Fano resonance by dipole-hexapole coupling in a χ-shaped plasmonic nanostructure.

We demonstrate that pronounced Fano resonance can be obtained by dipole-hexapole coupling in a χ-shaped plasmonic nanostructure. The confined local field in the vicinity of dipole-resonant nanorods excites and coherently couples to the hexapolar mode in the rod perpendicular to the polarization direction of the incident wave, leading to strong Fano resonance. By using the discrete dipole approximation, we numerically investigate the behavior of Fano resonance by dipole-hexapole coupling in the plasmonic nanostructure. We obtain the parameters relevant to Fano resonance by fitting the coupled oscillator model equation to the calculated extinction data and investigate their dependencies on the structural parameters. The results demonstrate the possibility of obtaining high-contrast Fano resonance with narrow features by using this plasmonic nanostructure, which has potential applications for highly sensitive biological sensing, low-loss metamaterials, and others.