The gain characteristics of a discharge-pumped ArF excimer amplifier are measured with ~700-fs-long pulses. The small-signal gain and saturation energy are found to be 0.17 cm-1 and 3.65 ± 0.15 mJ/cm2, respectively. The maximum output energy density extracted from the deeply saturated amplifier is as much as 10 mJ/cm2. The demonstrated femtosecond gain characteristics of ArF indicate that, by utilizing sufficiently high seed pulse energies, the ArF excimer laser is expected to show a performance similar to a femtosecond high-brightness KrF excimer laser system

Hofmann, T.

Mossavi, K.

Szabo, G.

Tittel, F.

English

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March 15, 1993 / Vol. 18, No. 6 / OPTICS LETTERS 435Femtosecond gain characteristics of the discharge-pumpedArF excimer amplifierK. Mossavi, Th. Hofinann, G. Szab6, and F. K. TittelDepartment of Electrical and Computer Engineering, Rice Quantum Institute, Rice University, P.O. Box 1892, Houston, Texas 77251Received October 8, 1992The gain characteristics of a discharge-pumped ArF excimer amplifier are measured with -700-fs-longpulses. The small-signal gain and saturation energy are found to be 0.17 cm-' and 3.65 + 0.15 mJ/cm 2,respectively. The maximum output energy density extracted from the deeply saturated amplifier is as much as10 mJ/cm2 . The demonstrated femtosecond gain characteristics of ArF indicate that, by utilizing sufficientlyhigh seed pulse energies, the ArF excimer laser is expected to show a performance similar to a femtosecondhigh-brightness KrF excimer laser system.In recent years we have witnessed fundamentalprogress in high-brightness ultrashort-pulse excimerlaser technology. High-power femtosecond gas lasersystems have been developed based on XeCl,1 KrF,2and, very recently, XeF (C - A) (Ref. 3) excimers.The ultrashort-pulse ArF excimer laser systems,however, have so far shown significantly lowerperformance.'-' This can be explained by two mainreasons. On one hand, the seed pulse generationbecomes increasingly difficult since the necessarywavelength (193 nm) lies outside the range thatis covered by currently available frequency-doublingcrystals; whereas, on the other hand, the lossesof the optical materials increase with decreasingwavelength. The design of the high-power ArF lasersystems was further impeded by the fact that, tothe best of our knowledge, the saturation energydensity, which is the most important design andscaling parameter of such systems, has not beenpublished in the femtosecond regime.In this Letter we present the gain characteristics ofa discharge-pumped ArF excimer amplifier measuredwith -700-fs probe pulses generated by a newly de-veloped, spectrally compensated sum-frequency mix-ing scheme8 and demonstrate that the femtosecondArF amplifier can provide the same high performanceas a KrF amplifier.The experimental apparatus is depicted in Fig. 1.The seed pulses are generated by using 50-ps266-nm pulses from a Nd:YAG laser and 300-fs707-nm pulses from a dye laser. 9 The two beamsenter the dispersively compensated frequency-mixingscheme,8 where their sum frequency at 193 nm is gen-erated. By using 3-mJ pulse energy at 266 nm and0.8 mJ at 707 nm, the pulse energy at 193 nm was12 ,uJ, with a typical spectral width of 0.2 nm, corre-sponding to an approximately 300-fs pulse duration.The energy of the seed pulses was too low to allowfor direct pulse-duration measurements. Instead,the pulse duration was measured after double-pass amplification in an ArF excimer laser, with aMichelson-type autocorrelator based on two-photonionization of Xe and was found to be 710 fs,assuming a Gaussian pulse shape. Since the corre-lation measurements were performed after the pulsestraveled four times through the 5-mm-thick CaF2window of the laser and approximately 8 m in air,one expects noticeable temporal pulse broadeningwith respect to the seed pulse duration. Consideringthat the seed pulse duration is expected to be thesame as that of the 707-nm pulse (i.e., 300 fs;Ref. 8), the assumed initial and the measured finalpulse durations are in