AD-A264 493 AiI(cid:2)IVI IlI Contract N000oo4-87-1%-20 16 I iI Naval Research Laboratory and Strategic Defense Initiative Organization Research Studies on Short Wavelength by Selective Auger Processes Final Technical Report for the period ending December 31, 1990 DTIC ELECTE m MAY 1 9 1993 Principal Investigator: S. E. Harris UEdward A L. Ginzton Laboratory Stanford University Stanford, CA 94305 Contract Monitor: Dr. Paul Kepple Itor COTR, Code 46720 Thim dcameni hbas been approv(cid:127)d Naval Research Laboratory public I).ecse and salt; Its 4555 Overlook Ave., S. W. disuiution is untimit. J Washington, DC 20375-(cid:127)6% January 1993 93-110i9 llI7 lll iIlI H ill lIllil €, " R" ' ." g 1. ENTODUCTION Efforts during this contract period are divided into two areas: (1) development of short wavelength coherent sources and (2) studies of lasers without inversion. In the first area, gain in the vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) was observed in the in Xe, Zn, Kr, Cs and H2 gases. Methods for saturating the laser transitions in Xe and Cs were developed and produced microjoule level fully saturated outputs at 109 nm and 96.9 nm, respectively. Efforts to construct the next generation laser driver for such studies were also completed during this time period. In the second area, theoretical studies of different methods for creating transparency in atomic vapors were explored. Various applications of these transparencies were investigated, including the possibility of systems which would exhibit laser gain without the necessity for population inversion and the enhancement of nonlinear optical processes in the presence of such a transparency. Experiments demonstrated enhanced sum-frequency generation of 104.8-nm radiation with a naturally occurring transparency in Zn vapor. 2 2. DEVELO)PN~ENT OF COHERENT SHORT WAVELENGTH SOURCES 2.1 Xenon 109-nm Laser Saturation of the Xe III 109-nm laser was accomplished using a traveling-wave laser-produced-plasma excitation. The laser was pumped by soft x-rays emitted from a laser-produced plasma. Using a 3.5-J, 300-ps, 1064-nm laser pump pulse, we measured a small-signal gain of exp (40). The laser was fully saturated and produced an output energy of 20 VJ in a beam with 10-r,-rad divergence. Studies of prepulsing to increase the efficiency of the laser-produced- plasma pumping were conducted. By using a low energy prepulse we were able to enhance the soft-x-ray emission of the laser-produced plasma in the parameter range used to pump the Xe III laser and other photoionization- pumped lasers. Conversion efficiency and output pulse duration as a function of input intensity, pulselength, and prepulse conditions were determined. With increased laser-produced plasma efficiency and better traveling-wave excitation it was possible to create a 2-Hz 109-nm mirrorless laser. This new device had a single pass gain of exp (33) and a saturated output energy of 1 gJ. The laser required only 500 mJ of 1064-nm energy in a 500-ps pump pulse. Because of the high repetition rate of this source it was possible to perform two slit coherence experiments and to test the focusability of the laser output. Intensities of greater than 109 W/cm 2 were produced at 109 nm. 3 Gain at 90 nm in analogous system in Kr III was also observed during several of these studies. However, the total gains were small and saturated output was never obtained. 2.2 Zinc I Laser Gain in a super Coster-Kronig-pumped Zn III laser system was demonstrated. Using a 5-J, 300-ps pump pulse of 1064-nm radiation, we observed gain on transitions in Zn III at 127.0, 130.6, and 131.9 nm with total gains of exp (2.4), exp (5.1), and exp (3.2), respectively. The large branching ratios of the rapid super Coster-Kronig decay into a small number of final levels made high-efficiency operation possible. However, because of the longer wavelength and the increased complexity of working with atomic Zn vapor, further study and saturation of this system was not attempted. 