Single-particle detection using high-refractive index film coated in microring resonator

Detection of the particle especially the nanoparticle attracts much attention in various fields from analysis of various biological materials to environmental monitoring. Microring resonator coated with high refractive index film provides a structure supporting whispering-gallery-modes (WGMs) due to the contrast of the refractive index. This structure can be used as a microfluidic detection and indication of the binding of nanoparticles. In this work, we numerically investigated and optimized this microring resonator, pursuing high quality factors 106, yielding the intense light and analyte interactions, as well as extremely high sensitivity and low detection limit. For a single particle adhered to the inner surface of the microring, the eigenmode were investigated. The symmetric and asymmetric modes located the particle at the antinode and node due to the backscattered light coupling between the clockwise and counter-clockwise propagation the WGMs. This leads to the original degenerate resonance mode splitting in frequency. The electric-field intensity distributions along the radius direction near to the film for the fundamental mode and higher order mode were simulated with and without particles. We found that the significant of splitting for the fundamental mode (similar to 0.010 THz) due to the scatter of the particles was very different with the separation between fundamental mode and high order mode (similar to 1 THz). In addition, the splitting in changed with the size and number of the particle were obtained and the sizes of the particles were estimated which consistent with those in design.