A spatial refractive index sensor using whispering gallery modes in an   optically trapped microsphere
  We propose the use of an optically trapped, dye doped polystyrene microsphere for spatial probing of the refractive index at any position in a fluid. We demonstrate the use of the dye embedded in the microsphere as an internal broadband excitation source, thus eliminating the need for a tunable excitation source. We measured the full width at half maximum of the TE and TM resonances, and their frequency spacing as a function of the refractive index of the immersion fluid. From these relations we obtained an absolute sensitivity of 5e-4 in local refractive index, even when the exact size of the microsphere was not a priori known. 
P. Zijlstra, K.L. van der Molen, A.P. Mosk