1148 J. Opt. Soc. Am. B/Vol. 9, No. 6/June 1992 Efficient LaF 3 : Nd 3 -based vacuum-ultraviolet laser at 172 nm M. A. Dubinskii,* A. C. Cefalas, E. Sarantopoulou, S. M. Spyrou, and C. A. Nicolaides Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 116-35, Greece R. Yu. Abdulsabirov, S. L. Korableva, and V V Semashko Kazan State University, 18 Lenin Street, 420008 Kazan, Russia Received June 18, 1991 Vacuum-ultraviolet (VUV) laser radiation at 172 nm has been obtained from a solid-state LaF 3 : Nd 3 +-based laser pumped by a pulsed-discharge molecular F 2 laser at 157 nm. The maximum slope efficiency of the solid- state laser described in this experiment was 21% (14% conversion efficiency), and the maximum output energy at 172 nm was 0.4 mJ for a nonoptimized optical cavity. This finding introduces serious prospects for realizing versions of active-medium-plus-source tunable VUV laser devices. INTRODUCTION Vacuum-ultraviolet (VUV) laser sources are useful devices for a variety of applications ranging from photochemistry to photolithography. However, until now only a few laser sources (such as H 2 and F 2 ), which have limited tunabili- ties, operate in this spectral region. Tunable VUV radia- tion has been obtained by a variety of methods, such as discharge excitation of atomic beams' and frequency mix- ing in gases and metal vapors, 23 but tunable VUV radia- tion can in principle be obtained from a wide class of solid-state active media, such as rare-earth-activated wide-band-gap fluoride dielectric crystals because of the allowed radiative d-f transitions. 4 Recent developments in new single-crystal wide-band-gap laser materials 57 offer encouraging prospects for the development of crystal- based efficient VUV laser devices. Among the well- known fluoride crystals LaF3 :Nd 3 (LaF:Nd) now seems to be the most suitable for VUV applications. The simpli- fied energy-level diagram of Nd 3 ions in LaF3 is shown in Fig. 1, and its excitation and emission spectra were stud- ied previously. 4 ''l 0 The strong and broad overlapping peaks of the excitation spectra (one of them centered at 159 nm) originate from transitions from the 43 ground state to the crystal field split 4f 2 5d levels of the Nd 3 ion in LaF3. However, among the five possible transitions that have been predicted by theoretical considerations concerning the C 2 site symmetry of the Nd 3 in LaF3, only four were observed. 4 The observed VUV fluorescence originates from the 4f 2 5d 4f3 electric dipole-allowed transitions. They have been assigned to the d -> 4I1/2 and 5d -- 4I13/2 interconfigurational transitions. 8 The fluorescence spectrum seems to have a simple structure at room temperature, 9 and the VUV quantum yield of the Nd3+ ions in the LaF3 host is more than 50%.8 The de- pression of the 4f 2 5d levels in LaF3 caused by the crystal field is the lowest among the known fluorides, and the onset of the 4f 2 5d-configuration levels is observed at fre- quencies as high as 60 000 cm-'. Therefore, the LaF: Nd laser should have one of the shortest wavelengths of all possible Nd3'-activated VUV lasers, provided that the ap- propriate pumping is available. Direct evidence of VUV laser action in LaF: Nd was obtained recently, 9 " 0 and this was the first solid-state laser in the VUV region of the spectrum. However, the method of excitation of the active medium that was used in Refs. 9 and 10 (the excita- tion of the active medium was due to fluorescence of Kr 2 * caused by an electron beam) was too complicated to be widely used at the laboratory scale. Therefore a simple and reliable pumping source could enhance the use of these sources for spectroscopic and other applications. Recently" we obtained laser action at 172 nm from LaF3: Nd 3 crystals pumped by a molecular F 2 laser at 157 nm with a 3% conversion efficiency. In this paper we report the observation of a 14% conversion efficiency with a 21% slope efficiency [defined as the tan(AE 0 /AE), where AE,, is the change of the output energy when the input energy changes by AEj near threshold] laser action at 172 nm from a LaF: Nd crystal transversely pumped by a molecular F 2 laser at 157 nm. The energy conversion of this system is determined, and the processes that limit the energy available from the crystal are discussed. EXPERIMENT The LaF: Nd crystals used in this experiment were grown from carbon crucibles by the Bridgman-Stockbarger method. Because the VUV transparency of the samples depends strongly on the amount and the types of impuri- ties, during the growth process all possible methods of initial products and growth atmosphere refining were used. The laser samples, cylindrical rods 35 mm long with 5-mm radii, were pumped transversely, and the ends of the laser rods were optically flat and parallel with 10 s of arc. A polished flat window was made on the side of the cylindrical surface of the crystal for efficient excita- tion by the pumping laser source. The concentrations of the Nd3+ ions used in these crystals were between 0.3% 0740-3224/92/061148-03$05.00 © 1992 Optical Society of America Dubinskii et al.