Spectroscopy of substrate thermal vibrational modes using an optomechanical sensor

We used an optomechanical sensor to record the vibrational spectrum of its own fused silica substrate. The coupling of the substrate modes was greatly enhanced by covering the sensor with an appropriately sized ethylene glycol droplet, and in that case even thermally populated modes could be clearly detected. We observed a rich spectrum of high-quality flexural modes for frequencies less than - 10 MHz, in good agreement with analytical theory and finite element simulations. We also observed signatures of acoustic waves resonantly confined by the droplet, over the entire - 0-40 MHz range of the experiment. This work could inform efforts to increase the quality factor of mechanical resonators, and the use of substrate phonon modes as information channels.