Zirconia-titania-doped tantala optical coatings for low mechanical loss Bragg mirrors

Abstract

The noise caused by internal mechanical dissipation in high refractive index amorphous thin films in dielectric mirrors is an important limitation for gravitational wave detection. The objective of this study is to decrease this noise spectral density, which is linearly dependent on such dissipation and characterized by the loss angle of Young’s modulus, by adding zirconia to titania-doped tantala, from which the current mirrors for gravitational wave detection are made. The purpose of adding zirconia is to raise the crystallization temperature, which allows the material to be more relaxed by raising the practical annealing temperature. The Ta, Ti, and Zr oxides are deposited by reactive magnetron sputtering in an Ar:O₂ atmosphere using radio frequency and high power impulse plasma excitation. We show that, thanks to zirconia, the crystallization temperature rises by more than 150°C, which allows one to obtain a loss angle of 2.5×10⁻⁴, that is, a decrease by a factor of 1.5 compared to the current mirror high-index layers. However, due to a difference in the coefficient of thermal expansion between the thin film and the silica substrate, cracks appear at high annealing temperature. In response, a silica capping layer is applied to increase the temperature of crack formation by 100°C.