All-space existence and dispersion of athermal directions in monoclinic KY(WO4)2

The analytical expressions for thermo-optic coefficients, dn/dT, for “fast” and “slow” light waves propagating along the arbitrary direction in a biaxial crystal are derived. On the basis of these expressions, the all-space analysis of existence of athermal directions is performed for monoclinic and biaxial KY(WO4)2 at 1.03 μm. The calculations are performed for an arbitrary light propagation direction and polarization (not restricted to the principal planes). The appearance of directions that can be athermal for both “fast” and “slow” waves is predicted. The dispersion of athermal directions is analyzed for visible and near-IR. The existence of upper and lower dispersion limits for athermal behavior of KY(WO4)2 is shown. It is shown that the optical indicatrix axes can be athermal itself at some light wavelengths. Wavelength- and temperature-dependent position of the optical axes of KY(WO4)2 is also determined.