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.