Subpicosecond vacuum-ultraviolet radiation at 155 nm with pulse energies above 0.2 mJ has been obtained by near-resonant four-wave difference-frequency mixing in a Xe gas jet. Laser fields for the mixing process have been generated by a short-pulse KrF dye excimer laser system and a Raman converter. The process permits tuning in a broad vacuum-ultraviolet range and can be scaled up to higher output energies

Fricke, L.

Liu, P.

Mossavi, K.

Wellegehausen, B.

English

Mossavi, Kasem

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Generation of high-power subpicosecond pulses at 155 nm

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June 15, 1995 / Vol. 20, No. 12 / OPTICS LETTERS 1403Generation of high-power subpicosecond pulses at 155 nmK. Mossavi, L. Fricke, P. Liu, and B. WellegehausenInstitut für Quantenoptik, Universität Hannover, Welfengarten 1, 30167 Hannover, GermanyReceived February 27, 1995Subpicosecond vacuum-ultraviolet radiation at 155 nm with pulse energies above 0.2 mJ has been obtained bynear-resonant four-wave difference-frequency mixing in a Xe gas jet. Laser fields for the mixing process havebeen generated by a short-pulse KrF dye excimer laser system and a Raman converter. The process permitstuning in a broad vacuum-ultraviolet range and can be scaled up to higher output energies.Frequency conversion of high-intensity laser radiationis a well-established technique for the generation of co-herent vacuum-ultraviolet (VUV), extreme-ultraviolet(XUV), and soft-x-ray radiation. Currently, short-pulse high-power lasers in the IR, visible, and UV spec-tral ranges are used as primary lasers. With theselaser systems wavelengths shorter than 10 nm havebeen obtained by high-order harmonic generation,1and high-order mixing has led to the generation oftunable XUV radiation.2 However the output powerand conversion efficiency of these high-order interac-tions are generally small. Therefore it seems to beadvantageous to start from high-power laser systemsoperating at VUV wavelengths and generate coherentradiation below 100 nm by eff icient low-order non-linear processes, such as frequency tripling and four-wave mixing. The development of powerful lasersources at VUV wavelengths with gigawatt outputpower, however, has been demonstrated only forArF excimer systems at 193 nm.3 – 8 By frequencyconversion techniques in metal vapors9 – 11 and LiB3O5crystals,12 VUV peak powers up to 11 MW have beenachieved.Recent investigations13 –16 have shown that tunablesubpicosecond VUV radiation can be eff iciently gener-ated by near-resonant four-wave difference-frequencymixing (FWDM) in Xe. The excitation of Xe by ashort-pulse KrF hybrid dye excimer laser17 (248.5 nm,20 mJ, 400 fs) close to the 5p6 –5p56pf1y2g0 two-photonresonance and additional injection of laser pulses withwavelengths between 330 and 1900 nm has resulted inshort-pulse radiation in the range of 200–133 nm, withseveral microjoules of output energy.14In this Letter we report on further progress to in-crease output power and conversion efficiency of theFWDM process in Xe. For experimental convenienceexperiments have been performed at the specific wave-length of 155 nm, produced according to v155 ­ 2v248 2v626. Output energies above 200 mJ and peak powersin the range of 0.5 GW have been achieved. To ourknowledge this is the first demonstration of a subpi-cosecond laser system in the mid-VUV spectral rangeoperating at energy levels well above 100 mJ.Efficient FWDM requires, in addition to the stronglaser field at 248 nm, a powerful injection field thatis synchronized to the KrF system. A straightfor-ward approach would be to use the 497-nm radiation(100 mJ, 360 fs) generated internally in the KrF laser0146-9592/95/121403-03$6.00/0system.17 Instead, we have frequency shifted thisradiation by Raman conversion in H2 to 626 nm (firstStokes line) to generate VUV radiation at a shorterwavelength and take advantage of a higher nonlinearsusceptibility at 155 nm.14 The experimental setupis shown in Fig. 1. Because Raman conversion ofsubpicosecond pulses becomes easily affected by otherpower-dependent processes, such as self-focusing andself-phase modulation,4 the Raman converter is opera-ted as a seeded Raman amplif ier, which signif icantlyimproves the spectral