Numerical research on whispering-gallery modes in a triple-layer-coated microsphere resonator

We numerically demonstrate that whispering-gallery modes(WGMs) in a microsphere resonator with three layers of high, low and high refractive index(RI) are analyzed by using the finite difference time domain(FDTD) method. To make the light couple in and out of the microsphere, a phase matched waveguide is used to overlap the WGMs evanescent radiation field. By changing the gap between the microsphere and waveguide, the WGM s of two high-RI layers are efficiently excited. The stored energy and the mode volume are optimized for sensing applications. The coupling structure reveals a good sensitivity of 38.29 nm/RIU(RI unit).