Thermal noise reduction for future gravitational wave detectors

Abstract

The coating thermal noise limits the sensitivity in gravitational wave detectors from few tens to hundreds Hz, where first gravitational signals were detected and others are expected. In view of future upgrades, the increase in the mechanical performances of reflective coatings, retaining their outstanding optical and morphological properties, is fundamental. In the Virgo collaboration a coating R&D group working with this aim is born. One of the research lines regards the mechanical characterization of both substrates and coatings, looking for materials with low mechanical loss angle. To perform a precise coating mechanical characterization, the substrate on which it is deposited must be characterized as well and must be stable with respect to its dissipative behaviour. Commercial SiO2 substrates are subject to effects that change their mechanical condition during time, compromising the characterization. The source of these spurious losses is related to the rough lateral surface: after its polishing, this behaviour is largely reduced. We designed and assemble a facility for the CO2 laser polishing of the substrates barrel, to provide a reliable heat treatment, reducing spurious losses to a negligible level. Other further treatments have been tested, with the aim of taking under control ϕ deterioration. We develop a procedure to prepare SiO2 samples for coating deposition. Procedure steps, mechanical characterizations and first results are shown.