and spatial quality of theRaman beam. Seed photons for the Raman ampli-fier are generated by a tunable dye laser (DCM inmethanol) that is tuned to 626 nm. The dye laseris pumped by 1.7 mJ of a frequency-doubled YAGlaser (Quanta-Ray DCR-11) and generates seed pulsesat 626 nm with an energy of 240 mJ and a pulseduration of 8 ns. The YAG laser is synchronized tothe KrF laser with a temporal jitter of 62 ns betweenthe 497-nm and the 626-nm dye laser pulses. The626-nm seed pulse and the 497-nm pulse are slightlynoncollinear overlapped in the Raman cell, whichpermits separation of the seed and Stokes pulse atmirror M1. The Raman amplif ier is operated at aH2 pressure of 32 bars, just below the self-oscillationthreshold. Pulse energies of ,16 mJ have been ob-tained for the first Stokes line. A second dye cellpumped by 50 mJ of 532-nm radiation is used for fur-ther amplification of the Stokes signal in a double-passconfiguration. The 626-nm output pulses from thedye amplif ier (DCM in methanol) have a typical energyof 300 mJ and a pulse duration of 330 fs (sech2 pulseshape). The spectral width of the amplif ied Stokespulse is , 3.7 nm, resulting in a time–bandwidthproduct of DnDt ­ 0.9. Although the Raman convertergenerates pulses only at a fixed wavelength and, corre-spondingly, the VUV output of the FWDM process isnot tunable; a set of VUV wavelengths is accessible bythe use of different gases for the Raman cell.Unlike in previous experiments,13 – 16 a Xe jet isused instead of a gas cell for the mixing process tominimize absorption of the generated VUV radiationat 155 nm. Mixing experiments in a gas cell havedemonstrated that VUV radiation at 155 nm is ab-sorbed mainly by two-photon ionization of Xe and two-photon absorption at the exit window (MgF2) of the gascell. The VUV transmission of the exit window is fur-ther affected by color-center formation. For a MgF2 1995 Optical Society of America1404 OPTICS LETTERS / Vol. 20, No. 12 / June 15, 1995Fig. 1. Schematic layout of the laser system. L1, f ­80 mm cylindrical lens; L2, f ­ 35 mm cylindrical lens;M2, uncoated quartz plate; HR 155, dielectric mirror; SHG,second-harmonic generator.window with a thickness of 2 mm a degradation of thetransmission from 80% to 50% has been observed afterseveral laser shots. Another advantage of the gas jetis the localized interaction region defined by the lengthof the nozzle (see Fig. 1). The input fields for the mix-ing process propagate without distortion to the inter-action region, which further increases the conversioneff iciency. A comparison of the VUV output betweenthe gas jet and a gas cell (equipped with a new MgF2window) has yielded more than twice the energy forthe gas jet. A vacuum of 1023 mbars has been main-tained within the propagation region of the input fieldsand the VUV radiation. The injection of Xe gas intothe nozzle is controlled by a pulsed gas valve (Gen-eral Valve Corporation). The valve is operated witha backing pressure of 5 bars and triggered approxi-mately 700 ms before the lasers. Optimum VUV out-put has been obtained for a nozzle length of 5 cm and adiameter of 3 mm.Phase matching of the FWDM process is accom-plished by a noncollinear mixing geometry with anangle of 1± between the 248- and 626-nm beams. AXe pressure of 54 mbars in a gas cell corresponds to anangle of 1± and can be used as an estimate of the pres-sure in the nozzle. The 248-nm beam with a pulse en-ergy of 15 mJ is focused by a 2-m focal-length lens intothe nozzle. A 1-m focal-length lens is used for focusingof the 626-nm beam. The generated VUV radiation isseparated by a 155-nm high-ref lecting dielectric mirrorfrom the input fields.Figure 2 shows a spectrum of the generated VUVradiation. Because of the multishot recording tech-nique, however, no information can be extracted aboutthe spectral structure. The spectrum suggests an up-per limit of ,1 nm (FWHM) for the spectral bandwidth.Although no pulse duration measurement at 155 nmhas been performed yet, autocorrelation of 193 nm ra-diation generated by FWDM in Xe (Ref. 14) has indi-cated comparable pulse durations between the FWDMsignal and the input pulses. This is further supportedby a recent pulse duration measurement of 147-nm ra-diation generated by four-wave parametric oscillationin Xe.18 