Cavity Optocapillaries

TL;DR

This paper demonstrates a highly sensitive optical method to measure thermal fluctuations of micro-droplet interfaces, enabling detailed study of capillary waves and potential optical control of droplet modes, especially in water.

Contribution

It introduces an optical resonator technique with high finesse to probe and quantify droplet radius fluctuations at unprecedented sensitivity and frequency.

Findings

Measured Brownian capillary fluctuations at 100 kHz

Achieved amplitude sensitivity of 1±0.025 Å

Observed fluctuation spectrum cutoff at the droplet's lowest eigenfrequency

Abstract

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.