-based mixed oxide coatings for gravitational wave detectors*

Thermal noise is a fundamental limitation in optical interferometry experiments as thermally driven fluctuations cause variations in the optical path. The sensitivity of gravitational wave detectors, such as Advanced LIGO and Advanced Virgo, is affected by the thermal noise in the high-reflectivity mirrors of the end masses. These mirrors are multilayer stacks of alternating layers of TiO₂:Ta₂O₅ and SiO₂, with the main source of thermal noise being the mechanical loss of TiO₂:Ta₂O₅.
Towards the goal of identifying other potential dopants that can further decrease the thermal noise, we carried out an extensive study of Ta₂O₅ doped with different oxides: Al₂O₃, HfO₂, Nb₂O₅, Sc₂O₃, SiO₂, TiO₂, Y₂O₃, ZnO, and ZrO₂. Films were grown by reactive sputtering with a 20% dopant cation concentration. The influence of the dopant on the structure, optical properties, crystallization temperature and mechanical loss was evaluated. We found that oxides with the lowest Gibbs free energy of formation, such as Sc₂O₃, inhibit the complete oxidation of the Ta₂O₅ phase. We identify dopants for which the material crystallizes as a ternary compound, such as TiO₂ and ZnO, which can induce atomic mixing and reduce mechanical loss.