The AdCoat project coatings for gravitational antennas of second and third generation

A relevant example where systems simultaneously capable of quasi ideal reflectivity and extremely low mechanical dissipation play a critical role, is provided by the test masses used in the gravitational waves detectors interferometers. Thermal noise generated by dissipation in the test-masses is the dominating factor in a frequency band, between 50 Hz and a few hundreds Hz, where a high relative sensitivity of the instrument meets a high astrophysical interest. The choice of interferential mirrors (e.g. λ/4 Ta
₂
O
₅
-SiO
₂
multilayers) negatively impacts on thermal noise as the mechanical losses in the coating are relatively large. Only a substantial reduction of the thermal noise of the mirrors will allow GWDs to reach and possibly exceed the limit represented by the quantum noise. This contribution aims at illustrating AdCoat a nation-wide research program recently launched by INFN, which attempts to gather all groups working in Italy on the reduction of thermal noise in coatings for gravitational antennas of second and third generation.