Speaker
Description
Crystalline AlGaAs/GaAs coatings are a promising low-noise material candidate for high-precision optical metrology due to their low mechanical loss. However, recent studies have revealed excess Brownian thermal noise in optical cavities, which appears to be linked to photoinduced effects. Additionally, there are indications that illumination influences the mechanical loss in GaAs, leading to variations in Brownian thermal noise.
To investigate these effects, we studied the mechanical loss characteristics of GaAs at $80~\mathrm{K}$, $200~\mathrm{K}$, and $295~\mathrm{K}$ under controlled illumination with wavelengths near the GaAs bandgap. Our results show that the first two mechanical resonance modes exhibit decreasing losses with increasing illumination intensities, whereas higher-frequency modes display the opposite trend. Furthermore, the behavior of mechanical loss changes with decreasing temperature.
Beyond mechanical loss, the elastic properties are also affected, with relaxation time constants ranging from seconds to minutes or even longer. We experimentally verified that changes in elasticity are linked to shear stress in our samples, as supported by COMSOL simulations. In addition, we qualitatively compared our measurements with birefringence relaxation observed in silicon cavities, suggesting that this mechanism may be a primary contributor to the observed effects.
Our findings indicate that photoinduced changes in mechanical loss and elasticity may contribute to variations in birefringence observed in AlGaAs/GaAs coatings and could be relevant for understanding excess thermal noise in mirror coatings. While this study does not fully explain all observed effects in GaAs, it represents an important step toward a more comprehensive understanding of photoinduced mechanical changes in this material.