Stimulated Brillouin Cavity Optomechanics in Liquid Droplets.

Liquid droplets are ubiquitous in nature wherein surface tension shapes them into perfect spheres with atomic-scale smooth surfaces. Here, we use stable droplets that cohost equatorial acoustical and optical resonances phase matched to enable the exchange of energy and momentum between sound and light. Relying on free-space laser excitation of multiple whispering-gallery modes, we harness a triple-resonant forward Brillouin scattering to stimulate optomechanical surface waves. Nonlinear amplification of droplet vibrations in the 60-70 MHz range is realized with spectral narrowing beyond the limit of material loss, thereby activating the droplet as hypersound-laser emitter.