Phase resonances in compound metallic gratings

We study the phase resonances that appear in infinite conducting gratings comprising a finite number of grooves in each period (compound gratings), when illuminated by a p-polariztd plane wave. In particular, we investigate a surface that separates a lossy conductor from a dielectric isotropic medium. The resonances appear when a particular distribution of the phase of the electromagnetic field inside the cavities takes plate, and are identified as peaks in the specularly reflected efficiency. These resonances are accompanied by an intensification of the internal field. The diffraction problem is solved by using the modal method. We use the surface impedance boundary condition, which has been proven to be reliable for metals with high conductivity, and simplifies the numerical treatment.