Split-cross antenna based narrowband mid-infrared absorber for sensing applications

We have investigated numerically a narrowband near unity mid-infrared absorber based on a periodic array of gold split cross antenna backed by a dielectric spacer and a gold backmirror. We systematically studied the spectral dependence on the antenna parameters and explored the optimized parameters for nanofabrication. The optimized structure has a linewidth of 39 nm at 3.17 mu m and the peak absorption is 96.5%. This can be explained in terms of surface lattice resonance of the periodic structure. The investigated structure can be devised as a mid-infrared refractive index sensor. Due to the strong near field enhancement and spectral dependence on the surface dielectric conditions, the narrow linewidth arises from the coupled plasmonic-photonic modes in the structure and has potential applications in plasmonic biosensing.