Development of amorphous silicon and amorphous silicon carbide thin-film coatings for gravitational wave detectors*

Interferometer optics plays an important role in increasing the sensitivity of gravitational wave detectors (GWD). Finding ultra-low noise mirror coating materials with low optical and mechanical loss is a priority research topic. Tunneling two level states (TLS) are known to dominate the mechanical loss, whereas optical loss is related to local defects. For low temperature detectors, where longer wavelength lasers will likely be used, amorphous silicon (a-Si) and amorphous silicon carbide (a-SiC) are promising substitutes to the currently used high refractive index coating material due to their lower mechanical loss, which will improve the sensitivity of the present and future GWD.

a-Si and a-SiC films were prepared using magnetron sputtering. We investigated the correlation between growth conditions and structure and properties of the resulting films. Characterization techniques were employed to study the structure, and mechanical loss was measured via Gentle Nodal Suspension (room temperature) and Double Paddle Oscillators (1-100 K). We also measured their optical absorption. This systematic study contributes to a better understanding of how the growth process affects the structure and how this should be optimized to minimize the optical and mechanical loss.