Variation in whispering gallery mode of resonance in a trapped and levitated liquid micro cavity

Whispering gallery mode (WGM) of resonance occurs when a traveling wave faces grazing reflection and confinement at the inner surface of a spherical cavity. Such a resonance was observed with micrometer sized liquid droplet and with light of sub-micron wavelength. The resonance occurs when specific boundary conditions are fulfilled, following a relation between optical path length (that is related to radius of the droplet) and resonating wavelength. Therefore, a change in WGM wavelength will be related to dimension of the droplet. We observed a shift in such a resonance spectra. For a smaller droplet the observed blue shift in the WGM were 1.5, 0.7, 3.7 nm. Following the resonance condition, it was estimated that such a shift corresponds to a reduction in radius of the droplet by 1.3, 0.6, 3.3 nm respectively. This was obtained with a droplet of radius about 600 nm. The droplet was created from a solution of glycerol, methanol and rhodamine 6G dye, and was trapped and levitated in a modified Paul trap. The WGMs were created by optically exciting the dye material from an external 532 nm cw laser beam. A shift in the WGM was observed with time, during a gradual increase in power of the excitation laser, and a reason for such a shift was thought to be thermal evaporation of the liquid from the droplet. For a larger droplet an initial 0.1 nm thermal expansion was also estimated because initially a red-shift of the WGM was observed, probably because of thermal expansion, which was negligible for a smaller droplet. For the smaller droplet, the estimated rate of change of WGM with radius, was 1.129. For larger droplet, this rate is lower.