiments to address the various issues just discussed, and we present here some of our most recent results. 2. SHORT-WAVELENGTH RADIATION FROM ULTRASHORT-LASER PLASMAS

The experiments were carried out with a table top terawatt (T 3) laser system which employs the chirped pulse amplification (CPA) technique (25, 26). This technique which requires a lot of pulse manipulations gives the possibility to generate ultraclean, very short, and ultra-intense laser pulse (27). The laser was delivering up to 2 J in a 500-fs pulse at the wavelength of 1.053 ~zm. The intensity peak-to-background contrast ratio was measured to be 5 × 105 at 1.053 t~m. The contrast ratio has been further approximately squared by frequency doubling (0.53 t~m) the laser pulse using a KDP crystal. The beam was focused either in normal incidence with an f/6 spherical lens or with a 45 ° incidence angle off-axis f/3 parabola and the sizes of the focal spots were, respectively, 80 and 10 t~m, giving laser intensities up to 4 × 1018 W/cm 2. The x ray generated in the interaction region were detected and analyzed using a variety of diagnostics. High resolution keV spectroscopy was realized with Johann and Von Hamos spectrometers. Picosecond time-resolved spectra were obtained with a modified Kentech X-ray streak camera, having a 2-ps temporal resolution, coupled to a Von Hamos spectrometer (23). A transmission grating spectrometer and a set of x-ray diodes were used to measured x-ray conversion efficiencies (15, 28) in the keV and sub-keV ranges. X-UV spectra were recorded with grazing incidence fiat field spectrometers (29). Finally, the size of the emitting plasma was determined with filtered

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camera, and x-ray monochromatic images of the plasma were obtained with the Johann spectrometer used in imaging mode (30).

2.1. High Resolution keV Spectroscopy High resolution keV spectroscopy is used to follow the ionization dynamics and also to characterize some plasma parameters. Figures 1-3 present some spectral signatures, respectively K-, L-, and M-shell spectra, of ultrashort plasmas for various experimental conditions. The effect of a prepulse on the K-shell line emission is illustrated with Fig. 1, which shows microdensitometer traces taken across time integrated spectra recorded from

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FIG. 2. Time integrated spectra from Ge targets: (a) 1-#m irradiation (400-fs pulse) at l016 W / c m 2 with a prepulse at 3 × 10~°W/era2; (b) 0.5-/~mirradiation (400-fs pulse) at 1016W/cm2 with no prepulse. A1 targets irradiated with a 1-1zm pulse at 1018 W / c m 2, with a prepulse at 3 × 1012 W / cm 2 (Fig. la), and with a 0.5-~zm pulse at 10 TM W / c m 2, with no prepulse (Fig. lb). When there is no preplasma the intercombination He~ line is absent which qualitatively indicates that the He-like emission is coming from a dense plasma. In these conditions, H % and Hev line profiles indicate that He-like emission takes place at around 6 × 1022 cm -3 (24, 31). The Li-like satellite emission appears also to be very sensitive to the irradiation conditions. In presence of a preplasma the a-d, q, r inner shell Li-like satellites (gabriel notation (32)) are strongly enhanced relative to the k, j dielectronic satellites, and the (a-d, q, r)/(k, j) line intensity ratio can be as high as three times the LTE value (Fig. la). This situation is really different from what has been previously observed with long pulses and seems to be related to non-steady-state population

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kinetics and to non-Maxwellian electron distribution, such as the one produced by nonlocal transport or nonlinear processes at nc or nc/4 (18, 33). Non-steady-state effects produce a delayed appearance of the He-like ions which limits the dielectronic capture on the upper level of the k, j transitions. On the other hand, nonthermal electrons mainly increase the population of the collisionaly excited level producing an enhancement of the core-excited a-d lines. When the Li-like emission takes place at very high density (larger than 1 × 1023 cm-3), the ls2121'level populations are at the LTE and line broadening, as seen in Fig. lb, start to be significant (33). Calculations of the satellites emission profile (33) indicate that with 0.5-urn irradiation at 1018 W/cm 2 (Fig. lb) the Li-like emission is produced at around 3 × 1023 cm -3, which demonstrates for the first time the generation of a hot emitting solid density plasma at these very high laser intensities (34). The observation of K~ emission (Fig. 1) reveals the presence of a hot electron component produced by nonlocal transport or/and by nonlinear processes at the critical density and in the plasma corona. A portion of the L-shell Ge spectrum (8-8.5 A) is shown in Fig. 2. Ne-like 3-2 emission dominates the 8-10 A spectral region, but F-like transitions are also observed (F-like 2s-3p lines in Fig. 2). In presence of a prepulse, we observed (Fig. 2a which corresponds to a 1-#m irradiation) the Ne-like E2 quadrupole line at 8.031 A (2s2p63d ( J -= 2) - 2s22p 6 (J = 0) transition) we previously observed and identified in our Ge x-ray laser experiments with nanosecond pulses (35). The E2/3A line ratio has been shown to be a sensitive density diagnostic in Ne-like laser-produced plasmas (36) because at low density radiative decay dominates the collisional deexcitation of the 3d level, while at high density collisional mixing of the n = 3 levels drives the ratio down. In our 1-#m experiments, the E2/3A line ratio roughly indicates, assuming an optically thin plasma in the calculations (37), that Ne-like emission takes place at density between 5 × 102o and 1 × 1021 cm -3. In the prepulse-free, experiments (Fig. 2b) the E2 line disappears and some O-like emission (2p-3d transitions) increases. These results qualitatively suggest that, in the absence of any preplasma, the emitting

