Scattering in sapphire test masses for gravitational wave detectors

Crystalline sapphire is proposed as a test mass material for advanced and cryogenic gravitational wave detectors. While having many advantageous properties, its optical properties are not well understood. Here we present observations of the angular dependence of the amplitude of optical scattering in single crystal sapphire samples. We show that the scattering is a strong function of the incident light polarization relative the to crystal axis. Although the data are suggestive of dipole radiation associated with the polarizability of scattering centres in anisotropic crystals, we found that the scattering matrix is neither diagonal nor symmetric. Results show that for test masses with the incident beam along the a-axis of the crystal it is advantageous to align the incident light polarization perpendicular to the crystal optical axis (the c-axis) to minimize the total scattering.