Angular momentum-induced circular dichroism in non-chiral nanostructures

Circular dichroism (CD), i.e. the differential response of a system to left and right circularly polarized light, is one of the only techniques capable of providing morphological information of certain samples. In biology, for instance, CD spectroscopy is widely used to study the structure of proteins. More recently, it has also been used to characterize metamaterials and plasmonic structures. Typically, CD can only be observed in chiral objects. Here, we present experimental results showing that a non-chiral sample such as a sub-wavelength circular nano-aperture can produce giant CD when a vortex beam is used to excite it. These measurements can be understood by studying the symmetries of the sample and the total angular momentum that vortex beams carry. Our results show that CD can provide a wealth of information about the sample when combined with the control of the total angular momentum of the input field.