We present a method to calculate the power spectral density of Brownian noise in complex optomechanical systems using Levin’s approach of virtual pressure and present first mechanical loss
measurements for high-purity GaAs over a wide temperature range from 7K to 250 K. The loss reveals three Debye loss peaks. Each peak corresponds to an Arrhenius-like relaxation process with activation energies of 17.9 meV, 65.4 meV and 123 meV respectively. Additional light induced damping was observed for photon energies below and above the fundamental gap of GaAs
in contrast to observations by Okamoto et al.