Negative thermo-optical coefficients and athermal directions in monoclinic KGd(WO_4)_2 and KY(WO_4)_2 laser host crystals in the visible region

摘要

The temperature dependence of three principal refractive indices of the biaxial KGd(WO_4)_2 and KY(WO_4)_2 crystals in the temperature range of 288-673 K was measured at wavelengths of 438.5 and 632.8 nm. Their thermo-optical coefficients (TOC) of the first and second orders were determined on the basis of the experimental data obtained. TOC were found to be negative for the n_g and n_p refractive indices of the KY(WO_4)_2 crystal and for the n_g refractive index of the KGd(WO_4)_2 crystal. It was established that in a fairly wide temperature range contributions of the positive and negative TOC to the refractive index of a light wave propagating in these crystals in certain directions compensate each other. Such athermal directions were determined for the index ellipsoid principal planes of these crystals on the basis of experimental data.