Optically thin metal gratings of high filling factor can support surface plasmon polariton modes at their horizontal metal boundaries and vertical cavity modes inside their slits. These modes are coupled through the metal stripes and the slits. This letter shows that the resonant interactions between the modes critically influence the optical field distribution near the grating and substantially enhance several phenomena such as low reflection, low transmission, and high absorption of the electromagnetic radiation. A high degree of energy confinement is obtained, especially when the energy is stored almost exclusively on the far side of the grating, away from the incident radiation. It is numerically shown that this phenomenon originates from the interaction of field modes with the collective motion of free electrons, as considered in the Drude model.

Abstract.A new configuration of a reflective one-dimensional surface-relief metallicgrating structure for unidirectional excitation of surface plasmon polaritons (SPPs) is proposed and numerically explored for plane wave normal incidence. The structure is embedded between two different dielectric media and composed of two layers, each of them consisted of periodically placed rectangular metal stripes. It is shown that even a small horizontal shift between these two layers, or a change of dielectric contrast of the grating fillings, may redirect energy flow propagation in a vicinity of the grating structure. From these two cases of counter propagating energy flows only one shows the field behaviour typical for SPP excitation. This evident directivity phenomenon exists together with high concentration of the electromagnetic field at the periodic structure. The configurations analysed may be useful in designing optical devices like photodetectors or, in general, in any case where efficient control of energy propagation directivity is of primary importance.

An analysis of phenomena leading to high transmission at a one-dimensional metal grating is presented. It is shown that high resonant transmission can be obtained either for thick or thin gratings for a wide range of filling factors and that the origins of the enhanced transmission are different in each case. We analyse the optical response of structures with subwavelength slits and of various thicknesses. The role of different pure (dielectric cavity modes, surface plasmon polaritons) and coupled resonances on the enhanced transmission is presented.

Abstract. Extraordinary optical transmission and good focusing properties of a two-dimensional scattering structure is presented. The structure is made of Fresnel zone plates periodically arranged along two orthogonal directions. Each plate consists of two ring-shaped waveguides supporting modes that match the symmetry of a circularly polarized incident plane wave. High field concentration at the focal plane is obtained with the short transverse and long longitudinal foci diameters. Optical vortex excitation in a paraxial region of the transmitted field is also observed and analysed in terms of cross-polarisation coupling. The structure presented may appear useful in visualization, trapping and precise manipulations of nanoparticles.