Vortex Circular Dichroism: An experimental technique to assess the scalar/vectorial regime of diffraction

Background: In classical electrodynamics, light-matter interactions are modelled using Maxwell equations. The solution of Maxwell equations, which is typically given by means of the electric and magnetic field, is vectorial in nature. Yet it is well known that light-matter interactions can be approximately described in a scalar (polarization independent) way for many optical applications. While the accuracy of the scalar approximation can be theoretically computed, to the best of our knowledge, it has never been determined experimentally. Here, we show that the vectoriality of diffraction can be probed with a new technique: Vortex Circular Dichroism(VCD). Methods: We measure the differential transmission of left and right circularly polarized vortex beams through a set of single circular nano-apertures with diameters ranging from 150 to 1950 nm. We observe that VCD > 0 for smaller diameters, VCD ≲ 0 for intermediate values and VCD ≈ 0 for larger values of the diameter. We also carry out Mie Theory simulations for spheres with the same diameters as the nanoholes and observe that the theoretical and experimental VCD values follow the same trend line. Results: We relate VCD ≠ 0 to a vectorial diffraction, and VCD ≈ 0 to a scalar one. This is corroborated by the simulations, which show that a diffraction process characterized by a VCD ≈ 0 (VCD ≠ 0) is polarization-independent (polarization-dependent). Conclusions: Overall, our results give a wealth of evidence that VCD allows for the experimental assessment of the scalar/vectorial regime of diffraction.