Simulation of internally referenced resonance in a three-layer-coated microsphere resonator

With the finite difference time domain (FDTD) method, whispering gallery modes (WGM) in a microsphere coated with three layers of high, low, and high refractive index (RI) are simulated. In the simulation, the coupling system includes a coating microsphere, a waveguide and a nanoscale gap separating the waveguide and the microsphere. A pulse with ultra-wide bandwidth that spans over several resonant modes of the resonator is used for simulation. Via waveguide coupling, the relative intensity spectra of the three layers and the transmission spectrum of the coupling system are obtained. We investigate the effects of the waveguide RI and the thickness of the low-RI layer on resonance characteristics. It is found that each of the two high-RI layers can sustain its own WGM if the values of RI and thickness of the three layers are appropriate. Furthermore, the effect of the RI of the surrounding medium on resonance characteristics is also studied. The simulation results show that a RI change of the surroundings will only change the resonance wavelength of the outer layer, and will not affect the WGM of the inner layer. Such property makes it feasible for a potential application in high-precision RI and temperature sensing.

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