Circular dichroism in a plasmonic array of elliptical nanoholes with square lattice

Chiral properties of plasmonic metasurfaces, especially related to different absorption of left and right circularly polarized light leading to circular dichroism (CD), are a research hot topic in nanophotonics. There is often a need to understand the physical origin of CD for different chiral metasurfaces, and to get guidelines for the design of structures with optimized and robust CD. In this work, we numerically study CD at normal incidence in square arrays of elliptic nanoholes etched in thin metallic layers (Ag, Au, Al) on a glass substrate and tilted with respect to the symmetry axes. Strong CD arises in absorption spectra at the same wavelength region of extraordinary optical transmission, indicating highly resonant coupling between light and surface plasmon polaritons at the metal/glass and metal/air interfaces. We elucidate the physical origin of absorption CD by a careful comparison of optical spectra for different polarizations (linear and circular), with the aid of static and dynamic simulations of local enhancement of the electric field. Furthermore, we optimize the CD as a function of the ellipse parameters (diameters and tilt), the thickness of the metallic layer, and the lattice constant. We find that silver and gold metasurfaces are most useful for CD resonances above 600 nm, while aluminum metasurfaces are convenient for achieving strong CD resonances in the short-wavelength range of the visible regime and in the near UV. The results give a full picture of chiral optical effects at normal incidence in this simple nanohole array, and suggest interesting applications for chiral biomolecules sensing in such plasmonic geometries.