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 droplet to probe its radius fluctuation. Our droplet benefits from a finesse of 520 that accordingly boosts its sensitivity in recording Brownian capillaries at 100-kHz rates and 1±0.025 ångström amplitudes, in agreement with natural-frequency calculation and the equipartition
theorem. A fall in the fluctuation spectrum is measured below cutoff at the drop's lowest-eigenfrequency. Our device facilitates resonantly-enhanced optocapillaryinteractions that might enable optical excitation (/cooling) of capillary droplet-modes, including with the most-common and important liquid -water.Single sentence: We activate a droplet as a hybrid optocapillary resonator, and access its thermal capillaries while benefiting from resonantly-enhanced sensitivity.Thermal capillary waves [1][2][3][4][5][6] were first suggested by Smoluchowski [7] and are most
: We experimentally demonstrate trapping a droplet with an optical tweezer and then enabling it as a microresonator by bringing it close to a tapered fiber coupler. Our tweezers facilitated the tuning of the coupling from the under-coupled to the critically coupled regime with an optical Q of 12 million and microresonator size at the 85 m scale.
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