Appl. Phys.15, 239--24I (1977) Applied Physics 9by Springer-Verlag1978 Generation of Coherent Radiation at 89.6 nm Through Two-Photon Resonant Phase-Matched Tripling of Fourth-Harmonic Nd" Glass Laser Radiation in Hg Vapors V. V. Slabko, A. K. Popov, and V. F. Lukinykh L. V. Kirensky Institute of Physics, USSR Academy of Sciences Siberian Branch, SU-660036 Krasnoyarsk, USSR Received 24 August 1977/Accepted4 October 1977 Abstract. Coherent radiation at 89.6nm has been obtained through third-harmonic conversion of the 268.8 nm pulses in mercury vapors. Tunability of the fourth-harmonic frequency of the Nd: glass laser radiation to the 6slSo-8saSo double quantum resonance of Hg atoms has been used for the resonant enhancement of the twelfth-harmonic output. Possibility to phase match the generated radiation and driving polarisation in pure vapors without employing of bufer gas has been demonstrated. PACS: 42.65, 32 The generation of coherent vacuum ultraviolet (VUV) radiation by means of optical frequency upconversion using third- and higher-order nonlinearities of atomic and molecular gas-phase media has attracted consider- able interest in recent years. By these means the shortest-wavelength coherent radiation in the VUV and soft X-range (XVUV) at 88.7, 53.2 and 38 Enm] was produced to date [1,2]. This radiation has been ob- tained through nonresonant harmonic generation and without phase matching with a conversion efficiency of the order of 10-5+ 10-7. Further improvements and potentialities of the frequency conversion in gases as a practical source of coherent XVUV light are de- termined by achieving phase matching and resonant enhancement [33 of nonlinearities espessially on the basis of familiar and available pumping lasers. It has been proposed recentIy [4-6] to generate 42XVUV wavelengths in the range from 180.5 to 70.8 [nm] due to phase matched two-photon resonant sum and difference frequency mixing on the basis of only 1 + 5 harmonics of Nd : glass laser radiation, and third-order nonlinear processes in Hg vapors and Kr gas. These harmonics of high power can be obtained with high efficiency using contemporary techniques of nonlinear optics. The proposed method rests on the feasibilities to tune the frequency of the fourth har- monic of this laser to the strong 6slSo-8slSo double- quantum resonance of Hg atoms and the fifth harmonic-through the four 4p 6 1S0-2p5,6,7,8 double- quantum resonances of Kr atoms. It is shown that a quantum conversion efficiency of about 50 % for the each wavelengths can, in principle, be achieved in the mentioned resonant media [5]. The employment of the high-order resonant nonlinearities based on these res- onances extends generated radiation to the soft x-ray range. Using an additional beam of dye-laser radiation pumped by the harmonics ofa Nd: glass laser provides the possibility of producing tunable XVUV radiation. In this paper we report on preliminary results of the experimental examination of the proposed conversion schemes for the shortest wavelength predicted in Hg. In this first step we have not tried to get spectral narrowing of the incident fundamental light and good conversion efficiency from 1075.5 nm to 268.8 nm. A block diagram of the experiment is shown in Fig. I. The frequency of self mode-locked Nd glass laser 1 is tuned within its gain bandwidth in the range of 0340-3793/78/0015/0239/$01.00