Analysis and application of whispering gallery modes of the triple-layer-coated microsphere resonator

Microsphere resonators supporting whispering gallery modes (WGMs) have been extensively applied for considerable fields including narrow linewidth filters, high-sensitivity sensors, and nonlinear optics. We numerically demonstrate a coated microsphere resonator with three layers of high, low, high refractive-index (RI) from inside to outside. A phase matched waveguide is used to overlap the WGMs evanescent radiation field. Eigen-mode, relative intensity spectra, electromagnetic (EM) field distributions are observed to analyze resonant characteristics of WGMs by using the finite difference time domain method. As a result, two brilliant rings with strong EM fields distribute in two high-RI layers. By optimizing the gap distance between the microsphere and waveguide, the WGMs of two high-RI layer are efficiently excited. More energy is stored in such a structure rather than a single-layer-coated microsphere. Our approach provides the RI sensing application with such a triple-layer-coated structure