Theoretical Evaluation of Terahertz Sources Generated From SnGa₄ Q₇ (Q=S, Se) as Infrared Nonlinear Optical Materials

We theoretically evaluated the integrated knowledge that contributes to conversion efficiency, including the phonon, photon, and electron properties of infrared nonlinear optical materials such as SnGa₄ Q₇ (Q=S, Se), which are terahertz (THz) sources. Specifically, we developed a new formula to calculate the susceptibility of the difference frequency generation (DFG) optical process. By evaluating the characteristics of the materials themselves in the THz region, we found that a larger nonlinear susceptibility or a large figure of merit resulted in a large efficiency of the THz source by comparing the findings of SnGa₄ Se₇ and SnGa₄ S₇ under the same experimental conditions; furthermore, THz absorption was found to reduce the efficiency of the THz source for the two SnGa₄ Q₇ (Q=S, Se) materials. The efficiency of the THz source also depended on the experimental conditions. A large crystal size, strong pump intensity, and small THz wavelength resulted in better efficiency of the THz source based on the DFG process. The efficiency was found to be a comprehensive index to evaluate the THz source based on the DFG process.