Internal symmetries in conical diffraction from metallic gratings

Previous investigations in conical diffraction have shown that for a metallic grating illuminated under conditions of resonant excitation of surface plasmons, the maximum efficiency of polarization conversion is obtained when the plane of incidence forms an angle phi =45 degrees with the main section of the grating. This fact has been understood as a consequence of the evident symmetry imposed by the incident wave and it has not been questioned up to now. Here we show that, apart from this highly symmetrical configuration, the polarization conversion efficiency has local maxima also in other cases which do not exhibit such an evident symmetry. We demonstrate that the occurrence of both the local maxima for the new configurations and the maximum corresponding to phi =45 degrees are a consequence of internal symmetries imposed by the propagation direction of the resonantly excited surface plasmons.