Resonances on metallic compound transmission gratings with subwavelength wires and slits

We investigate the response of gratings formed by subwavelength wires in a dielectric medium. For gratings with a single wire per period, most of the incident power is transmitted through the structure. If the distance between adjacent wires is small enough, the excitation of waveguide modes can lead to reflection resonances. However, only when the period is formed by several wires (compound grating), new degrees of freedom are introduced for the phase distribution of the field inside the slits, allowing a new class of resonances analyzed in this letter. We give numerical examples illustrating the combined effect produced by both types of resonances in the grating response.