Theoretical material properties of the nonlinear optical crystals: disordered barium gallium chalcogenides

Disordered barium gallium chalcogenides, including BaGa 2 MQ 6 (M=Si, Ge, Q=S, Se) and Ba 2 Ga 8 GeS 16 (B2GGS), are becoming emerging nonlinear optical (NLO) materials capable of generating mid-infrared pulses up to 18 µm. Though showing great potential in powerful frequency conversion applications, the key parameters of the NLO coefficients of those materials are still in dispute, and most other relevant physical properties are yet to be determined. In the present first-principles theoretical study, a simple approach of treating the statistical mixing of the Ga/M sites was developed by lowering the crystal symmetry to the primitive unit cell and then transforming the calculated property tensor matrices back to the reported structures. The calculated NLO coefficients agree with the most recent measurements for BaGa 2 GeSe 6 (BGGSe) and BaGa 2 GeS 6 (BGGS) quite well after rotations of certain angles from the X axis, therefore providing a baseline to solve the confusions of different measurements and the previous theoretical work. Furthermore, physical properties of dielectric, piezoelectric, elastic, and electro-optical (EO) constants are all obtained simultaneously for the above disordered systems. It is found that crystals in the BaGa 2 MQ 6 system have reasonably large piezoelectric constants, the largest being d 33 =−15.53pC/N for BGGS, and B2GGS possesses a pretty large EO constant, γ 33 S =4.67pm/V, showing potential in terahertz (THz) generation and sampling applications.