Resonant response of metallic compound gratings with subwavelength slits

We investigate the response of gratings formed by subwavelength wires and slits in a dielectric medium. Transmission metallic gratings with subwavelength slits are known to produce enhanced transmitted intensity for certain resonant wavelengths. One of the mechanisms that produce these resonances is the excitation of waveguide modes inside the slits. We show that when the period of the structure is formed by several wires and slits (compound grating), new degrees of freedom are introduced for the phase distribution of the field inside the slits, allowing a new class of resonances. Phase resonances appear as sharp features in the transmitted/reflected response and are characterized by a significant enhancement of the interior field. We investigate the response of such systems under normal and oblique illumination and show that new phase modes appear in this last case. Contrary to the effect produced by a defect in a photonic crystal, these systems exhibit forbidden channels within a permitted band. We also found that the appearance of these resonances is not highly dependent on the slits width and thickness, even though these parameters modify the overall transmittance. We give numerical examples illustrating the effect produced by these resonances in the grating response