Plasmonic Cup Resonators for Single-Nanohole-Based Sensing and Spectroscopy

We fabricate a new type of plasmonic resonator, called a cup resonator, consisting of a single nanohole and a cylindrical surrounding reflector. This device is made using template stripping, which creates a smooth silver surface for the base along with a sidewall mirror in one step to form a compact plasmonic microcavity. When a cup resonator is illuminated, surface plasmon waves, launched by a nanohole in all directions, are reflected off of the cylindrical sidewall, generating cavity resonances that can be observed as interference patterns in the optical transmission spectra. Since the resonances inside the cup depend on the local refractive index, this device can function as a compact optical sensor. With this sensor we observe a bulk index sensitivity of 390 nm/refractive index unit (RIU). A major advantage of this system over other propagation-based plasmonic sensors is that the energy is confined within a single cup, which is on the order of 1 μm². This means that large arrays can be fabricated and used for parallel ensemble sensing and imaging applications.