Confined collective excitations of self-standing and supported planar periodic particle arrays.

We find the conditions for the existence of trapped modes in planar periodic particle arrays. Confined excitations of TE and TM symmetry are observed in symmetric environments, originating in lattice resonances that are signalled by the onset of new diffraction beams. This mechanism of mode formation is shown to be inhibited by the presence of a dielectric interface in an asymmetric configuration. Modes can still exist above a threshold finite distance from the interface. Both rigorous numerical simulation and analytical modeling are used to elucidate the origin and systematics of this unexpected difference in the behavior of trapped modes in self-standing and supported particle arrays.