Temperature-Dependent In-Situ Analysis to Enhance the Performance of Optical Coatings in Gravitational Wave Detectors
December 11, 2024 12:00 am
Gravitational-wave detectors (GWD) are ultra-sensitive, large-scale facilities whose successful operation depends critically -among various factors- on the performance of high-reflective mirrors. These consist of doublets of high- and low-refractive-index amorphous oxide coatings deposited by ion beam sputtering (IBS). Their performance, defined in terms of high reflectivity, low optical absorption and low thermal noise, can be enhanced by optimizing the constituent materials, the deposition and post-deposition processes such as the thermal annealing. In this contribution, an implementation of real-time spectroscopic ellipsometry is proposed as a convenient tool to understand the evolution of coatings properties during the post-deposition thermal annealing. The amorphous titania-tantala coating and the annealing protocol considered here match those currently used in mirrors for GWD. In-situ analysis shows the evolution of the coating refractive index and thickness throughout the annealing, including the heating and cooling ramps. Results indicate that the current annealing protocol leaves room for further possible modifications in the coatings properties and suggest ways to optimize it. The in-situ analysis discussed here can be beneficial to screen and validate other coating materials as well as to test new annealing protocols to enhance the properties of mirror coatings for GWD applications.