An Achiral Secondary Grating Metasurface for Quasi-Bicenhanced Chiral Response and Sensing

This paper reports a reflective secondary grating metasurface in the visible spectrum for chiral sensing. Distinct from conventional chiral metasurfaces, the proposed design is geometrically achiral, while its chiral response is externally induced by tuning the incident light. The azimuth-dependent asymmetric distribution of the electromagnetic field generates a pronounced circular dichroism (CD) response. In the visible regime, the metasurface exhibits up to a ninefold enhancement in optical chirality density. Experimentally, an amplified CD signal is demonstrated for L-phenylalanine at concentrations as low as$50 \mu ~\mathrm{g} / \text{mL}$. Owing to its low optical loss, reduced background noise, and high sensitivity, this metasurface provides a promising and practical platform for molecular chirality detection.