Silicon nitride

Amorphous solid

Detector

Materials science

Coating

Absorption (acoustics)

Computer science

Silicon

Wavelength

Metrology

Crystalline silicon

Laser

Physics

Nanotechnology

Organic chemistry

Optics

Image (mathematics)

Artificial intelligence

Composite material

Amorphous silicon

Chemistry

Noise (video)

Optoelectronics

Optical coating

Physical Review Letters

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 2 μm instead of the more commonly used 1550 nm. Our silicon-based coating offers a factor of 12 thermal noise reduction compared to the performance possible with silica and tantala at 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 (≈12^{3}).

Silicon-Based Optical Mirror Coatings for Ultrahigh Precision Metrology and Sensing