Hydrogen-induced ultra-low optical absorption and mechanical loss in amorphous silicon for gravitational-wave detectors*

The sensitivity of gravitational-wave detectors is limited by the optical absorption and mechanical loss associated with the amorphous coatings of the detectors' mirrors. Optical absorption (at laser wavelengths of 1064, 1550 and 2000 nm) and mechanical loss (at temperatures of 1, 100 and 290 K) has been measured for amorphous silicon films grown at different temperatures and in different states: as-deposited, aged, annealed and hydrogenated. The addition of hydrogen to the silicon network yields an ultra-low optical absorption (below 10 ppm) for 500 nm-thick films, and the already low mechanical loss is further reduced at all temperatures. These results show that hydrogenation is a promising strategy to reduce both optical absorption and mechanical loss in amorphous silicon, and may help to fabricate the next generation of gravitational-wave detectors with improved sensitivity.