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.
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