Cryogenic Mechanical Loss of GaAs/AlGaAs Crystalline Coatings
Optical coatings with minimal thermal noise are essential for high-precision, laser-based metrology experiments. Substrate-transferred crystalline GaAs/Al$_{0.92}$Ga$_{0.08}$As coatings have the lowest thermal noise for high-reflectivity mirror coatings with $\lambda =900$ nm -- 12 $\mu$m. Time-frequency stabilization experiments have demonstrated that GaAs/AlGaAs mirror coatings have a lower noise floor than amorphous mirror coatings. These mirrors are now being developed at sizes $\ge 300$ mm for gravitational-wave detectors. In order to understand the coating's fundamental loss mechanisms, which cause thermal noise, we have performed the first measurement of the mechanical loss at cryogenic temperatures using a cryogenic, multimodal, gentle nodal suspension system, which operates from 12 K to room temperature. This initial measurement used a silicon substrate whose asymmetry produced excess friction with the nodal support. Accounting for this friction and Akhiezer loss in the substrate and coating, the mechanical loss agrees well with the thermal noise observed in experiments using cryogenic fixed-cavities with GaAs/AlGaAs mirrors.