Dispersion properties of GaSe1-x S x in the terahertz range

Abstract

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-

Doped GaSe crystals for laser frequency conversion

“…Six types of frequency interactions can be realized in such an OPG, providing rich possibilities for maximizing generation efficiency, especially if we bear in mind the strong anisotropy of the mid-IR 82 and THz 51 absorption, and the possibility of temperature controlled tuning. Moreover, the uncommon ee-e type of three-wave interaction can be realized 80 . Figure 13b shows the PM for ee-e type DFG, by a two-frequency CO 2 laser, is realized at large PM angles, i.e., with low efficiency due to the GaSe lattice symmetry.…”

Model studies of THz-range generation by down-conversion in GaSe and GaSeS crystals

“…This difference in absorption loss leads to a higher efficiency of THz e-wave generation 8,34 . It was also predicted and confirmed experimentally that the uncommon ee-e type of interaction can be realized in pure and S-doped GaSe crystals 15,16 . Successful design of THz sources calls for adequate data on PM possibilities and potential efficiencies for all possible three frequency interactions.…”

“…It allowed easier processing at arbitrary directions and improves frequency conversion efficiency. Modified properties and improved efficiencies are reported for a number of doped crystals: light (GaSe:S) and heavely S-doped GaSe crystals that also referred to as solid solution crystals GaSe:GaS (GaSe 1-x S x , where x is mixing ratio) 12,13,14,15,16 , GaSe:In and Ga 1-x [36][37][38][39][40] , which consider some other double element doped GaSe crystals. Strengthened structuregives opportunity of the application in out-of-door applied systems 41 .…”

Aspects for efficient wide spectral band THz generation via CO2laser down conversion

“…This difference in absorption loss leads to a higher efficiency of THz ewave generation 9,28 . It was also predicted and confirmed experimentally that the uncommon ee-e type of interaction can be realized in pure and S-doped GaSe crystals 14,15 . Successful design of THz sources calls for adequate data on PM possibilities and potential efficiencies for all possible three frequency interactions.…”

“…Strengthened structure gives opportunity of the application in out-of-door applied systems 10 . Modified properties and improved efficiencies are reported for a number of doped crystals: light (GaSe:S) and heavely S-doped GaSe crystals or so called solid solution crystals GaSe 1-x S x (GaSe:GaS) 11,12,13,14,15 , GaSe:In and Ga 1-x In x Se 16,17,18,19,20 , GaSe:Te and GaSe 1-x Te x 18,21,22 , doped GaSe:Er 23,24 and GaSe:Al 25 crystals. Increased frequency conversion efficiency is recorded for frequency conversion into both mid-IR 12,16,17,19 and THz 26,27,28 range.…”