The optical characteristics of the nonlinear optical single crystal CdSiP2 in the terahertz and infrared ranges

Abstract

Experimental study of mechanisms forming absorption of Terahertz (THz) radiation in the advanced nonlinear-optical crystal CdSiP2 has been performed by backward-wave oscillator, time-domain and infrared spectroscopy. The classical oscillator model and Lyddane–Sachs–Teller relation were used to calculate the dispersion parameters of phonon modes. The spectra of complex permittivity and absorption coefficient have been determined based on the calculated dispersion parameters. The application of the Fresnel formulas for the absorption analysis in the THz region together with the results of the fit of the phonon modes made it possible to obtain quantitative data on the magnitude of the additional radiation losses in the THz range. Additional loss mechanisms give rise to increase the total absorption by at least one order of magnitude compared to the phonon contribution. The increase of the polariton absorption in the low-frequency dispersion branch is typical for crystals with ionic and ion-covalent bond and is composed of extrinsic losses and difference multi-particle processes. The level of total losses in the THz region in CdSiP2 single crystal is comparable to the values in other nonlinear-optical crystals, which makes this material promising for nonlinear-optical conversion of the infrared radiation into the THz region.