2.3 Cesium 96.9-nm Laser Fully saturated operation of a 96.9-nm laser transition in neutral Cs vapor was demonstrated. This was the first operation of a new class of XUV lasers in which the upper level of the laser transition was embedded in the continuum of the valence electron. The laser employed a grating- assisted traveling-wave geometry that created an - 20-ps-long pulse of laser-produced soft x-rays traveling synchronously with the generated 96.9-nm radiation. In order to create such a shG*,t pumping pulse, a new short pulse laser source for producing excitation plasmas was developed. This laser system was based on the concept of chirped pulse amplification 4 and was the first demonstration of multijoule output using this concept. Typical system parameters were 3 J in 10 ps at 1064 nmn. With this pumping source the gain coefficient of the Cs 96.9-nm laser system was as high as 4.9 cm"1 over a 17 cm length. This results in a very high extrapolated small signal gain of exp (83). After about 4 cm, the output energy grew linearly with length indicating that the laser transition was fully saturated. The total output at 96.9 nm was - 1.5 ýiJ. Several studies were conducted to investigate the spectroscopy and atomic physics of this system. Other experiments investigated the nature of the pumping mechanism. It was concluded that, unlike the Xe, Kr and Zn systems, this laser was pumped by photo-generated electrons which were the by-product of the partial photoionization of the Cs vapor by soft x-rays from the laser- produced plasma. This new electron pumping scheme is capable of producing very high excitation current densities on the order 106 A/cm2 . Implementation of this pumping scheme to traditional electron-pumped molecular lasers resulted in saturated output in the VUV and deep UJV when hydrogen and nitrogen were used in place of Cs. 2.4 Molecuar Hydron l6-nmL Using the photoionization electron source similar to that described above, a 116-nm laser in the Werner band of molecular hydrogen was demonstrated. Studies indicated that, even though the free electrons have an average temperature of - 10 eV, the hydrogen molecules retain an ambient temperature of - 0.01 eV. This allowed an extrapolated small- signal gain of exp (43) with a 1064-nm pumping energy of 580 mJ in 200 ps. Several longer-wi-vclcngth transitions in thp Werner ari Iyzrnýr baads also exhibited saturated output. 5 2.5 High Pek Power Laser Development The next generation laser plasma driver was developed. This system was based on amplification in Ti-doped sapphire. At its completion it was one of only two in the world to be able to produce terawatt level 125-fs infrared pulses. The system was again based on chirped pulse amplification. The laser output at 807 nm contained 60 mJ of energy in a 125 fs pulse. The system produced a nearly diffraction-limited beam that was focusable to an intensity of 1018 W/cm2 with an ff6 optic. 6 3. STUDIE OF LASER WrTOUT LNNERSION 3.1 Interference of Dressed A~fetime-Broadened States The use of a coupling electromagnetic field to provide a general method for producing inversion-free lascr systems was investigated. The interference between dressed states produces a zero in absorption while allowing gains of that of the uncoupled system. This important concept allows interferences between nearly arbitrary states to be created with a tunable coupling laser, thus remnoving the atomic constraints from many experiments. 3.2 Singe-Atom Transient Resvnse The effect of the transient response on the dynamics of lifetime- broadened lasers that operate without the need for population inversion was investigated. A relationship between the steady-state absorption transition probability rate and the transient gain and loss was determined. This relationship often implies that, in order to achieve lasing without inversion, the rate into the lower level must be less than that into the upper level. 3.3 Closed Lifetime Systems A model three-state laser system which operates by an electro- magnetically induced interference was studied. Provided that an in version condition for the thermal radiation field is satisfied, the system lases without atomic population inversion in steady state. The system is pumped by incoherent radiation on the transition on which lasing occurs. 