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-

Abstract. There are only four lidar stations in the world which have almost continuously performed observations of the stratospheric aerosol layer (SAL) state over the last 30 years. The longest time series of the SAL lidar measurements have been accumulated at the Mauna Loa Observatory (Hawaii) since 1973, the NASA Langley Research Center (Hampton, Virginia) since 1974, and Garmisch-Partenkirchen (Germany) since 1976. The fourth lidar station we present started to perform routine observations of the SAL parameters in Tomsk (56.48° N, 85.05° E, Western Siberia, Russia) in 1986. In this paper, we mainly focus on and discuss the stratospheric background period from 2000 to 2005 and the causes of the SAL perturbations over Tomsk in the 2006–2015 period. During the last decade, volcanic aerosol plumes from tropical Mt. Manam, Soufrière Hills, Rabaul, Merapi, Nabro, and Kelut and extratropical (northern) Mt. Okmok, Kasatochi, Redoubt, Sarychev Peak, Eyjafjallajökull, and Grímsvötn were detected in the stratosphere over Tomsk. When it was possible, we used the NOAA HYSPLIT trajectory model to assign aerosol layers observed over Tomsk to the corresponding volcanic eruptions. The trajectory analysis highlighted some surprising results. For example, in the cases of the Okmok, Kasatochi, and Eyjafjallajökull eruptions, the HYSPLIT air mass backward trajectories, started from altitudes of aerosol layers detected over Tomsk with a lidar, passed over these volcanoes on their eruption days at altitudes higher than the maximum plume altitudes given by the Smithsonian Institution Global Volcanism Program. An explanation of these facts is suggested. The role of both tropical and northern volcanic eruptions in volcanogenic aerosol loading of the midlatitude stratosphere is also discussed. In addition to volcanoes, we considered other possible causes of the SAL perturbations over Tomsk, i.e., the polar stratospheric cloud (PSC) events and smoke plumes from strong forest fires. At least two PSC events were detected in 1995 and 2007. We also make an assumption that the Kelut volcanic eruption (Indonesia, February 2014) could be the cause of the SAL perturbations over Tomsk during the first quarter of 2015.

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