Hybridizing whispering gallery modes and plasmonic resonances in a photonic meta-device for bio-sensing applications

Abstract

High-Q dielectric whispering gallery mode (WGM) resonators emerged as promising sensing-platforms which enable ultra-sensitive detection of e.g. nanoparticles (NPs) [1] or single molecules [2]. Measuring the shift in the resonance wavelength of a high-Q dielectric whispering gallery mode (WGM) resonator induced upon changing the dielectric environment is thereby a prime sensing strategy. To increase the sensitivity of dielectric WGM resonators, strategies have to be developed to push the optical mode further outside the resonator to enhance its interaction with the surrounding. This contribution has presented a novel approach to improve the sensing capabilities of optical microresonators. We have studied theoretically and numerically a photonic meta-device consisting of a dielectric microdisk resonator decorated with a large number of randomly distributed and densely arranged gold nanoparticles. By relying on metamaterial concepts, we have considered the layer of gold NPs surrounding the resonator as a homogenous medium to which effective properties are assigned. This effective treatment allows for an efficient tuning of its properties and is the key to investigate resonator systems with huge NP densities. Careful spectral tuning of the plasmonic resonance relative to the WGM increases the sensitivity to a notable extent at an acceptable simultaneous decrease of the quality factor [3]. We have demonstrated that dielectric WGM resonators with ultra-thin homogeneous metal coatings with thicknesses <15 nm can be well mimicked with our effective medium approach. By replacing ultra-thin metallic films with NP composites hybrid devices with comparable optical properties can be fabricated with ease via self-assembly techniques. In contrast to the challenging fabrication of ultra-thin, ultra-smooth metallic coatings on micron-sized non-planar dielectric substrates, our metamaterial-inspired approach is robust against fabrication imperfection as the only critical parameter is the NP density.