Investigation on mechanical losses in TiO2/SiO2 dielectric coatings

Interferometric gravitational wave detectors use test masses made by large mirrors whose coating is usually made by multiple layers of dielectric materials, most commonly alternating layers of SiO2 (silica) and Ta2O5 (tantala). It is foreseeable that in future interferometric gravitational wave detector projects (LCGT, EGO, VIRGO,), the mirrors will be cooled down to cryogenic temperature in order to reduce the noise generated by the thermally activated motion of the masses. However, low temperature mechanical losses in the Ta2O5/SiO2 coatings might limit the design sensitivity for such cryogenic detectors by setting a lower limit for the expected thermal noise. Here we present some measurements of mechanical losses in the TiO2/SiO2 coatings at room and low temperature (80K-300K).