Plasmonic waves allow perfect transmission through sub-wavelength metallic gratings

We investigate the transmission properties of a metallic layer with narrow slits. Recent measurements and numerical calculations concerning the light transmission through metallic sub-wavelength structures suggest that an unexpectedly high transmission coefficient is possible. We analyze the time harmonic Maxwell's equations in the H-parallel case for a fixed incident wavelength. Denoting by η>0 the typical size of the complex structure, effective equations describing the limit η→ 0 are derived. For metallic permittivities with negative real part, plasmonic waves can be excited on the surfaces of the channels. When these waves are in resonance with the height of the layer, the result can be perfect transmission through the layer.