  Optically active achiral metasurfaces offer a promising way to detect chiral
molecules based on chiroptic methods. The combination of plasmonic enhanced
circular dichroism and reversible optical activity would boost the sensitivity
and provide enantiomerselective surfaces while using a single sensing site. In
this work, we use metasurfaces containing arrays of U-shaped resonators as a
benchmark for analyzing the optical activity of achiral materials. Although the
peculiar optical activity of these metasurfaces has 1 been quite well
described, we present here an experimental and numerical quantitative
determination of the different contributions to the measured optical activity.
In particular, it is shown that linear birefringence and retardance contribute,
but only marginally, to the apparent circular dichroism of the metasurface
associated with the excitation of magnetoelectric modes. We then numerically
demonstrate the peculiar near-field properties of the magneto-electric modes
and explain how these properties could be reflected in the far-field
polarimetric properties in the presence of chiral molecules. This work provides
alternatives for the detection scheme of chiral molecules using plasmonic
resonators.
