A study of coating mechanical and optical losses in view of reducing mirror thermal noise in gravitational wave detectors

Abstract\nCoating mechanical and optical losses in gravitational wave detectors Abstract Mirror coatings play a crucial role in the performance of laser interferometers devoted to gravitational wave detection such as Virgo and LIGO. Mechanical losses in the coating material limit the sensitivity of the detectors due to the associated mirror thermal noise. The absorption of light in the coating induces a thermal lens in the mirror substrate which reduces the quality of the optical interference and requires sophisticated thermal compensation systems. This paper describes the work ongoing at LMA in order to reduce mechanical losses and optical absorption in the coating. The results obtained by doping the Ta 2 O 5 layers and testing different high index materials are described. Finally the performances of different potential coatings are compared and the results obtained with a 40 kg mirror are reported. Titania doped Ta 2 O 5 shows mechanical losses of 2 10 -4 and absorption below 0.5 ppm. Nb 2 O 5 appears to be the best competitor from the thermal noise point of view but it has an optical absorption 4 to 5 five times larger.