Enhancing the sensitivity of a whispering-gallery mode microsphere sensor by a high-refractive-index surface layer

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Abstract
We examine properties of whispering gallery modes (WGMs) in a dielectric microsphere coated with a high-refractive-index layer. With an increase in layer thickness, the photonic field of a WGM moves near the sphere’s surface, while the peak in its radial distribution narrows, followed by backtracking and broadening. During these changes, the resonance frequency decreases. The radial compression exposes a stronger evanescent field to the surroundings, yet the mode retains a sharp resonance peak in the frequency domain. The high-refractive-index layer will enhance the sensitivity of WGM frequency-shift sensors when used to detect adsorption of molecules and a change in refractive index in the surroundings. A possibility to match resonance shifts of different radial modes in the measurement of refractive indices is proposed.