  We experimentally demonstrate the use of a novel liquid state,
whispering-gallery-mode optical resonator as a highly sensitive humidity
sensor. The optical resonator used consists of a droplet made of glycerol, a
transparent liquid that enables high optical quality factor, doped with
rhodamine 6G, which acts as fluorescent emitter. 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 modes wavelengths,
which modulates the emission of rhodamine 6G. This easily-made device has an
unpreceded sensitivity of 10-3 per relative humidity percent.
