Simulation and optimization of multilayer-coated microsphere in temperature and refractive index sensing

This study proposes an approach to simultaneously detect the refractive-index (RI) and temperature changes with a three-layer-coated microsphere resonator. The RIs of the three layers are high, low, and high from inside to outside. Using the perturbation theory and finite element method (FEM), a model is developed to calculate the RI and temperature sensitivities for the inner mode and outer mode, respectively. As a result, a second-order sensing matrix is defined to determine the RI and temperature changes. By optimizing the coatings thickness, the thermal noise can be eliminated with the differential frequency of outer and inner modes.