Whispering gallery modes (WGMs) in dielectric microcylinders are investigated numerically as sensitive probes of the surrounding medium, for applications in chemical sensing and biosensing. We consider a geometry where WGMs are exited via frustrated total internal reflection from a planar dielectric substrate, as reported in recent experiments. The optical coupling between the substrate and the cylinder yields a WGM broadening exponentially decreasing with increasing distance between the cylinder and the substrate. We also consider a separation layer between the substrate and the cylinder that results in a WGM broadening and shift depending on the index mismatch between the layer and the surrounding medium, and find that Q values are possible for a mismatch in the few percent range. For biosensing applications we calculate the effect of single and multiple cylinder-shaped particles of different sizes attached to the cylinder surface to simulate biological analytes. We find not only WGM shifts but also broadenings and splittings acting as sensitive indicators of different properties of the analytes. In particular, in the case of a single particle, both particle size and refractive index can be determined from the WGM shift and broadening, opening the perspective to a new modality of biosensing applicable to single objects such as viruses or bacteria. In the multiparticle case, the results are statistically analyzed in terms of their surface coverage.
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