  Liquid droplet whispering-gallery-mode microresonators open a new research
frontier for optomechanics and photonic devices. At visible wavelengths, where
most liquids are transparent, a major contribution to a droplet optical quality
factor is expected theoretically from thermal surface distortions and capillary
waves. Here, we investigate experimentally these predictions using transient
cavity ring-down spectroscopy. In this way, the optical out-coupling and
intrinsic loss are measured independently while any perturbation induced by
thermal, acoustic and laser-frequency noise is avoided thanks to the
ultra-short light-cavity interaction time. The measurements reveal a photon
lifetime ten times longer than the thermal limit and suggest that capillary
fluctuations activate surface scattering effects responsible for light
coupling.
