Thermo-optical resonance locking of an optically trapped salt-water microdroplet

We demonstrate that it is possible to lock the radius of an optically trapped salt-water microdroplet to the n = 1 whispering gallery resonances (WGRs) at the trapping laser wavelength. The optical properties of the droplet are determined using stimulated Raman scattering. The droplet is in thermodynamic equilibrium with the surrounding vapour and the proposed locking mechanism consists of a balance between bulk heating and WGR heating. Raman measurements allow the size parameter (nka) of the droplet to be determined with a precision of similar to 10(-5) and the resonance linewidth to be estimated.