We calculate the guided band diagram of a metallic film with a one-dimensional periodic arrangement of cut-through subwavelength slits. We find that this system supports two distinct types of guided modes propagating in a direction perpendicular to the slits when the metal obeys a plasmonic dispersion model. The first type is a well-known surface mode. The second type results from the presence of a subwavelength electromagnetic resonance inside the slits and closely resembles waveguide modes in a dielectric slab. We refer to them as effective dielectric slab modes. We show how the behavior of both modes is affected by film thickness and surface properties.
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