Dual ultrahigh-Q Fano Resonances of 3D gap metamaterials for slow light from ultraviolet to visible range

We theoretically report an approach to support multispectral ultranarrow Fano resonances (FRs) from ultraviolet (UV) to visible region in a 3D metamaterial composed of aluminium vertical split-ring resonators with a nanogap (Al VSRRG). The plasmonic hybridization between the 1st order broad localized surface plasmon (LSP) resonance of the Al VSRRG and the 1st order narrow optical diffraction mode propagated in the dielectric substrate caused by the Wood anomaly, and the plasmonic hybridization between the 2nd order LSP resonance and the 2nd order optical diffraction mode produce the dual ultranarrow FRs with quality (Q) factors of 118 and 437 in the visible and UV regions, respectively. The simulated dual FRs phenomenon of the proposed Al VSRRG array is well explained by the coupled oscillator theory. In comparison with the Al VSRR without a gap, both the Q-factors of the double FR peaks and the localized electromagnetic field intensities of the Al VSRRG are further increased. The positions and line shapes of the ultrahigh -Q double FRs tuned by different geometric parameters are also investigated. Our designed simple 3D gap metamaterial has large group indexes of 210 and 240 in visible and UV regions, suggesting promising applications in slow light nanodevices.