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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.
S. M. Iftiquar (Fri,) studied this question.