Intertwined Fano resonances in sub-wavelength metallic gratings: omnidirectional and wideband optical transmission

Metallic gratings can be used as infrared filters, but are limited by their narrow bandwidth due to metallic losses. Here, we propose a metallic groove-slit-groove (GSG) structure that exhibits an angularly independent extraordinary optical transmission of over 70% with a full spectral width at half maximum of 2 µm in the longwave infrared range. Each groove interferes with the central slit following a Fano profile, as confirmed by a pole study in the complex frequency plane. In addition, the two Fano profiles mirror each other, so that their association gives rise to a broadband transmission flanked by two zeros, improving the rejection rate. These results are extended to a two-dimensional GSG structure exhibiting unpolarized transmission of over 70%, for angles up to 50°.