Structure and morphology of low mechanical loss TiO2-doped Ta2O5

The exceptional stability required from high finesse optical cavities and high precision interferometers is fundamentally limited by Brownian motion noise in the interference coatings of the cavity mirrors. In amorphous oxide coatings these thermally driven fluctuations are dominant in the high index layer compared to those in the low index SiO2 layer in the stack. We present a systematic study of the evolution of the structural and optical properties of ion beam sputtered TiO2-doped Ta2O5 films with annealing temperature. We show that low mechanical loss in TiO2-doped Ta2O5 with a Ti cation ratio = 0.27 is associated with a material that consists of a homogeneous titanium-tantalum-oxygen mixture containing a low density of nanometer sized Ar-filled voids. When the Ti cation ratio is 0.53, phase separation occurs leading to increased mechanical loss. These results suggest that amorphous mixed oxides with low mechanical loss could be identified by considering the thermodynamics of ternary phase formation. (c) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement