Optical activity in diffraction from a planar array of achiral nanoparticles

We prepare diffractive planar arrays of metal nanoparticles that are chiral because of either the shape of individual particles (molecular chirality) or the orientation of achiral particles in the array (structural chirality). Both sorts of samples are shown to lead to comparable polarization changes in the diffracted light. For the case of structural chirality, one might assume that these effects can occur only through interparticle interactions, as would be the case for transmission measurements (zero-order diffraction). However, we show that the results can be explained by a simple model in which the polarization effects are based on independent scattering by individual particles, with no interparticle coupling, and with the array structure simply determining the direction of the diffraction maximum. We thus conclude that structural and molecular chiralities are indistinguishable in diffraction experiments.