The interaction of circularly polarized light with matter is the basis for molecular circular dichroism spectroscopy, optical spin manipulation, and optical torques. However, chiroptical effects are usually hampered by weak chiral light-matter interaction. Nanophotonic structures can enhance optical intensity to boost interactions, but magnifying chiral effects requires that the near field remains chiral in the process.
Here, we propose the conditions, limits and guidelines for enhancing different chiroptical effects near achiral metasurfaces with maximum chirality of the evanescent fields. We illustrate these conditions with arrays of metal and dielectric nanodisks and decompose their distinct electromagnetic metrics into propagating and evanescent Fourier orders. We prove that chirality metrics like circular dichroism and the degree of circular polarization, which go hand-in-hand in the far field for propagating plane waves, are incompatible in the evanescent near field. As a result, a nanostructure cannot be universally optimal for different chirality metrics and therefore applications. For example, arrays tailored for enhanced spin excitation with spatially uniform circular polarization destroy circular dichroism. Conversely, we predict a limit of maximum attainable circular dichroism in highly evanescent Fourier orders through a simple relation with the evanescent wavevector and polarization. We demonstrate that silicon nanodisk arrays can enhance chiral effects within these constraints.
Our results define the limits on the ability of nanophotonic platforms to enhance different chiral light-matter interactions simultaneously. Our work provides design rules for diverse chiroptical applications such as molecular spectroscopy, information technology and optical nanomanipulation.