Mode Splitting of High-Q Whispering-Gallery Modes in a Microring Resonator Coated With a Fluorescent High-Refractive-Index Film

A microring resonator coated with a fluorescent high-refractive-index film provides a structure that supports whispering-gallery modes (WGMs) via the contrast in the refractive index. In the past, number, size, and refractive index of nanoparticles were analyzed by the degree of resonant mode splitting in the cavity. Compared with the small mode shift, this splitting is easier to detect and has strong anti-interference ability. However, there are several mechanisms for the mode splitting due to the different sizes of nanoparticles. The magnitudes of different mode splittings are quite different, which easily leads to confusion in spectral analysis. In this work, the mechanisms of resonant mode splitting are studied by the FDTD method combined with light field distributions and spectra. Field distributions of the symmetric mode and the asymmetric mode relative to the position of the nanoparticle for the fundamental order and second radial order were simulated. We first distinguished the splitting of the fundamental mode (~0.010 THz) due to the backscattering of the particles from the separation between the fundamental mode and the high-order mode (~1 THz) for the same particle (~100 nm). In addition, fundamental mode splitting was used to estimate the size and number of particles, and these sizes were consistent with those in the design. This work demonstrates the potential application of microring cavities based on WGMs in single particle detection.