Unidirectional SPP excitation at asymmetrical two-layered metal gratings

A new configuration of a reflective one-dimensional (1D) surface-relief metallic-grating 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 consisting of periodically placed rectangular metal stripes. It is shown that even a small horizontal shift between these two layers, or a change in dielectric contrast of the grating fillings, may redirect energy flow propagation in the vicinity of the grating structure. From these two cases of counter propagating energy flows, only one shows the field behaviour typical of SPP excitation. This evident directivity phenomenon exists together with a 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.