Abstract. We report on the optical, mechanical and structural characterization of the sputtered coating materials of Advanced LIGO, Advanced Virgo and KAGRA gravitationalwaves detectors. We present the latest results of our research program aiming at decreasing coating thermal noise through doping, optimization of deposition parameters and postdeposition annealing. Finally, we propose sputtered Si 3N4 as a candidate material for the mirrors of future detectors.

Energy dissipation in amorphous coatings severely affects high-precision optical and quantum transducers. In order to isolate the source of coating loss, we performed an extensive study of Raman scattering and mechanical loss of a thermally-treated sputtered silica coating. Our results show that loss is correlated with the population of three-membered rings of Si-O4 tetrahedral units, and support the evidence that thermal treatment reduces the density of metastable states separated by a characteristic energy of about 0.5 eV, in favour of an increase of the states separated by smaller activation energies. Finally, we conclude that three-fold rings are involved in the relaxation mechanisms only if they belong to more complex chain-like structures of 10 to 100 tetrahedra.