Thermo-optic dispersion formulas for monoclinic double tungstates K Re(WO 4) 2 where Re = Gd, Y, Lu, Yb

Abstract

Thermooptic coefficients dn/ dT for monoclinic potassium (rare-earth) double tungstates K Re(WO 4) 2 (where Re = Gd, Y, Lu, Yb) were determined at eight wavelengths in the range of 0.3–1.1 μm by a laser beam deviation method. In the long-wavelength range all the dn/ dT values were found to be negative and near constant, while dn/ dT( λ) dependence is especially pronounced in the short-wavelength range where thermooptic coefficients change sign to positive. Linear thermal expansion coefficients were measured in K Re(WO 4) 2 along the optical indicatrix axes by a dilatometric technique. Volumetric thermal expansion effect was found to dominate over the bandgap change influence on temperature dependence of the refractive index. Thermooptic dispersion formulas were derived for K Re(WO 4) 2 as series in 1/ λ 2. The influence of Yb and Nd doping on the dn/ dT values was investigated, resulting in prediction of thermooptic coefficients evolution in the isostructural KY x Yb (1− x )(WO 4) 2 and KYb x Lu (1− x )(WO 4) 2 series.