Liquid whispering-gallery-mode resonator as a humidity sensor.

We experimentally demonstrate the high sensitivity of a novel liquid state, whispering-gallery-mode optical resonator to humidity changes. The optical resonator used consists of a droplet made of glycerol, a transparent liquid that enables high optical quality factor, doped with fluorescent material. As glycerol is highly hygroscopic, the refractive index and radius of the droplet change with ambient humidity. This produces a shift on the whispering gallery mode's wavelengths, which modulates the emission of the fluorescent material. This device shows an unpreceded sensitivity of 10^-3 per relative humidity percent.