We grew nonlinear crystals of the solid solutions GaSe 1-x S x (x ≤ 0.4) by the vertical Bridgman method. The increase in hardness from 8 kg/mm 2 for x = 0 to ~20 kg/mm 2 for x = 0.4 as a result of the presence of sulfur in the GaSe crystals allowed us to use a special technology to make working samples with position of the optic axis in the plane of the entrance surfaces, and for the first time to make direct measurements of the dispersion properties n e (λ) for the extraordinary wave and n o (λ) for the ordinary wave in the terahertz range of the spectrum by pulsed terahertz spectroscopy. We show that it is possible to realize an unconventional ee-e type of interaction in generation of terahertz radiation.Introduction. The unique set of physical properties in nonlinear GaSe crystals, responsible for the efficiency of parametric frequency conversion processes for converting radiation in the near IR and mid-IR ranges to the terahertz range of the spectrum [1, 2], has attracted steady attention from many researchers and developers. Unfortunately, the extremely poor mechanical properties (almost zero hardness on the Mohs scale and easy cleavage) limit use of these layered crystals of symmetry point group 6 _ 2m to intralaboratory applications. Recently, with the aim of expanding nonlinear optics applications, the mechanical properties of GaSe crystals have been significantly improved as a result of doping with Group III and Group IV elements of Mendeleev's Periodic Table (Al [3], S [4], In [5-7], Te [8,9], Er [10]) and growing the corresponding crystals of the solid solutions, and also by growing crystals from GaSe:AgGaSe 2 [6] and GaSe:AgGaS 2 [11] melts. In contrast to GaSe crystals, AgGaSe 2 and AgGaS 2 crystals have symmetry point group 4 _ 2m. It has been established that besides the mechanical properties, other key physical properties of GaSe crystals can be controllably modified by selection of the sulfur content. Introducing large concentrations of sulfur, indium, and tellurium (leading to a change in the lattice parameters) have the best possibilities in this regard; in other words, growing nonlinear crystals of solid solutions according to the chemical formulas GaSe:GaS → GaSe 1-x S x [4], [12][13][14][15][16][17][18][19], GaSe:InSe → Ga 1-x In x Se [5,7], and GaSe:GaTe → GaSe 1-x Te x [8,9]. Introducing small sulfur ions eliminates cleavage defects in the GaSe crystals and reduces linear optical losses in the region of maximum transparency (optimal doping level, 2-3 wt.%), and also increases the thermal conductivity severalfold orthogonal to the growth layers, as a result of substitution of selenium ions, filling vacancies, and intercalation in the interlayer space. As a result of these changes, the radiation resistance relative to nanosecond pump pulses increases by 20%-30%. The increase in the mixing ratio to x ≤ 0.4 shifts the short-wavelength edge of the transmission spec-

Second-harmonic output at 4.6-5.5 mum of the order of 6 mW with a 0.12% external conversion efficiency has been obtained by pumping a AgGaSe(2) crystal with a low-power cw CO(2) laser. The surface damage threshold of AgGaSe(2) for cw radiation was found to be inside the limit of 33-45 kW/cm(2) in the 9.2-10.8-mum wavelength region. Another important limitation of the pump power connected with a thermal lensing effect in crystal was determined experimentally. A comparison was made of AgGaSe(2) and ZnGeP(2) crystals as materials suitable for the efficient generation of the second harmonic of cw CO(2) laser radiation.

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