BaGa2GeX6(X=S, Se): New mid-IR nonlinear optical crystals with large band gaps

New middle infrared nonlinear optical crystals of BaGa2GeX6(X=S, Se) have been synthesized. The structures of both BaGa2GeS6 and BaGa2GeSe6 were found to be isomorphous exhibiting a three-dimensional framework constructed by the parallel alignment of Ga or Ge tetrahedron groups along the c-aix. The ultraviolet transmittance spectrum indicates the short-wavelength absorption edges of BaGa2GeS6 and BaGa2GeSe6 are 380 nm and 440 nm, respectively. The IR transmittance spectrum shows that the absorption edge of the BaGa2GeS6 crystal is 13.7 μm. Second-harmonic generation of the powder samples was measured with a 2.05 μm laser using the Kurtz and Perry technique, which showed that BaGa2GeS6 and BaGa2GeSe6 are phase-matched materials, and their measured SHG coefficient is ∼2.1 and ∼3.5 times as large as the coefficient of AgGaS2, or 26.3 pm/V and 43.7 pm/V.