Tunable Multipolar Fano Resonances and Electric Field Enhancements in Au Ring-Disk Plasmonic Nanostructures

Enhancing Fano resonances through coupling of dark modes in a dual-ring nanostructure

In this paper we investigate the Fano resonances of a ring-disc nanostructure that consists of two nanodiscs and two concentric nanorings. The dark modes of both nanorings can couple to the bright mode of the nanodiscs, leading to separate Fano resonances from the outer and the inner nanoring. The concentric arrangement of the two nanorings allows for a coupling between the dark modes of the outer and the inner nanoring, thus creating an additional interaction that influences the Fano resonances of the dual-ring nanostructure. This interaction is investigated by comparing the Fano resonances of the complete dual-ring structure with the isolated Fano resonances of the individual single-ring structures. The effect of the coupling between dark modes on the Fano resonances is verified using a model of coupled harmonic oscillators that describe the Fano resonances of this system in a classical analogy. Lastly we compare the sensitivity of a single-ring nanostructure with that of a dual-ring nanostructure to investigate the effects of a coupling between dark modes on the sensing performance.