Silicon-based optical mirror coatings for ultrahigh precision metrology and sensing

Thermal noise of highly-reflective mirror coatings is a major limit to the sensitivity of many precision laser experiments with strict requirements such as low optical absorption. Here we investigate amorphous silicon and silicon nitride as an alternative to the currently used combination of coating materials, silica and tantala. We demonstrate an improvement by a factor of ≈55 with respect to the lowest so far reported optical absorption of amorphous silicon at near-infrared wavelengths. This reduction was achieved via a combination of heat treatment, final operation at low temperature and a wavelength of \unit[2]{μm} instead of the more commonly used \unit[1550]{nm}. Our silicon-based coating offers a factor of 12 thermal noise reduction compared to the performance possible with silica and tantala at \unit[20]{K}. In gravitational-wave detectors, a noise reduction by a factor of 12 corresponds to an increase in the average detection rate by three orders of magnitude (≈123).