Fano resonances with a high figure of merit in silver oligomer systems

Abstract

A silver quadrumer consisting of four parallel aligned rectangular nanobars, with three at the bottom and one at the top, is proposed to provide two Fano resonances. These two resonances can be adjusted either simultaneously or independently simply by tuning the geometrical parameters. Due to the formation of the two resonances in a relatively short wavelength range, one of them can be spectrally squeezed to be very narrow, which induces a very high figure of merit ($\mathrm{FoM}=45$). By decomposing the scattering spectrum into bright modes and dark modes, the double Fano resonances are found to be originated from grouping the unit cells into two different groups. The evolution of the scattering spectrum with the central dimer position along the polarization direction suggests that the symmetry reducing induces the second Fano resonance and improves the FoM of the first one. By introducing one more nanobar into the quadrumer system, the FoM can approach the material’s limit, although the dip is relatively shallow. The ultrahigh FoM of the Fano resonance in the proposed quadrumer can provide ultra-sensitive refractive index sensing. Furthermore, the method for providing multiple independently tunable Fano resonances can offer new solutions to designing plasmonic-related nanolasers, photocatalysis, and biochemical sensors, etc.