Coupling of evanescent s-polarized waves to the far field by waveguide modes in metallic arrays

In this paper we show that enhanced optical transmission through a 1D periodic slit array comprising real metallic cylinders is possible for s-polarization when the array is near a dielectric interface. We investigate the behaviour of this structure under s-polarized illumination, in which case surface plasmons are not excited. Numerical results show that the transmitted intensity appears as a periodic function of the slit depth, for propagating as well as for evanescent incidence, suggesting that this behaviour is related to the excitation of waveguide modes in the slits. In particular, the coupling of evanescent to propagating electromagnetic waves is investigated. It is shown that s-polarized evanescent waves generated at the interface can be transformed into propagating waves if the optical width of the slits allows propagation of the first waveguide mode. As the interface approaches the array, the transmitted intensity increases for evanescent incidence.