good agreement with thevalues reported in Ref. 6 under similar conditions.Considering the supporting experimental evidencepresented in Ref. 6, we believe that the main pulse-broadening mechanism is coherent pulse propagationthrough 02- In this case, taking into account theeffective optical path lengths that the pulses traveledthrough air between the seed pulse generator and theexcimer amplifier and between the amplifier and theautocorrelator, the actual probe pulse duration inthe amplifier is expected to be approximately 500 fs.To obtain energies high enough to reach deepsaturation in the gain measurements, the seed pulseswere preamplified in a single pass through theexcimer laser. A Lambda Physik EMG-101-type ex-cimer laser rated at 200-mJ maximum output energywas used in the measurements. After single-passamplification the total energy was typically 500 ,uJ.Fig. 1. Experimental arrangement for measurementsof the femtosecond gain characteristics of ArF. PD,photodiode; E, energy meter; D, diaphragm; ND, neu-tral-density filter; BS, beam splitter; SFM, sum-frequencymixer.0146-9592/93/060435-03$5.00/0 © 1993 Optical Society of America436 OPTICS LETTERS / Vol. 18, No. 6 / March 15, 1993To avoid difficulties in the gain measurementscaused by spatial inhomogeneities of the probebeam,3 the most homogeneous central part of thespot containing 50 ,uJ of energy was selected by adiaphragm with a 3-mm diameter.After the diaphragm and the turning mirrors, asmall portion of the beam was coupled out with aCaF2 plate and sent onto a solar-blind vacuum pho-todiode (Hamamatsu R1193U-04) that provided theinput energy reference for the gain measurements.Since possible nonlinear optical losses in the CaF2beam-splitter plate and the absorption in air can leadto measured input energies that are higher than theactual energy entering the amplifier, we calibratedthe input energy reference by measuring the energydirectly in front of the input window of the excimerlaser relative to the photodiode signal. To providethe dynamic range that is necessary for the inputenergy measurements we also used neutral-densityfilters in front of the photodiode. These neutral-density filters had been calibrated individually byusing an energy meter and the femtosecond pulsesfrom the ArF laser. The input-output energy char-acteristics of the ArF amplifier were obtained bymeasuring the energy of the beam after the sec-ond pass through the amplifier with an energy me-ter (Laser Precision RJ 7620) equipped with a typeRJP-735 detector head.The use of the diaphragm was also important tominimize the amplified spontaneous emission (ASE)in the probe pulse. With the diaphragm in place, thetime-integrated ASE energy barely reached the sen-sitivity threshold of the energy meter; consequently,only an upper limit of 0.2% could be obtained.A clearly visible blue fluorescence indicated thepresence of the intensity-dependent losses in the win-dow materials.' 0 -'2 These losses, however, may dis-tort the measured gain characteristics. To accountfor this effect we measured the transmission of aCaF2 window similar to the ones used in the ex-cimer laser. For the transmission measurements,the CaF2 sample was inserted into the laser beamclose to the output window where the pulses exit theamplifier after the second pass. In this way, the in-tensity on the sample was the same as on the outputwindow. The measured transmission as a functionof the energy density is shown in Fig. 2. From thisfigure we can see that the output intensity range usedfor the gain measurements extends only up to theonset of the nonlinear processes; i.e., the transmissionof the CaF2 plate decreases only slightly from itsinitial value of 85% to 70% at the highest intensity.For comparison, the transmission as measured withnanosecond pulses when no nonlinear effects areexpected is indicated by a square on the vertical axis.Although there is some controversy in the literatureas to whether the main nonlinear loss mechanismin CaF2 at 248 nm is two-photon or three-photonabsorption,"",2 considering the 10-eV band gap ofCaF2, at 