摘要： Grazing diffraction orders on metal gratings give rise to peculiar optical effects that were contemplated by Wood, Rayleigh and Fano. With plasmonic nanoparticles as resonant grating elements, the phenomenology of such surface lattice resonances becomes quite rich, including spectrally narrow extinction peaks and optical band gap formation. It has been observed that at perpendicular incidence either the higher or lower energy branch corresponding to the first grazing diffraction orders is bright, i.e., couples strongly to light. Reviewing the literature, it appears that particle size is the factor determining which dispersion branch lights up. However, a consistent explanation for this effect is lacking. After revisiting the effect experimentally and by numerical simulation, we clarify the underlying physics by analyzing nanoparticle gratings in terms of, first, an oscillator model and, second, a photonic crystal description. Both approaches reveal the central role of a particle-size-dependent phase shift in the back-scattering of grazing light fields by the particle grating. This phase shift determines the symmetry of the resulting field profiles corresponding to the dispersion branches and thus their ability to couple to the exciting light. This physical understanding could considerably simplify the dispersion engineering of plasmonic nanoparticle gratings for specific applications as sensing or lasing.