Coating thermal noise issue for Gravitational-Waves detectors

Thermal noise in amorphous coatings is a limitation for a wide range of precision experiments such as gravitational-wave detectors. Gravitational-wave detectors are Michelson interferometers with mirrors placed at the ends of two Fabry-Pérot cavities, in order to measure the relative motion of the two arms. Mirrors are composed of multiple thin layers of dielectric materials deposited on a substrate: the stack is made of layers with a high refractive index material interleaved with layers of low refractive index materials. The main goal is to obtain high reflectivity and low thermal noise at the same time. In this work I introduce the topic of thermal noise and thermo-optic noise and I show some experimental facilities that I have built during my PhD period and some experimental results concerning the metrology, fluoride coatings and thermo-optic characterization of tantala coatings. Moreover, in the last chapter I show the results of finite element analysis simulations performed at Caltech regarding the behaviour of coatings that are not deposited on the whole substrate surface.