A dispersively compensated scheme for sum-frequency mixing of broadband ultrashort laser pulses is reported. An increase of the bandwidth of the sum-frequency mixing process by 12 times compared with the noncompensated bandwidth of the given crystal has been demonstrated. Mixing radiation at 266 and 707 nm in a 1-mm-thick beta-barium metaborate crystal by using the compensated scheme results in an output bandwidth of 0.6 nm at 193 nm, which corresponds to a minimum output pulse duration of 90 fs

Hofmann, T.

Mossavi, K.

Szabo, G.

Tittel, F.

English

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December 1, 1992 / Vol. 17, No. 23 / OPTICS LETTERS 1691Spectrally compensated sum-frequency mixing schemefor generation of broadband radiation at 193 nmTh. Hofinann, K. Mossavi, and F. K. TittelDepartment of Electrical and Computer Engineering, Rice Quantum Institute, Rice University, P.O. Box 1892, Houston, Texas 77251G. SzaboDepartment of Optics and Quantum Electronics, Attila J6zsef University, H-6720 Szeged, Dom ter 9, HungaryReceived August 10, 1992A dispersively compensated scheme for sum-frequency mixing of broadband ultrashort laser pulses is reported.An increase of the bandwidth of the sum-frequency mixing process by 12 times compared with the noncom-pensated bandwidth of the given crystal has been demonstrated. Mixing radiation at 266 and 707 nm in a1-mm-thick beta-barium metaborate crystal by using the compensated scheme results in an output bandwidthof 0.6 nm at 193 nm, which corresponds to a minimum output pulse duration of 90 fs.The interest in various applications of far-UV andVIJV laser pulses, such as ultrafast photodissociationstudies of molecules or laser-induced fluorescencespectroscopy, has led to the development of methodsto generate intense, coherent light in this part ofthe spectrum. A particularly important wavelengthin this range is 193 nm, which is the wavelengthof the ArF excimer laser. Ultrashort seed pulses at193 nm can be amplified to gigawatt power levels.1-3Frequency tripling of the ArF laser has been re-ported to be a useful source of 64-nm radiation,4which can find important applications in biology andlithography. Short-pulse high-brightness ArF lasersystems can be considered as a promising pumpsource for the production of x rays and laboratory-scale x-ray laser schemes,5 but they may also playa fundamental role in extending the range of theultrafast photodynamical studies of molecules' intothe VU1V region.Different methods of generating 193-nm radiationhave been reported: sum-frequency mixing (SFM) ofUV and IR wavelengths in beta-barium metaborate2 ,9(BBO) and potassium pentaborate,'0 third-harmonicgeneration in Sr vapor,' and nonresonant3 and near-resonant" difference-frequency mixing in Xe. Fre-quency mixing in gases generally requires extremelyhigh pump intensities to achieve output energiessufficient for practical applications.3For short pulses, the high dispersion of BBO makesit difficult to achieve high-efficiency SFM in the VUVrange. By using the Sellmeier constants reported byKato,12 the spectral bandwidth for SFM at 193 nmusing wavelengths of approximately 266 and 707 nmfor a 1-mm-thick BBO crystal can be calculated to beapproximately 0.05 nm, which corresponds to an ap-proximately I-ps minimum pulse duration, assuminga Gaussian pulse shape. Thus to generate 193-nmradiation with pulse durations in the subpicosecondrange the crystal thickness should be well below1 mm, which in turn results in decreased conversionefficiency.2Several schemes for efficient frequency doubling'3-6and tripling17 of broadband ultrashort pulses bymeans of compensating the chromatic dispersion ofthe nonlinear crystal by angular spectral dispersionhave been reported. The methods described inRefs. 13-16 were developed for frequency doubling;consequently, they cannot be used directly for SFM.The tripling scheme of Ref. 17 could also be usedfor SFM, but its single-beam input design wouldconsiderably increase the risk of damage to thedispersive element, and restriction of the inputenergy in order to avoid damage would limit theefficiency. In addition, as discussed below, the com-pensated mixing bandwidth is smaller when theinput beams are collinear.In this Letter we present a dispersive scheme es-pecially designed for SFM that increases the spectralacceptance of a given crystal by more than a factorof 10. All the experimental details discussed belowrefer to a system that is designed to mix picosecondfourth-harmonic pulses from a Nd:YAG laser withfemtosecond pulses from a dye laser (i.e., the UVinput pulses are nearly monochromatic while thered input pulses are broadband); however, this ar-rangement can be adopted to other combinations ofwavelengths and pulse durations by using the samedesign considerations. Furthermore, the scheme canbe used not only with BBO in the VUV range, butalso with other nonlinear crystals anywhere in thespectrum, including the far IR, where the spectralacceptance of the SFM process limits the conversionefficiency.The experimental arrangement is depicted inFig. 