Enhancement of circular dichroism by a planar non-chiral magnetic metamaterial

We investigate the polarization properties of an elliptical nanohole array (ENA) penetrating through a metal/insulator/metal (MIM) film in the visible wavelength region. The calculated transmission spectra reveal that such a non-chiral magnetic planar metamaterial exhibits a strong transmission circular dichroism (CD). The proposed structure exhibits two surface plasmon polariton (SPP) modes polarized along the short and long diameters of the elliptical nanoholes at oblique incidence. When the elliptical nanoholes form a chiral arrangement with off-normal incidence, the two different polarization modes introduce a net polar vector that forms a chiral triad resulting in transmission CD, even though the MIM perforated with an ENA is a non-chiral structure. We also show that the transmission CD can be enhanced in an MIM-ENA compared with a single metal layer-ENA. This effect is due to the MIM-ENA allowing local magnetic dipole moments, hence increasing the polarization effects compared with a single metal layer-ENA where only electric dipole moments occur. The magnitude of the transmission CD can be tuned by modifying the geometrical parameters of the ENA.