Polarization dependent enhanced infrared transmission through complementary nanostructured gold films

A pair of complementary-structured gold films, with periodic rectangular nanoscale patches and rectangular holes in the complementary layer arranged in a stretched hexagonal lattice and spaced apart by 200 nm of a photoresist film, were fabricated by laser interference lithography and subsequent physical vapor deposition of gold. The pair of complementary films showed a polarization-dependent extraordinary transmission (EOT) at mid-infrared frequencies, evidenced by a resonant dip in reflectance and strong enhancement of the transmittance for light polarized perpendicular to the long axis of the rectangular structures. Numerical simulations confirm the enhanced transmission and indicate the involvement of the TE01 wave-guide mode resonance of the rectangular structures in the resonant transmittance. The enhanced transmittance in the complementary pair of structured films separated by sub-wavelength distances, which is otherwise be expected to be opaque, is surprising. The Poynting vector maps show that the energy flow weaves across the openings in the two structured films. Dependence on the metal thickness and period of the structures have been investigated. Sensitivity of the EOT peak to the surrounding medium's refractive index is studied by simulations to reveal its potential for sensor applications.