Circular dichroism in low-cost plasmonics: 2D arrays of nanoholes in silver

Arrays of nanoholes in metal are important plasmonic devices proposed for applications spanning from biosensing to communications. In this work we show that in such arrays the symmetry can be broken by means of the elliptical shape of the nanoholes, combined with the in-plane tilt of the ellipse axes away from the array symmetry lines. The array then differently interacts with circular polarizations of opposite handedness at normal incidence, i.e. it becomes intrinsically chiral. The measure of this difference is called circular dichroism (CD). The nanosphere lithography combined with tilted silver evaporation was employed as a low-cost fabrication technique. We demonstrate intrinsic chirality and CD by measuring the extinction in the near-infrared range. We further employ numerical analysis to visualize the circular polarization coupling with the nanostructure. We find good agreement between simulations and the experiment, meaning that the optimization can be used to further increase CD.