“Optical activity produces a differential absorption or refraction according to the handedness of circular polarized light, but the inverse is not necessarily true: a differential absorption of refraction of circular polarized light does not imply optical activity”
This simple statement has been sometimes overseen and there exist some confusion among researchers studying optical activity in several fields of science, from chemistry to material science.
Last year I wrote a short comment addressing this topic and with the focus on metamaterials and nanostructures. It was under consideration in Nature Photonics for a long time, but finally they decided that this topic was already clear and had been discussed elsewhere. Just in case I make it now available as a preprint at: http://arxiv.org/abs/1508.02422
Abstract: It has been assumed that optical activity can be measured by illuminating alternatively a material with left- and right- handed circular polarized light and analyzing the differential response. This simple and intuitive approach is in general incorrect, and has led to misleading idea that extrinsic chirality involves optical activity.