| Summary: | 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 ultrastable lasers. The main contribution—Brownian thermal noise—is related to the mechanical loss of the coating material. Al_{0.92}Ga_{0.08}As/GaAs crystalline mirror coatings are expected to reduce this limit. The first measurements of cryogenic silicon cavities revealed the existence of additional noise contributions exceeding the expected Brownian thermal noise. We describe a novel, nonthermal, photoinduced effect in birefringence that is most likely related to the recently discovered birefringence noise. Our studies of the dynamics and power dependence are an important step toward uncovering the underlying mechanisms. Averaging the anticorrelated birefringent noise results in a residual noise that is shown to be substantially different from Brownian thermal noise. To this end, we develop a new method for analyzing the coating noise in higher-order transverse-cavity modes, which makes it possible for the first time to determine the contribution of Brownian thermal noise to the total cavity noise. The new noise contributions must be considered carefully in precision interferometry experiments using similar coatings based on semiconductor materials.
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