Optical activity and circular dichroism of 3D-chiral holes: Symmetry, causality, reciprocity and reversibility aspects

We report substantial and broadband optical activity and circular dichroism of subwavelength arrays of 3D-chiral holes in a freely suspended silver film. We explain our observations in terms of general physical principles. In particular, we consider the implications of the array in-plane 4-fold symmetry and the difference of the interfaces, verify the appropriate form of the Kramers-Kronig relations, and show how the Lorentz reciprocity lemma connects the circular dichroism with irreversibility due to dissipation. We suggest a possibility of asymmetric transmission through arrays of lower in-plane symmetry.