Gallium selenide nonlinear optical crystals are effective up- and down-converters of terahertz and infrared frequencies. Their nonlinear characteristics at the telecommunication wavelength have not received enough attention, particularly for sulfur-doped crystals. In order to close this gap, we put forward data on the optical and electro-optical characteristics of GaSe(1-x)Sx crystals (where x = 0, 0.03, 0.12, 0.16, and 0.22) at the wavelength of 1.55 micrometers and in the terahertz range. This study provides an estimate of the GaSe:S crystal’s nonlinear coefficient d22(ω1, ω2, ω3) responsible for the terahertz wave generation by means of optical rectification and difference frequency generation, where ω1 ≈ ω2 – corresponds to the near infrared range, and ω3 belongs to the subterahertz frequencies. We assume that the value of d22 lies within the range from 13.9 to 20.5 pm/V for the wavelengths of 0.63 to 1.55μm. The potential of S-doped GaSe crystals to be used as nonlinear optical converters for photonic devices operating at telecom wavelengths is discussed.
Based on electro-optical measurements in the THz region and comparison with data from other authors, estimates of the nonlinear optical coefficient of GaSe crystals with varied degrees of sulfur doping are provided. The d22 value for GaSe was 13.9 pm/V at a laser wavelength of 1.55 microns. The acquired data are compared with the values of nonlinear coefficients for undoped GaSe samples provided in the works of other authors. The qualitative correspondence of experimental measurements of the nonlinearity coefficient and the empirical model based on Miller's rule in the range of 0.63 – 1.55 microns is shown. The inaccuracy of theoretical curve in comparison to the experimental one is around 10%. The techniques described in the paper for calculating the nonlinear optical coefficient's magnitude and the results obtained will make it possible to establish the foundation for the creation of radiophotonics devices based on unalloyed and sulfur-doped GaSe crystals.
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