Internal symmetries in conical diffraction from metallic gratings

Previous investigations in conical diffraction have shown that for a metall ic grating illuminated under conditions of resonant excitation of surface p lasmons, the maximum efficiency of polarization conversion is obtained when the plane of incidence forms an angle phi =45 degrees with the main sectio n of the grating. This fact has been understood as a consequence of the evi dent symmetry imposed by the incident wave and it has not been questioned u p to now. Here we show that, apart from this highly symmetrical configurati on, the polarization conversion efficiency has local maxima also in other c ases which do not exhibit such an evident symmetry. We demonstrate that the occurrence of both the local maxima for the new configurations and the max imum corresponding to phi =45 degrees are a consequence of internal symmetr ies imposed by the propagation direction of the resonantly excited surface plasmons.