Periodic Nanostructures: Spatial Dispersion Mimics Chirality

Polarization rotation in isotropic materials is commonly associated with chirality, i.e., structures with a handedness which are not identical with their mirror image. We observe this effect in the visible and near-IR regions at oblique incidence in the optical response of a subwavelength square array of holes. Mapping the complete (k) over right arrow space via Mueller-matrix spectroscopic ellipsometry, we find that in specific directions the rotary power is orders of magnitude larger than that observed for chiral molecules. Although experimentally indistinguishable, the physics behind the two phenomena is fundamentally different: While optical activity is a consequence of magnetic interactions, nanostructures on a square lattice rotate the polarization due to spatial dispersion.