193 nm the nonlinear losses are expected tobe due to two-photon absorption. Assuming that theoverall losses are small, i.e., aLL, a 2,LI02 << 1 (wherea1 and a2, are the linear and two-photon absorptioncoefficients of CaF2, respectively, L is the thicknessof the sample, and Io is the peak intensity on thesample), the transmission, defined as the ratio of theenergy of the pulse entering and leaving the CaF2plate, can be written asJ [IoA(t) - aILIoA(t) - a2,LIo2A2(t)]dtIo A(t)dtwhere A(t) is the pulse envelope. From Eq. (1)we expect a linear decrease of the transmissionwith increasing peak intensity. The measured curveclearly indicates this tendency. The solid curve inFig. 2 represents the best linear fit to the data points.This curve was used to correct the measured gaindata for window transmission. Furthermore, byusing Eq. (1) and assuming a Gaussian pulse shape,the slope of the line provides an approximatevalue of 3.1 X 10-"1 cm/W for the two-photonabsorption coefficient of CaF2 at 193 nm. It shouldbe noted, however, that the intensity-dependenttransmission can be different with samples fromdifferent suppliers. The losses in this pulse durationrange seem to be tolerable, but with decreasing pulseduration they may become prohibitive. Minimizingthese losses by using different optical materials suchas BaF2 (Ref. 6) is a critical issue in the design ofhigh-power short-pulse ArF laser systems.The measured output energy density of the ArFexcimer amplifier as a function of the input energydensity can be seen in Fig. 3. Since the small-signalgain depends on the timing of the seed pulse with re-spect to the excimer amplifier, the shot-to-shot timingjitter, if it were large enough to cause a notice-able (shot-to-shot) change in the small-signal gain,introduces some uncertainty in the gain character-istics. This uncertainty can be decreased by aver-aging a sufficiently large number of shots (in thefigure each point represents an average of 100 shots),and the reliability of the gain curves can addition-ally be checked by performing measurements for dif-ferent temporal delays. Accordingly, the measure-ments were made for two different time delays. Theexperimental points represented by circles were ob-tained for operating conditions such that the seed0.9 -C0.ECuH0.8-0.70.6 -0.5 -.. . ., . . ., . . . .0 2 4 6Energy Density (mJ/cm')8 10Fig. 2. Intensity-dependent transmission of a CaF2 laserwindow at 193 nm. The filled square on the vertical axisindicates the transmission measured by a nanosecondpulse.6 0~~~~~1v.zMarch 15, 1993 / Vol. 18, No. 6 / OPTICS LETTERS 437a-EEZI-C}U]2Qa,1412-10-8-50 100 150Input Energy Density (gJ/cm2)Fig. 3. Input-output energy characteristics of the ArFamplifier. The circles and triangles represent data fortwo different time delays of the probe pulse with respectto the amplifier. The solid and dashed curves indicatethe best Frantz-Nodvik fits to the experimental data.pulse entered the amplifier when its ASE, and conse-quently the gain, was at a maximum. The trianglesrepresent data belonging to a temporal delay thatis larger by 4 ns (which is two times the 2-ns tim-ing jitter), corresponding to a situation when theintensity of the ASE decreased to approximately halfof its maximum value. Both the input and outputenergy values are corrected for window transmis-sion by using the transmission curve in Fig. 2. Thesolid and dashed curves in Fig. 3 represent the bestFrantz-Nodvik fits to the experimental points byusing the expression given as Eq. (1) in Ref. 13. Therespective best-fit parameters for the small-signalgain coefficient and saturation energy are 0.17 cm'land 3.8 mJ/cm' (circles and solid curve) and 0.16cm'1 and 3.5 mJ/cm' (triangles and dashed curve).The small-signal gain coefficients of 0.17 and 0.16cm-' correspond to small-signal gains of 934 and 626,respectively, considering the 40-cm gain length of theexcimer laser. A comparison of the measured small-signal gains and saturation energies shows that forthe time delays used, the small-signal gain differsconsiderably, whereas the saturation energy, as ex-pected, remains the same within the experimentalaccuracy. This also demonstrates the reliability ofthe measured saturation energy values.The saturation energy density of 3.65 ± 0.15mJ/cm2 obtained for ArF is considerably higher thanthe value of 2.0 mJ/cm2 reported in Ref. 14 for KrF.This is somewhat surprising considering that the twomolecules are spectroscopically rather similar.15 Byusing the stimulated emission cross sections given inRef. 