Estimating the Nonlinear Optical Coefficient Responsible for Interaction between Telecom and Terahertz Waves or GaSe:S Crystals

GaSe crystals as a particular case of van der Waals materials attract increased interest of researchers and developers in the fields of terahertz optoelectronics and integrated photonics. From the engineering viewpoint, the unambiguous knowledge of the nonlinear optical coefficient, in particular, at telecom wavelengths, is important. Low attention has been paid to its study, and the available data are strongly diverse. In this paper, the independent estimation of the nonlinear optical coefficient d(22)(eo) responsible for interaction between the frequencies of optical (including telecom) and terahertz ranges has been performed for GaSe1-xSx crystals (where x = 0, 0.03, 0.12, 0.16, and 0.22). Based on the set of data obtained by electooptical measurements and the predictions based on the Miller's rule, it has been shown that d(22)(eo) for crystals with different doping is less than 20 pm/V for near-infrared wavelengths. This value is up to 2 times lower than the value reported by other authors.