Symmetry and reciprocity constraints on diffraction by gratings of quasi-planar particles

Symmetry and reciprocity constraints on the polarization state of a field diffracted by gratings of quasi-planar particles are considered. It is shown that the optical activity effects observed recently in arrays of quasi-planar plasmonic particles on a dielectric substrate are due to the reflection of the higher spatial harmonics produced by the grating at the air-dielectric slab interface.