1. The 266-nm laser beam enters the BBO crys-tal directly, while the 707-nm beam is directed ontothe reflection grating, where it is spectrally dispersed.The surface of the grating is imaged onto the crystal0146-9592/92/231691-03$5.00/0 ©- 1992 Optical Society of America1692 OPTICS LETTERS / Vol. 17, No. 23 / December 1, 1992REDuvBBO L2 PrismGratingFig. 1. Experimental arrangement for dispersively com-pensated SFM of broadband, ultrashort pulses.by the lens (Ll) with a magnification that is chosen sothat each of the spectral components of the red beamfalls on the crystal at an angle that phase matchesthe 266-nm beam. The necessary magnification canbe calculated by comparing the angular dispersionrequired by the crystal with that of the grating. Byusing the Sellmeier constants of Ref. 12 we obtaina crystal dispersion of 9.16 mrad/nm, whereas thedispersion of the 1200-line/mm grating used in theexperiment is 1.672 mrad/nm, assuming an angle ofincidence of 8.75°. (The incidence angle was chosento achieve the highest efficiency with the givengrating.) From these values we obtain 5.5 for thenecessary magnification.To achieve the largest output bandwidth, the cen-tral wavelength of the red beam was set slightlynoncollinear with the 266-nm beam. The reason forthis can be understood with the help of Fig. 2. (Inthe figure the indices of the k vectors are used inthe following sense: the subscripts 1, 2, and 3 cor-respond to the central wavelength of 266, 707, and193 nm, respectively, and the plus and minus, ifpresent, indicate that the given vector belongs to awavelength that is slightly longer or shorter thanthe central wavelength.) If the central component ofthe red and the UV beam were collinear, the lengthof the sum of the red and UV k vectors would bemaximum practically at the central wavelengths [seeFig. 2(a)]. This would mean, however, that phasematching could only be achieved at wavelengths thatare equal to or longer than the central VWJV wave-length, because the length of the VWY k vector is amonotonic function of the frequency. This effect canbe compensated for by aligning the central componentof the red beam slightly noncollinear with the UVbeam as indicated in Fig. 2(b).As a result of the angular dispersion at the in-put of the crystal, the 193-nm sum-frequency out-put beam is also dispersed. It is recollimated bymeans of a lens (L2) and a 600 fused-silica prismthat is set to minimum deviation. Here the lens isagain positioned so that the crystal plane is imagedonto the prism with the appropriate magnification tomatch dispersion. From the angular dispersion of33.57 mrad/nm at 193 nm and a prism dispersion of2.707 mrad/nm the required magnification is 12.4.The imaging of the grating onto the crystal andthe crystal onto the prism ensures (as can be seenby using Fermat's principle) that the optical pathis equal for any of the red and V1TV spectral com-ponents, provided that the lens aberrations can beneglected. This also means that the output pulseduration is expected to be the same as that of the redinput pulse. This is, in fact, exactly what is neededin the present experiments because the system isdesigned to mix picosecond fourth-harmonic pulsesfrom a Nd:YAG laser with femtosecond red pulsesfrom a dye-laser system. If, however, the 'U inputpulse duration were shorter, the scheme could bemodified by dispersing the UV beam instead of thered beam.In the arrangement depicted in Fig. 1 we used anf = 100 mm lens as L1, an f = 50 mm fused-silicalens as L2, and a 1-mm-thick BBO crystal cut to 760phase matching, while the angle between the 266-nmbeam and the central spectral component of the redbeam was set at 7°.The performance of the dispersively compensatedSFM scheme was tested by simulating the broadbandwidth of the short input pulse with a tunablenarrow-band laser. In this case the SFM energyis measured as a function of the VUV wavelengthwhile the red laser is tuned, with the energy heldconstant. The advantage of this method is that theoutput bandwidth can be measured directly even if itis larger than that of the currently available shortestpulses at the given wavelength.The measurements have been performed by usingtwo nanosecond dye lasers pumped by the sameXeCl excimer laser so as to avoid temporal jitterbetween the pulses. One of the dye lasers operatedat a constant wavelength of 532 nm while the otherwas tuned around 707 nm. The dyes used in thered- and green-dye laser were Rhodamine 700and Coumarin 540A (both dissolved in methanol),respectively. The output of the green-dye laser wasfrequency doubled by a 6.5-mm-thick BBO crystalthat was cut for a 51° phase-matching angle and(a) A(b) AN k2 A k 2kI k2+~~~~~~~2k3 - k'1.3I Il UlI Ii III~~~~~~3+I 1lFig. 2. Wave vectors showing the phase matching fordispersively compensated SFM scheme with central com-ponent of the 707-nm beam (a) collinear and (b) non-collinear with the 266-nm beam. The 1, 2, and 3 indicesof the k vectors refer to the central wavelengths of 266,707, and 193 nm, respectively. The plus and minus,if present, indicate that the given vector belongs to awavelength that is slightly larger or smaller than thecentral wavelength.December 1, 1992 / Vol. 17, No. 23 / OPTICS LETTERS 1693.,C,_._ACC0Cc,CrEU,1.210.80.60.40.2Wavelength (nm)Fig. 3. Measured and calculated SFM spectral accep-tance of a 1-mm-thick BBO crystal, uncompensated(squares and dashed curve) and compensated (circles andsolid curve).generated 200 AuJ of energy at 266 nm. The outputenergy of the red-dye laser was 1 mJ.For the intensity measurements at 193 nm, weused a solar-blind vacuum photodiode, whereas thewavelength was measured with a grating mono-chromator equipped with an optical multichannelanalyzer.The measured SFM bandwidths are shown inFig. 3. The squares show the output of an uncom-pensated 1-mm-thick BBO crystal, and the circlesrepresent the output of our dispersive scheme usingthe same crystal. Figure 3 also indicates the corre-sponding calculated SFM bandwidths by the dashedand solid curves, respectively. The calculations,showing good agreement with the measurements,have been performed in a low-conversion-efficiencylimit simply by substituting the calculated phasemismatch Ak intosin2 (AkL/2)SFM (AkL/2) 2where L is the crystal length.From Fig. 3 it is evident that the dispersively com-pensated scheme increases the uncompensated SFMbandwidth of 0.05 to 0.6 nm (i.e., by a factor of12), corresponding approximately to 90-fs minimumpulse duration. This also means that for a givenoutput bandwidth (or pulse duration) the maximumallowable crystal length is 12 times longer than thatwithout compensation. This increase in the usefulcrystal length is of fundamental importance becausethe SFM energy in the low-conversion-efficiency limit,which is generally valid for ultrashort pulses, is pro-portional to the square of the crystal length.In preliminary experiments with 300-fs, 707-nmpulses we succeeded in generating 10 /uJ of energyat 193 nm with the broadest measured bandwidthof 0.22 nm, corresponding to 250-fs pulse duration.This energy is approximately two orders of magnitudehigher than the seed pulse energy generated by SFMin Ref. 2, with comparable pulse duration.In summary, we have developed a dispersively com-pensated scheme for SFM that increases the band-width of the SFM process by more than a factor of 10compared with the noncompensated bandwidth of thegiven crystal. By using a 1-mm-thick BBO crystalwe have measured an output bandwidth of 0.6 nm at193 nm, corresponding to an output pulse durationof 90 fs.The support of the Robert Welch Foundation (grantC-0586) and the U.S. Office of Naval Research isgratefully acknowledged. G. Szab6 also acknowl-edges support from the OTKA Foundation of theHungarian Academy of Sciences (grant 3056). Theauthors are indebted to R. Sauerbrey and J. F. Youngfor critical reading of the manuscript.References1. H. Egger, T. S. Luk, K. Boyer, D. F. Muller, H.Pummer, T. Srinivasan, and C. K. Rhodes, Appl. Phys.Lett. 41, 1032 (1982).2. S. Szatmari and F. P. Schafer, J. Opt. Soc. Am. B 6,1877 (1989).3. J. H. Glownia, M. Kaschke, and P. P. Sorokin, Opt.Lett. 17, 337 (1992).4. H. Pummer, T. Srinivasan, H. Egger, K. Boyer, T. S.Luk, and C. K. Rhodes, Opt. Lett. 7, 93 (1982).5. M. Steyer, F. P. Schafer, S. Szatmari, and G. Kiihnle,Appl. Phys. B 50, 265 (1990).6. M. J. Rosker, M. Dantus, and A. H. Zewail, Science241, 1200 (1988).7. J. H. Glownia, J. A. Misewich, and P. P. Sorokin,J. Chem. Phys. 92, 3335 (1990).8. T. R. Gosnell, A. J. Taylor, and J. L. Lyman, J. Chem.Phys. 94, 5949 (1991).9. W. MUckenheim, P. Lokai, B. Burghardt, and D. Bast-ing, Appl. Phys. B 45, 259 (1988).10. R. E. Stickel and F. B. Dunning, Appl. Opt. 17, 981(1978).11. A. T~innermann, K. Mossavi, and B. Wellegehausen,Phys. Rev. A 46, 2707 (1992).12. K. Kato, IEEE J. Quantum Electron. QE-22, 1013(1986).13. G. Szab6 and Zs. Bor, Appl. Phys. B 50, 51 (1990).14. 0. E. Martinez, IEEE J. Quantum Electron. 25, 2464(1989).15. T. R. Zhang, H. R. Choo, and M. C. Downer, Appi. Opt.29, 3927 (1990).16. R. A. Cheville, M. T. Reiten, and N. J. Halas, Opt.Lett. 17, 1343 (1992).17. M. D. Skeldon, R. S. Craxton, T. J. Kessler, W. Seka,R. W. Short, S. Skrupsky, and J. M. Soures, IEEEJ. Quantum Electron. 28, 1389 (1992).

Spectrally compensated sum-frequency mixing scheme for generation of broadband radiation at 193 nm

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Spectrally compensated sum-frequency mixing scheme for generation of broadband radiation at 193 nm

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