Measured limits on amplitude dependence of mechanical loss in substrate-transferred GaAs/Al0.92Ga0.08As coatings

The standard approach for predicting thermal noise in optical mirrors using the fluctuation-dissipation theorem requires knowledge of the level of all significant sources of mechanical loss occurring at the oscillation amplitudes of thermal noise. Using a gentle nodal suspension system read out by a Michelson interferometer, we tested the amplitude-dependence of loss in GaAs/Al0.92Ga0.08As multilayer optical coatings on silica substrates in the range from just above the rms thermal noise amplitude up to amplitudes typical of ringdown measurements: 10-1-103 picometers. None of the three samples tested showed any significant amplitude dependence over this range.