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plasma is mainly overdense, which is consistent with the measurements of the aluminum K-shell emission. A similar behavior is observed with higher Z targets (38, 39) and an example of Ta M-shell spectrum is shown in Fig. 3, where unresolved transition array (UTA) emission lines of Ni-, Cu-, Zn-, and Ga-like ions are clearly observed (3d-5ftransitions). As 5f level energy is much higher than the ionization threshold, the continuum may be interpreted as an evidence of relatively large density (40).

2.2. X-Ray Yield With 0.5-gin irradiation, in the absence of any prepulse, we observed an increase of the x-ray diode signal and of the line intensity with the laser intensity. A power law dependence of I~5, where IL is the laser intensity, is observed for the A1 H% line for 1017 W / c m 2 < IL ~< 1018 W / c m 2. This scaling is different from scaling observed at lower laser intensities (I 43 observed in the 10 TM W / c m 2 range (14) and 12.2 observed in the 1016 W / c m 2 range (41)) but is similar to the scaling observed in other experiments at similar laser intensity (13). This could eventually indicate that the slope of the xray yield power law decreases with increasing laser intensity for IL > 1016 W / c m 2

(15, 41). Figure 4 shows some spectra obtained with the transmission grating spectrometer with a 1-/~m irradiation (600 fs pulse) at 5 × 1016 W / c m 2 (prepulse intensity of 1011 W/cm2). The conversion efficiency decreases as the photon energy increases but this trend strongly depends on the target material due to the different shells involved in the emission. Conversion efficiencies of about 0.4% in 27r srd and 4% in 27r srd can be deduced from these spectra for respectively the keV (0.8-1.5 keV) and the sub-keV (0.1-0.8 keV) range for Ta (42). It is interesting to note that the conversion efficiency in A1 H% line (0.5-~m irradiation without prepulse) is around 2 × 10.3 % in 2~r sr at 10 TM W / c m 2 in our experiments. The x-ray conversion efficiency is affected by the presence of a prepulse, by the laser intensity and focusing conditions. For instance, conversion in A1 He~ line of

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FIG. 6. Time-resolved AI spectrum (7.70-8.0 A) for a l-#m irradiation (400-fs pulse) at 3 × 1016W/cm 2 with a prepulse at 8 X 101° W/cm 2. The temporal resolution is 2 ps and the spectral resolution is 20 mA.

8 X 10-3 % in 2~r sr is obtained with a 1-#m irradiation at 1018 W/cm 2 with a prepulse at 3 × 1012 W/cm 2. A systematic investigation of the effects of these parameters is ongoing.

2.3. Temporal Evolution The time history of the x-ray emission has been studied with an x-ray streak camera having a 2-ps temporal resolution (23). The camera has been calibrated directly on keV x rays emitted by two adjacent plasmas produced by two low intensity beams that have a known relative time delay between them. The x-ray pulses emitted in the keV range (through Be filters) by one laser beam incident on various solid thick targets have been measured for 1- and 0.5-~tm (prepulse free) irradiations at 3 X 1016 W/cm 2 and are shown in Fig. 5. For 1-~m irradiation, the prepulse intensity was 8 × 101° W/cm 2. At a laser wavelength of 1 #m, the signal rise time is typically a few ps and the F W H M is around 10 ps. Furthermore, the signal presents a long tail, where duration (from 20 to 40 ps) depends on the target material. The situation is dramatically different at shorter wavelengths where the prepulse is absent, as illustrated with Ti result in Fig. 5b. In this condition, the rise time is 2 ps (instrument limited) and F W H M is around 3 ps. Then the presence of a prepulse is a critical issue for obtaining the shortest x-ray pulses.

FIG. 5. X-ray pulses emitted in the keV range (through 12.5-~zm Be filters). The temporal resolution is 2 ps and the laser intensity is 3 × 1016 W/cm 2. (a) AI target and 1-um irradiation (prepulse at 8 X 101° W/ cm2). (b) Ti target and 1-gm irradiation (prepulse at 8 N 101°W/cm 2) and 0.5-gm irradiation (no prepulse). (c) Cu target and l-#m irradiation (prepulse at 8 × 101° W/cm2).

