Conditions for the occurrence of k gaps for surface polaritons on gratings.
We obtain a formula for the change of the reflectivity from a symmetric metallic grating due to the excitation of surface polaritons. We find that reflectivity measurements at near-normal incidence can show a k gap in the dispersion curves of the surface polaritons even when direct coupling between two polaritons at the zone boundary is present. We discuss the limit in which this can occur and examine the reflectivity surface as the direct coupling is varied. The formula permits a simple discussion of the k gaps at 2npi/a for any n.
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