FDTD Simulation: Refractive Index and Single-Object Sensing Using a Whispering-Gallery-Modes Microring Resonator

Sensing of whispering-gallery-modes in a microring resonator coated with a fluorescent high-refractive-index film is modeled using finite-difference time-domain method. A short pulse with ultrawide bandwidth in the high-index film spans the circumference of the ring. The spectra of the high-index film and the pulse propagation in time domain were simulated. The results indicate redshifts of the resonance wavelengths with an increasing refractive index inside the ring. A sensitivity of 28.5 nm/RIU is found for the index of 1.48. Considering the effects of scattering and absorption of a single object adhered to the inner surface of the microring, an obvious splitting or broadening of resonance modes is found for sensing. For single particles with a radius of 3 μm, the mode splitting is increased with increasing number of particles from one to nine. The estimation of the size of the particles is also studied based on the mode splitting of the optical field in the ring depends on the scattering of the particles. This work reveals the potential applications of whispering-gallery-modes in a microring resonator coated with a fluorescent high-index film for sensing one or multiple objects and even a single molecule.