Higher order fano resonances in plasmonic nanostructures with two disks outside one ring

Abstract

A kind of plasmonic nanostructure with two disks placed outside a ring is introduced. When incident light is perpendicular to the surface of the structure, the extinction spectra in the observed wavelength range show that octupolar modes are reduced as the radii of two-disks are different. In the case of two disks with the same size symmetrically around the ring (the SDDR structure), octupolar modes are suppressed absolutely. For SDDR structure, the larger the size ratios of the ring to the disks, the easier higher order dark modes in the ring are excited; as higher order resonances appear, lower order resonances are reduced. The decapolar mode of the SDDR structure with the ring radius of 250nm and two-disk radius of 60nm is in the near-infrared wavelength region, its contrast ratio (CR) and figure of merit (FOM) can reach 87% and 22.5, respectively, which demonstrates application potentials for high-performance biochemical sensing.