Cavity Optocapillaries

Droplets, particularly water, are abundant in nature and artificial systems. Thermal fluctuations imply that droplet interfaces behave like a stormy sea at the sub-nanometer scale.Thermal capillary-waves have been widely studied since 1908 and are of key importance in surface science. Here we use an optical mode of a micro-droplet to probe its radius fluctuation. Our droplet benefits from a finesse of 520 that accordingly boosts its sensitivity inrecording Brownian capillaries at 100-kHz rates and 1+-0.025 angstom amplitudes, in agreement with natural-frequency calculation and the equipartition theorem. A fall in the fluctuation spectrum is measured below cutoff at the drop'slowest-eigenfrequency. Our device facilitates resonantly-enhanced optocapillary interactions that might enable optical excitation (/cooling) of capillary droplet-modes, including with the most-common and important liquid - water.