Excess noise in highly reflective crystalline mirror coatings

Thermodynamically induced length fluctuations of high-reflectivity mirror coatings put a fundamental limit on sensitivity and stability of precision optical interferometers like gravitational wave detectors and ultra-stable lasers. The main contribution - Brownian thermal noise - is related to the mechanical loss of the coating material. Owing to their low mechanical losses, Al\textsubscript{0.92}Ga\textsubscript{0.08}As/GaAs crystalline mirror coatings are expected to reduce this limit. At room temperature they have demonstrated lower Brownian thermal noise than with conventional amorphous coatings. %However, no detailed study on the noise constituents from these coatings in optical interferometers has been conducted. We present a detailed study on the spatial and temporal noise properties of such coatings by using them in two independent cryogenic silicon optical Fabry-Perot resonators operated at 4 K, 16 K and 124 K. We confirm the expected low Brownian thermal noise, but also discover two new noise sources that exceed the Brownian noise: birefringent noise that can be canceled via polarization averaging and global excess noise (10 dB above Brownian noise). These new noise contributions are a barrier to improving ultra-stable lasers and the related performance of atomic clocks, and potentially limit the sensitivity of third-generation gravitational wave detectors. Hence, they must be considered carefully in precision interferometry experiments using similar coatings based on semiconductor materials.