Study of amorphous coatings for mirrors of gravitational waves detectors

The recent detection of gravitational waves has opened a new era for astronomical observations, posing new challenges for interferometric detectors. Current gravitational wave detectors require exceptionally high sensitivity, which, within the detectable frequency range of 40-350 Hz, is mainly limited by the thermal noise of the coating (CTN) of main mirrors. Coating R&D programs across the gravitational wave community aim to identify solutions able to reduce CTN using different strategies. These strategies include researching new materials, optimizing deposition techniques, and post-production treatments, as well as identifying correlations between structural properties and thermal noise of relevance for the design of noise mitigation approaches. This thesis addresses both these aspects. It focuses on indirect evaluations of CTN by measuring the mechanical dissipations of the coatings, which are directly linked to CTN through the Fluctuation-Dissipation Theorem and summarized by the loss angle parameter. Additionally, it investigates the effects of post-deposition treatments, such as annealing at high temperature, on the atomic structure of selected amorphous coatings. Specifically, the mechanical dissipation measurements were carried out using a single nodal suspension system (GeNS) and involved both measurements to evaluate the loss angle of specific coatings, and measurements to optimize the substrates to be used. The structure of current coating materials (amorphous silica a-SiO2 and mixed titania-tantala a-TiO2:Ta2O5) was studied mainly by X-ray Absorption Spectroscopy (XAS) experiments, conducted at different beamlines of large scale synchrotron facilities, to get high-sensitivity insights into the short and medium range order. In particular, the evolution of the structure of silica coatings was investigated varying post-deposition annealing temperature up to 1000°C. The crystallinity and density of the coatings have been also studied. This is of interest in relation to the possibility of annealing future high index materials well above the 500°C, currently used for TiO2:Ta2O5. For tantala coatings this work focused on the structure changes due to annealing up to 500°C or adding increasing amounts of TiO2. Finally, this thesis includes also some research activities on a possible novel material: amorphous GaAs. Deposition parameters were optimized to grow it in an amorphous form, and its mechanical, morphological and optical (in collaboration with Maastricht University) properties were measured.