Low Noise Mirror Coatings for Next-Generation Gravitational-Wave Detectors

Nowadays, operational gravitational-wave detectors are Michelson-like interferometers and the most limiting source of noise in the frequency range of maximum sensitivity (30-500 Hz) is thermal noise produced inside mirror coatings. Thermal noise is strictly related to mechanical properties of coating materials through the Fluctuation-Dissipation theorem and it is owing to energy dissipation mechanisms. These mechanisms consist in structural relaxations which, from recent studies, seem to involve the motion of atomic groups formed by 10 to 20 units. An attempt to reduce thermal noise, within the Virgo collaboration, is focused on blocking as much structural relaxations as possible. This goal can be obtained by inducing the formation of nanocrystals inside amorphous Ta2O5, which is the most problematic coating material when dealing with thermal noise. Within this master thesis, the crystallization kinetic of 500nm-thick films of a- Ta2O5 produced by ion beam sputtering has been studied, with the aim of producing samples to be optically and mechanically characterized. Another path to block structural relaxations is by using different amorphous materials with high-coordination-number atoms: according to Phillips’ conjecture, structural relaxations should be less favoured in high-coordination-number glasses. In this regards, an interesting material in SiC, particularly appealing for applications in infrared frequencies. Within this master thesis, many hyper-pure and stoichiometric thin films of a-SiC have been produced by rf magnetron sputtering and they have been characterized.