Mode broadening induced by nanoparticles in an optical whispering-gallery microcavity

We theoretically investigate mode broadening of a high-Q optical whispering-gallery microcavity coupled to a single or multiple dielectric or plasmonic subwavelength particles. The result shows that backscattering contributes dominantly to the mode broadening in both transmission and reflection spectra for dielectric particles binding on the microcavity surface, while absorption also plays an important role for lossy nanoparticles. The mode broadening induced by nanoparticles holds great potential in optical biosensing. For instance, by monitoring the change of mode linewidth, a single 11-nm-radius spherical polystyrene nanoparticle is detectable. This detection breaks through the detection limit of the mode-splitting method using a passive cavity and remains immune to various noises, such as thermal fluctuations and frequency drifts of the probe laser. Finally, the mode broadening is demonstrated to be particularly suitable for detecting lossy nanoparticles, e.g., plasmonic particles.