A new method of probing mechanical losses and comparing the corresponding deposition processes of metallic and dielectric coatings in 1-100 MHz frequency range and cryogenic temperatures is presented. The method is based on the use of extremely high-quality quartz acoustic cavities whose internal losses are orders of magnitude lower than any available coatings nowadays. The approach is demonstrated for Chromium, Chromium/Gold and a multilayer tantala/silica coatings. The Ta2O5/SiO2 coating has been found to exhibit a loss angle lower than 1.6 × 10 −5 near 30 MHz at 4 K. The results are compared to the previous measurements.

International audienceWe present a free-space optomechanical system operating in the 1-K range. The device is made ofa high mechanical quality factor micropillar with a high-reflectivity optical coating atop, combinedwith an ultra-small radius-of-curvature coupling mirror to form a high-finesse Fabry-Perot cavityembedded in a dilution refrigerator. The cavity environment as well as the cryostat have beendesigned to ensure low vibrations and to preserve micron-level alignment from room temperatur