If we assume a Fourier-limited VUV pulsewith a duration of 400 fs and a sech2 pulse shape, wecan calculate a minimum bandwidth of 0.06 nm.A maximum output energy of 195 mJ at 155 nm hasbeen measured by a pyroelectric energy detector, cali-brated at 200 nm. Because the spectral sensitivity ofpyroelectric detectors tends to decrease at wavelengthsbelow 200 nm, the actual pulse energy is higher. Theloss of sensitivity of the pyroelectric detector hasbeen estimated in the following way. According toManley–Rowe relations,16 the FWDM process ampli-fies the 626-nm pulse. By measuring the gain in en-ergy of the 626-nm pulse after FWDM, we can calculatethe number of generated 626-nm photons and in turnthe number of generated photons at 155 nm. Thisgives an estimate of the generated VUV energy, if weassume no significant absorption of the VUV radia-tion within the nozzle. If we compare this value withthe measured VUV energy, we obtain a drop in sensi-tivity at 155 nm of ,25% compared with the sensi-tivity at wavelengths above 200 nm. The correctedmaximum output energy would be 260 mJ, which corre-sponds to a peak power of 650 MW for a pulse durationof 400 fs.Fig. 2. Multishot spectrum of FWDM radiation at155 nm.June 15, 1995 / Vol. 20, No. 12 / OPTICS LETTERS 1405Fig. 3. Output energy at 155 nm versus input energy at626 nm (detector calibration at 200 nm).As indicated in Fig. 3, the energy dependence be-tween the 155- and 626-nm radiation of the presentsetup shows no saturation. Therefore even higherVUV output energies can be expected at higher inputenergies at 626 nm. This can be achieved by a morepowerful YAG laser for the dye amplif ier.In conclusion, we have developed a high-power lasersystem at 155 nm based on four-wave difference-frequency mixing in Xe. The system generatessubpicosecond pulses with an energy of more than200 mJ and can be scaled up to higher VUV ener-gies. Experiments are now in progress to replace theRaman converter by an optical parametric generator tointroduce continuous tunability of the VUV radiation.With further improvements the mixing process has thepotential of providing tunable VUV powers in excessof 1 GW.The authors acknowledge financial support from theDeutsche Forschungsgemeinschaft.References1. A. L’Huillier, L.-A. Lompre, G. Mainfray, and C.Manus, in Atoms in Intense Laser Fields, M. Gavrila,ed. (Academic, New York, 1992), pp. 139–206, andreferences therein; M. D. Perry and G. Mourou, Science264, 917 (1994).2. H. Eichmann, S. Meyer, K. Riepl, C. Momma, and B.Wellegehausen, Phys. Rev. A 50, R2834 (1994).3. H. Egger, T. S. Luk, K. Boyer, D. F. Muller, H.Pummer, T. Srinivasan, and C. K. Rhodes, Appl. Phys.Lett. 41, 1032 (1982).4. S. Szatmari and F. P. Schäfer, J. Opt. Soc. Am. B 6,1877 (1989).5. J. H. Glownia, M. Kaschke, and P. Sorokin, Opt. Lett.17, 337 (1992).6. J. Ringling, O. Kittelmann, and F. Noack, Opt. Lett.17, 1794 (1992).7. K. Mossavi, T. Hofmann, G. Szabo, and F. K. Tittel,Appl. Phys. Lett. 62, 1203 (1993).8. C. Momma, H. Eichmann, H. Jacobs, A. Tünnermann,H. Welling, and B. Wellegehausen, Opt. Lett. 18, 516(1993).9. M. H. R. Hutchinson and R. J. Manning, Opt. Commun.55, 55 (1985).10. C. H. Muller III, D. D. Lowenthal, A. V. Smith, andM. A. DeFaccio, Opt. Lett. 13, 651 (1988).11. M. K. Kodirov, S. A. Myslivets, A. K. Popov, and V. V.Slabko, Sov. J. Quantum Electron. 22, 195 (1992).12. J. Ringling, O. Kittelmann, G. Korn, J. Squier, and F.Noack, Opt. Lett. 18, 2035 (1993).13. A. Tünnermann, K. Mossavi, and B. Wellegehausen,Phys. Rev. A 46, 2707 (1992).14. A. Tünnermann, C. Momma, K. Mossavi, C. Windolph,and B. Wellegenhausen, IEEE J. Quantum Electron.29, 1233 (1993).15. J. H. Glownia, D. R. Gnass, and P. P. Sorokin, J. Opt.Soc. Am. B 11, 2427 (1994).16. S. P. Le Blanc, Z. Qi, and R. Sauerbrey, ‘‘Generation offemtosecond vacuum ultraviolet pulses,’’ Appl. Phys. B(to be published).17. S. Szatmari and F. P. Schäfer, Opt. Commun. 68, 196(1988).18. S. P. Le Blanc, Z. Qi, and R. Sauerbrey, Opt. Lett. 20,312 (1995).19. P. N. Butcher and D. Cotter, The Elements of Non-linear Optics (Cambridge U. Press, Cambridge, 1991),p. 127.

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Generation of high-power subpicosecond pulses at 155 nm

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