Thermal noise reduction in ion-beam sputtered HfO2:Ta2O5 thin films via high-temperature treatment

Reducing coating thermal noise (CTN) in mirrors for gravitational wave (GW) interferometers is pivotal to improving sensitivity in the mid-frequency range. Current mirror coatings are heat-treated (annealed) after deposition in order to partially relax their microstructure and to improve their optical and mechanical properties. The maximum annealing temperature is an important parameter in this respect as a higher thermal energy allows the system to relax to more stable configurations, which is often beneficial for the thermal noise performances of the coatings. However, the useful temperature range is limited by the stability of the amorphous microstructure, since excessive heating eventually leads to the formation of crystalline grains which are detrimental from both the mechanical and optical viewpoints. In this work, inspired by the possibility to improve glass stability in alloys by a careful choice of mixing ratios, we studied ion-beam co-sputtered amorphous HfO2:Ta2O5 thin films with different HfO2 concentrations, so as to identify conditions that would lead to a higher glass stability in order to explore the effects of a thermal annealing over an extended temperature range. We then deposited a multilayer mirror, alternating layers of HfO2:Ta2O5 with composition providing the highest crystallization temperature and SiO2 layers. The thermal Brownian noise of the mirror coating was found to decrease with increasing heat-treatment temperatures, reaching losses comparable to the Ti-doped Ta2O5 coatings of Advanced LIGO when heated at the highest possible temperature. Our results demonstrate the critical importance of optimizing the film composition and annealing procedure in order to improve the coating performances and the sensitivity for the next generation of GW detectors.