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.
© 2006 Optical Society of America
Original Manuscript: October 14, 2005
Revised Manuscript: January 16, 2006
Manuscript Accepted: February 7, 2006
Vol. 1, Iss. 8 Virtual Journal for Biomedical Optics
Iwao Teraoka and Stephen Arnold, "Enhancing the sensitivity of a whispering-gallery mode microsphere sensor by a high-refractive-index surface layer," J. Opt. Soc. Am. B 23, 1434-1441 (2006)