Opto-mechanical oscillator in a nanoliter droplet

Droplets are very simple physical systems, whereby surface tension shapes liquids into ideal opto-mechanical devices. This has recently enabled low-viscosity liquid samples to serve as miniature acoustic resonators harnessing optical generation of bulk vibrations, capillaries, or surface waves. Uniquely, a simple room-temperature pendant droplet can be activated as a hypersound-laser emitter when illuminated by a free-space, low-power visible laser thanks to stimulated Brillouin scattering of optical and acoustic whispering-gallery modes. Here, we demonstrate continuous operation of a liquid polymer opto-mechanical resonator and characterize its quality factor and long-term frequency stability. Our results point to the feasibility of all-liquid micro-mechanical oscillators working in the 50-100 MHz range. The stimulated generation of high-quality surface waves on nanoliter droplets gives momentum to new optical schemes for characterization of material viscous-elastic properties, laboratory investigation of atmospheric phenomena, and mass sensing for direct analysis of biological fluids based on ultrasound-hypersound coherent generation and detection.