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However, even with strong prepulse, ultrashort x-ray pulse (eventually shorter than the laser pulse itself) can be generated by judiciously selecting the emission coming from an ionization state produced at low or intermediate temperature and which has a very brief lifetime during the laser pulse rise time. This is illustrated by the Fig. 6 (1 ~m irradiation at 3 × 1016 W/cm2), which shows a time-resolved spectrum (with a 2-ps temporal resolution and a 20-mA spectral resolution) near the AI He~ resonance line (7.757 A ls 2- ls2p transition). The H G rise time and F W H M are similar to what is observed with the direct x-ray pulse (Fig. 5a). The tail on the resonance line seems to indicate that the three-body recombination plays an important role in populating the A1XII excited levels in these conditions (1-t~m irradiation at 3 × 1016 W/cm 2, prepulse at 8 X 10 l° W/cm2). In contrast, the Li-like satellite emission lasts only a few ps (the F W H M is 4 ps) and the Be-like emission at 7.94 A is still shorter, limited by the 2-ps instrumental resolution. Then by controlling the prepulse or the emitting ionization state, we can control the source duration. However, the source brightness is lower (at constant laser intensity) when the x-ray pulse is shorter. 3. PERSPECTIVES AND CONCLUSIONS The development of an x-ray source adapted to some planned applications (x-ray laser, molecular dynamics) requires simultaneously an increase of the source brightness and a decrease of the x-ray pulse duration. Using higher intensities (1018-1019 W / c m 2) will allow the production of hotter plasmas, increasing the source brightness while maintaining an ultrashort gradient scale length due to the ponderomotive force (43). In this intensity regime, the ponderomotive force will balance the thermal pressure and one could expect to produce radiation pressure confined plasmas (34). In these conditions, emission will take place at near solid density, and x-ray pulse durations are thus expected to be ultrashort. We have started to explore this new regime and we recently showed that the mean density at which the emission takes place in a plasma produced by a high contrast ratio subpicosecond pulse remains very high (a few times 10 23 cm 3), close to the solid density, after the laser intensity is increased from 1016 to 1018 W / c m 2 (24, 31, 34). This behavior, which is not expected from scaling laws based on thermal expansion of the plasma, could indicate the presence of a strong ponderomotive pressure. Furthermore, it was recently demonstrated that the x-ray yield could be increased 10-fold by using thin foil targets in which electronic diffusion is limited (44). Then ultimately it would be necessary to irradiate the two faces of a thin target--thickness close to the skin depth--at very high intensity to maintain the plasma at solid density during the laser pulse by radiation confinement and then achieve a solid density isochore heating up to a few keV. In summary, the prospects for the generation of a bright ultrafast x-ray source is very good, and by choosing the appropriate emitting target, laser intensity, and pulse shape, the conversion efficiency as well as the x-ray duration may be optimized. ACKNOWLEDGMENTS The authors wish to thank Y. Beaudoin,C. Y. C6t6, S. Coe, Z. Jiang, and J. F. PeUetierfor assistance with the laser systemsand for help during experiments,and P. Jaanimagi,T. W. Johnston,J. P. Matte, G.

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Mourou, O. Peyrusse, and D. Umstadter for many helpful discussions. This work is supported by the Minist&e de l'Education du Qu6bec and by the Natural Science and Engineering Research Council of Canada. REFERENCES 1. K. M. GLIBERT, J. P. ANTHES, M. A. GUSINOW, M. A. PALMER, R. R. WHITLOCK, AND D. J. NAGEL, J. Appl. Phys. 51, 1449 (1980). 2. M. CHAKER, H. PIPPIN, V. BAREAU, B. LA FONTAINE, I. TOUHANS, R. FABBRO, AND B. FARAL, J. Appl. Phys. 63, 892 (1988). 3. D. NAGEL, M. PECKERAR, R. R. WHITLOCK, J. R. GREIG, AND R. E. PECHACEK, Electron. Lett. 14, 781 (1978). 4. U. PI~PIN, P. ALATERRE, M. CHAKER, R. FABBRO, B. EARAL, I. TOUBHANS, D. J. NAGEL, AND M. PECKERAR, J. Vac. Sci. Technol. B 5, 27 (1987). 5. B. YAAKOBI,H. KIM, J. M. SOURES, H. DECKMAN, AND J. DUNSMUIR,Appl. Phys. Lett. 43, 686 (1983). 6. G. D. KUBIAK et al., J. Vac. Sci. Technol. B 9, 3184 (1991). 7. N. M. CEGLIO AND A. M. HAWYLUK, Proe. Opt. Soc. Am. 12, 5 (1991). 8. M. CHAKER, B. LA FONTAINE, C. Y. C~ST~, J. C. KIEFFER, H. PIPPIN, M. H. TALON, G. D. ENRIGHT, AND D. M. VILLENEUVE,J. Vac. Sci. Technol. B 10, 3239 (1992). 9. 10. 11. 12. 13.

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