Chiral sensing with achiral isotropic metasurfaces

Metasurfaces, the two-dimensional analogs of metamaterials, are ideal platforms for sensing molecular chirality at the nanoscale, e.g., of inclusions of natural optically active molecules, as they offer large accessible surfaces and can provide the necessary strong resonances for coupling the probing radiation with the chiral inclusions. Here, by employing a less familiar but more convenient formalism, we examine chiral inclusions in achiral isotropic metasurfaces; the latter sustain the electric and magnetic resonant dipoles to enhance the chirality features. We derive analytically, and verify numerically, expressions that provide insight to the enhancement mechanism of the magnetoelectric coupling and explain why circular dichroism signals (difference in absorption between left and right circularly polarized waves) can arise from both the real and the imaginary part of the chirality parameter kappa, correcting thus existing misconceptions. Our analysis demonstrates distinct chiroptical signals where the contributions from both the real and imaginary part of kappa can be independently observed and, based on such measurements, we propose a scheme for the unambiguous determination of an unknown chirality.