Resonant effects in periodic gratings comprising a finite number of grooves in each period

We give numerical evidence of a kind of resonance that appears in infinite perfectly conducting gratings comprising a finite number of grooves in each period when illuminated by a normally incident p-polarized plane wave. This phenomenon is intimately connected with the particular distribution of the phase of the electromagnetic field inside the cavities, which is automatically generated by waves of certain resonant wavelengths. The resonances appear as sharp peaks in the specularly reflected efficiency and are accompanied by a significant intensification of the interior field. We study the particular case of rectangular cavities and consider configurations with different numbers of grooves per period. The diffraction problem is solved for s and p polarization by using the modal method, which proves to be especially suitable for this profile.