Optical Nonlinearity in Cu<sub>2</sub>CdSnS<sub>4</sub> and α/β-Cu<sub>2</sub>ZnSiS<sub>4</sub>: Diamond-like Semiconductors with High Laser-Damage Thresholds

Cu<sub>2</sub>CdSnS<sub>4</sub> and α/β-Cu<sub>2</sub>ZnSiS<sub>4</sub> meet several criteria for promising nonlinear optical materials for use in the infrared (IR) region. Both are air-stable, crystallize in noncentrosymmetric space groups, and possess high thermal stabilities. Cu<sub>2</sub>CdSnS<sub>4</sub> and α/β-Cu<sub>2</sub>ZnSiS<sub>4</sub> display wide ranges of optical transparency, 1.4–25 and 0.7–25 μm, respectively, and have relatively large second-order nonlinearity as well as phase matchability for wide regions in the IR. The laser-damage threshold (LDT) for Cu<sub>2</sub>CdSnS<sub>4</sub> is 0.2 GW/cm<sup>2</sup>, whereas α/β-Cu<sub>2</sub>ZnSiS<sub>4</sub> has a LDT of 2.0 GW/cm<sup>2</sup> for picosecond near-IR excitation. Both compounds also exhibit efficient third-order nonlinearity. Electronic structure calculations provide insight into the variation in properties.