Light diffraction by a nanograting with bimetallic metamaterial

Diffraction optical spectra for a periodic array of bimetallic metamaterial bars, overlying a dielectric substrate, are calculated and analysed within the Fourier formalism. The optical response of the metamaterial is here described by a local frequency-dependent permittivity tensor according to the effective medium approach (EMA). The zero- and first-order diffraction spectra, predicted by EMA, are compared with exact numerical calculations considering the real nanostructure of the bimetallic metamaterial, as well as realistic metals permittivities. In particular, results for a nanograting with Al–Ag layered metamaterial are presented. It is demonstrated that the diffraction spectra, for incident light polarized along the metamaterial bars, strongly depend on the orientation of the metal layers. For this reason, EMA can only provide a qualitative description of diffraction spectra. For a nanograting with Al–Ag-metamaterial layers, being perpendicular to the bar axis, the enhancement of the intensity of zero-order transmitted wave, accompanied by the strong suppression of the specular reflectivity and all the nonzero-order diffraction spectra, is predicted to be observed near a matching frequency.