Mechanical ringdown studies of large-area substrate-transferred GaAs/AlGaAs crystalline coatings

We investigated elastic loss in GaAs/AlGaAs multilayers to help determine the suitability of these coatings for future gravitational wave detectors. We measured large (≈70-mm diameter) substrate-transferred crystalline coating samples with an improved substrate polish and bonding method. The elastic loss, when decomposed into bulk and shear contributions, was shown to arise entirely from the bulk loss, ϕbulk=(5.33±0.03)×10−4, with ϕshear=0.0−0.0+5.2×10−7. These results predict the coating loss of an 8-mm diameter coating in a 35-mm long cavity with a 250-μm spot size (radius) to be ϕcoating=(4.78±0.05)×10−5, in agreement with the published result from direct thermal noise measurement of ϕcoating=(4±4)×10−5. Bonding defects were shown to have little impact on the overall elastic loss.