Optical and Electro-Optical Properties of Sn2P2S6

We report on the rotation of the indicatrix in the mirror plane and the refractive indices of Sn₂P₂S₆ single crystals in the wavelength range 550-2200 nm at room temperature. Electro-optical properties of Sn₂P₂S₆ were also measured by direct interferometric technique for electric fields applied parallel to the crystallographic x-axis that lies near the spontaneous polarization vector. The room temperature free electro-optical coefficient r₁₁₁ ^T reaches 174plusmn10 pm/V at lambda=633 nm and shows only weak dispersion in the wavelength range between lambdaap0.6... 1.3mum. The temperature dependence of the electro-optical coefficients near the structural phase transition at T_C=65degC is well described by a Curie-Weiss law with a peak value r₁₁₁ ^Tcong4500 pm/V