Abstract
Localized surface plasmon resonances (LSPRs) in metallic nanostructures have been studied intensely in the last decade. Fano interference is an important way to decrease the resonance linewidth and enhance the spectral detection resolution. The concentric ring/disk cavities (CRDCs) support multipolar dark resonance modes and are desired nanostructures for realizing super-narrow, high-contrast-ratio Fano resonances. In this paper, the optical properties of CRDCs are analyzed by conducting systematic numerical studies. The multipolar dark modes can be excited directly by horizontal incident light and also by the bright dipolar mode of an external nanodisk. The relationship between the dark modes and the resulting Fano resonances are discussed, and the formation mechanism of the narrow dark modes is illustrated by the electrostatic lightning rod effect that causes large field enhancement in the ring gap of CRDCs.
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