Dual effect of surface plasmons in light transmission through perforated metal films

The light transmission through square and rectangular holes in a metal film has been studied. By taking account of plasma response of real metal on hole walls as well as metal surface, an analytical result for the transmission has been deduced, which agrees well with the experiments. We show that the light transmission involves both the diffraction modes and the surface plasmons. Strong coupling of diffraction modes to surface plasmon polariton results in the transmission minima, whereas the coupling to cavity surface plasmon leads to the transmission maxima. The results suggest a dual-effect of the surface plasmons.