Fano Resonances in Thin Metallic Grating for Refractive Index Sensing with High Figure of Merit

We theoretically present a high figure of merit (FOM) tunable surface plasmon resonance sensor device by using multiple resonance modes generated from a prism-based thin metallic grating structure at the oblique incidence in this paper. This sensor includes a thin gold film periodically pierced by narrow nanostructured slits deposited on the prism structure. The excitation of Fano-like resonance in different surfaces of metallic grating structure creates two new plasmon modes with narrow bandwidths and unique spectra features, resulting in two obvious dips in the reflection spectrum. These Fano-like resonances in the designed structure originate from the interference between surface plasmons polariton (SPP) resonance modes with different orders on the surface of metallic grating and the narrow dark resonance mode induced by asymmetric charge distribution in the metal slit. The dependence of two resonance dips on structural parameters further explains their physics mechanism, which indirectly verify that the excitation of Fano-like resonance on the top and down surfaces of metallic grating generates the two resonance dips. High bulk sensitivity and high FOM of the designed structure are also analyzed, where the narrow FWHM (full width at half maximum) of resonance dip plays an important role. We also analyze the selections of specific structure parameters to obtain high sensitivity and FOM for different refractive index range. This thin metallic grating based on surface plasmon resonance (SPR) device can be used for biochemical measurements in a wider refractive index range.