Whispering-gallery modes in a triple-layer-coated microsphere resonator for refractive index sensors

In this study, a highly sensitive refractive index (RI) sensor is designed by functionally coating three layers of high, low, and high RI from inside to outside on the external surface of an optical microsphere resonator. The properties of whispering-gallery modes, confined in normal microspheres, as well as triple-layer-coated microspheres are presented theoretically and numerically through electromagnetic analyses. The whispering-gallery modes in such a triple-layer-coated microsphere resonator are found to be efficiently excited when optimizing the gap between the microsphere and waveguide. Two cladding modes, referred to as the inner mode (IM) and outer mode (OM), are exhibited in two high-RI layers. Potential RI-sensing applications stemming from the coupling for the IM and OM are investigated thereafter using the perturbation theory and the finite difference time domain method. It is found that the sensing properties of the OM can exceed single-layer-coated microspheres in terms of sensitivity. Specifically, an increased sensitivity of 49.5 nm/RIU (Refractive Index Unit) can be achieved by tailoring the film thickness of the middle low-RI layer. When applying a thick coating in the middle layer, there is a mode bonding in the sensitivity for the IM and OM. The detection limit, or quality factor, is also investigated by using an approximate theory. Results demonstrate that it is possible to reach a detection limit with a variation of ∼10−6. The study's theoretical and numerical predictions provide guidelines for the improved design and fabrication of microsphere RI sensors.