Dielectric microsphere sizing using fluorescence microscopy of whispering gallery mode resonances from adsorbed dyes

Whispering gallery mode (WGM) resonances in dielectric microspheres are very sensitive to their size and environment, which can be used for sensing but also as an indirect proxy to determine their size. By coating them with suitable fluorescent dyes and using fluorescence microscopy, we show that the WGM resonances of individual microspheres in solution can be easily studied with a high throughput. Brownian motion ensures that a representative sample is probed over time in the scattering volume. To analyze these WGM-imprinted fluorescent spectra, we propose a simple algorithm based on monitoring the spacing between resonances and comparing it to Mie theory predictions to infer their size. This allows us to measure the size distribution of typical polystyrene microsphere solutions. We also discuss the potential effects of dye concentration and choice of particle refractive index on the analysis. This method can be used, for example, for quality-testing microsphere solutions.