7 3.4 Nonlinear Optics Within an Absorption Window Besides the possibility of lasers without inversion, the types of transparency described above also have applications in nonlinear optics. Two autoionizing levels which are separated by a few decay widths may exhibit a sharp interference or window in their absorption profile and also make canceling contributions to the refractive index at the absorption minimum. A correct choice of intermediate mixing levels prevents a similar cancellation in the nonlinear susceptibility. Using UV lasers with energies of about a millijoule and pulse lengths of 5 ns, 0.23 AJ per pulse at 104.8-nm radiation was generated in zinc vapor. 3.5 Nonlinear Optics and Electromagnetica&l Induced Transnarency While the previous nonlinear optics experiment relied on a naturally occurring transparency this need not be the case. By applying a strong coupling field between a metastable state and the upper state of an allowed transition to ground one may obtain a resonantly enhanced third-order susceptibility while, at the same time, inducing transparency of the media. Theoretical studies indicated that an improvement in conversion efficiency and parametric gain, as compared to weak-coupling field behavior, of many orders of magnitude is possible. 8 4. LISEO F PUBLICATIONS 1. M. H. Sher, J. J. Macklin, J. F. Young, and S. E. Harris, "Saturation of the Xe III 109-nm Laser Using Traveling-Wave Laser-Produced- Plasma Excitation," Opt. Lett. 12, 891-893 (November 1987). 2. D. J. Walker, C. P. J. Barty, G. Y. Yin, J. F. Young, and S. E. Harris, "Observation of Super Coster-Kronig-Pumped Gain in Zn III," Opt. Lett. 12, 894-896 (November 1987). 3. C. P. J. Barty, D. A. King, G. Y. Yin, K. H. Hahn, J. E. Field, J. F. Young, and S. E. Harris, "12.8 eV Laser in Neutral Cesium," in Short Wavelength Coherent Radiation: Generation and Applications, edited by Roger W. Falcone and Janos Kirz (Washington DC, Optical Society of America, 1988), vol. 2, pp. 13-20. 4. M. H. Sher and S. J. Benerofe, "Prepulsingt o Increase the Efficiency of Laser-Produced-PlasmaP umped Lasers," in Short Wavelength Coherent Radiation: Generation and Applications, edited by Roger W. Falcone and Janos Kirz (Washington DC, Optical Society of Aj ýrica, 1988), vol. 2, pp. 60-64. 5. C. P. J. Barty, D. A. King, G. Y. Yin, K. H. Hahn, J. E. Field, J. F. Young, and S. E. Harris, "12.8-eV Laser in Neutral Cesium," Phys. Rev. Lett. 61, 2201-2204 (November 1988). 6. S. E. Harris and J. J. Macklin, "Lasers Without Inversion: Single- Atom Transient Response," Phys. Rev. A 40, 4135-4137 (October 1989). 7. A. Imamoglu and S. E. Harris, "Lasers Without Inversion: Interference of Dressed Lifetime-Broadened States," Opt. Lett. 14, 1344-1346 (December 1989). 8. A. Imamoglu, J. J. Macklin, and S. E. Harris, "Interference of Lifetime Broadened Resonances: Nonreciprocal Gain and Loss Profiles," in New Frontiers in Quantum Electrodynamics and Quantum Optics, edited by A. 0. Barut (New York, Plenum Press, 1990), pp. 223-229. 9. C. P. J. Barty, G. Y. Yin, J. E. Field, S. J. Benerofe, J. F. Young, and S. E. Harris, "Photo-Electrona nd Photoionization Pumping of XUV Lasers by Laser Produced Plasmas,"i n X-Ray Lasers 1990, edited by G. J. Tallents (New York, Institute of Physics, 1990), pp. 21-28. 10. S. E. Harris, J. E. Field, and A. Imamoglu, "Nonlinear Optical Processes Using Electromagnetically Induced Transparency," Phys. Rev. Lett. 64, 1107-1110 (March 1990). 9 11. K. H. Hahn, D. A. King, and S. E. Harris, "Nonlinear Generation of 104.8-nm Radiation Within an Absorption Window in Zinc," Phys. Rev. Lett. 65, 2777-2779 (November 1990). 12. M. H. Sher, S. J. Benerofe, J. F. Young, and S. E. Harris, "2-Hz 109- nm MirrorlessL aser," J. Opt. Soc. Am. B 8, 114-116 (January 1991). 13. J. D. Kmetec, J. J. Macklin, and J. F. Young, "0.5-TW, 125.fs Ti:Sapphire Laser," Opt. Lett. 16, 1001-1003 (July 1991). 14. A. Imamoglu, J. E. Field, and S. E. Harris, "Lasers Without Inversion: A Closed Lifetime Broadened System," Phys. Rev. Lett. 66, 1154-1156 (March 1991). 15. S. J. Benerofe, Guang-Yu Yin, C. P. J. Barty, J. F. Young, and S. E. Harris, "116-nm H2 Laser Pumped by a Traveling-Wave Photoionization Electron Source," Phys. Rev. Lett. 66, 3136-3139 (June 1991). 10