Spatially Localized Enhancement of Evanescent Coupling to Whispering-Gallery Modes at 1550 nm Due to Surface Plasmon Resonances of Au Nanowires

We demonstrate a highly localized enhancement of evanescent coupling between a tapered optical fiber and a high-
Q
dielectric microresonator owing to the resonant evanescent excitation of localized surface plasmons of a gold nanowire placed on the microresonator's surface. This localized enhancement, at 1550 nm, is due to nanowire fragments that are confined to specific regions of the surface. The enhancement (of photon-tunneling probability) is measured by comparing properties of the whispering-gallery modes' throughput dips before and after deposition of the nanowire. The experimental results presented here show coupling enhancement by a factor of 10-700, depending on the position of fiber-microresonator contact in relation to the nanowire. These Au-nanowire-coated microresonators could provide enhanced sensitivity in various types of surface-plasmon-based gas, chemical, and biological-sensing applications within the telecommunication frequency bands.