This Letter presents a plasmonic nanostructure consisting of a nanodisk and a nanoring. The nanodisk is outside of the nanoring. The quadrupolar, hexapolar, and octupolar resonance modes of the nanoring are excited easily by the bright dipolar mode of nanodisks. This nanostructure shows strong interaction and deep Fano dips. In addition, the resonance frequency, depth, and line width of Fano dips can be tuned by changing the geometrical parameters of the nanodisk and nanoring. These plasmonic nanostructures show both high contrast ratio and high figure of merit. Such characters make them suitable for chemical and biological sensing.

This paper reports a spaser based on Fano resonance of a plasmonic nanostructure consisting of a rod and concentric square ring-disk structure coated with a layer of gain media. The amplification of the dark quadrupolar mode at the Fano resonance wavelength causes the spaser with a high Purcell factor of 3.24 × 107, a high signal to noise ratio of 4.4 × 106, and a lower threshold of 0.02086. These significant optical properties are attributed to the greatly enhanced spontaneous emission and depressed radiation loss supported by the strong localized dark mode at the Fano resonance wavelength.

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