Excitation of plasmonic and photonic modes in metallic lamellar gratings

We study the optical properties of metallic lamellar gratings with sub-wavelength period as a function of the groove depth and width, in situations in which there is only one propagating order (order zero or specular). The reflected light is calculated with an integral equation method for periodic surfaces. Under p-polarized illumination, different kinds of anomalies are observed as the angle of incidence or the grating parameters are changed. These are due to the excitation of surface plasmon polaritons (SPPs) and their interaction with the plasmonic and photonic modes supported by the grating grooves. We find that the propagation constant of the SPPs supported by shallow gratings is affected by the presence of the corrugations. As the groove depth increases, the formation of an energy gap inhibits the excitation of SPPs. However, for grooves deeper than about half a wavelength, new resonances appear. These are due to the coupling of the SPP excitations with the photonic and plasmonic modes supported by the grating grooves.