15, the calculated energy densities are also simi-lar; 3.6 and 4.2 mJ/cm2 for ArF and KrF, respectively.The difference in the measured values is mainlydue to the fact that the values presented here arecorrected for window transmission. The saturationenergy density is 2.3 mJ/cm2 for ArF, if uncorrecteddata are used, which is close to the value for KrF.A comparison of the femtosecond gain character-istics of KrF and ArF and the recently measuredgain recovery time of ArF (Ref. 16) indicates that,by utilizing sufficiently high seed pulse energies, theArF excimer laser is expected to show a performancesimilar to a femtosecond high-brightness KrF lasersystem. This is supported by the fact that wesucceeded in generating -700-fs pulses with 8-mJenergy (ASE < 3%) with double-pass amplificationwhen the full cross section of our discharge-pumpedamplifier was used, and by adding an e-beam-pumpedArF excimer power amplifier to the system, an outputpulse energy of 60 mJ was achieved.9In conclusion, we have measured the gain charac-teristics of a discharge-pumped ArF excimer amplifierby using -700-fs pulses generated by a dispersivelycompensated sum-frequency mixing scheme. Thesmall-signal gain and the saturation energy densityhave been found to be 0.17 cm-' and 3.65 mJ/cm2, re-spectively. The maximum energy density extractedfrom the saturated amplifier was 10 mJ/cm2. Sincethe demonstrated gain characteristics of ArF comparevery well with those of KrF, it seems to be feasiblethat terawatt-class femtosecond laser systems cannow be designed for 193 nm.The support of the Robert Welch Foundationgrant C-0586 and the U.S. Office of Naval Re-search is gratefully acknowledged. G. Szab6 alsoacknowledges support from the OTKA Foundationof Hungarian Academy of Sciences grant 3056 andNational Science Foundation grant INT-9020541.References1. A. J. Taylor, C. R. Tallman, J. P. Roberts, C. S. Lester,T. R. Gosnell, P. H. Y. Lee, and G. A. Kyrala, Opt.Lett. 15, 39 (1990).2. A. Endoh, M. Watanabe, N. Sarukura, and S. Watan-abe, Opt. Lett. 14, 353 (1989).3. Th. Hofmann, T. E. Sharp, C. B. Dane, P. J. Wisoff,W. L. Wilson, F. K. Tittel, and G. Szab6, IEEE J.Quantum Electron. 28, 1366 (1992).4. H. Egger, T. S. Luk, K. Boyer, D. F. Muller, H.Pummer, T. Srinivasan, and C. K. Rhodes, Appl. Phys.Lett. 41, 1032 (1982).5. S. Szatmari and F. P. Schaffer, J. Opt. Soc. Am. B 6,1877 (1989).6. J. H. Glownia, M. Kaschke, and P. Sorokin, Opt. Lett.17, 337 (1992).7. I. V. Tomov, T. Anderson, and P. M. Rentzepis, Appl.Phys. Lett. 61, 1157 (1992).8. Th. Hofmann, K. Mossavi, F. K. Tittel, and G. Szab6,Opt. Lett. 17, 1691 (1992).9. K. Mossavi, Th. Hofmann, F. K. Tittel, and G. Szab6,"Ultrahigh-brightness, femtosecond ArF excimer lasersystem," Appl. Phys. Lett. (to be published).10. A. J. Taylor, R. B. Gibson, and J. P. Roberts, Opt. Lett.13, 814 (1988).11. P. Simon, H. Gerhardt, and S. Szatmari, Opt. Lett.14, 1207 (1989).12. K. Hata, M. Watanabe, and S. Watanabe, Appl. Phys.B 50, 55 (1990).13. A. J. Taylor, T. R. Gosnell, and J. P. Roberts, Opt.Lett. 15, 118 (1990).14. S. Szatmari and F. P. Schafer, J. Opt. Soc. Am. B 4,1943 (1987).15. T. H. Dunning and P. J. Hay, J. Chem. Phys. 69, 134(1978).16. C. Momma, H. Eichmann, H. Jacobs, A. TInnermann,H. Welling, and B. Wellegehausen, "Short-pulse ampli-fication and gain dynamics of the ArF excimer laser,"Opt. Lett. (to be published).

Femtosecond gain characteristics of the discharge-pumped ArF excimer amplifier

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Femtosecond gain characteristics of the discharge-pumped ArF excimer amplifier

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