High Tunability Multipolar Fano Resonances in Dual-Ring/Disk Cavities

Metallic nanostructures that support multipolar Fano resonances have drawn much attention in recent years. Such structures are applicable especially to enhanced nonlinear optics, where two resonance wavelengths need to be modulated simultaneously. However, how to tune multipolar Fano resonances independently remains a challenge. In the paper, the plasmonic nanostructure consisting of two ring/disk cavities (RDCs) is investigated using the finite element method. The dark multipolar modes of each RDC are excited, and sharp multipolar Fano resonances are induced. The multipolar modes supported by different RDCs can be tuned independently by changing the sizes. The line-widths of such Fano resonances nearly keep below 0.05 eV, and the contrast ratio (CR) of the two quadrupolar Fano dips mostly maintain above 50 %. In addition, the exciting bonding modes of different RDCs make the selective storage of resonance energy available. Such plasmonic nanostructures may find applications in enhanced nonlinear optics or nano-optical elements.