Achiral Magnetic Photonic Antenna as a Tunable Nanosource of Chiral Light

Sensitivity to molecular chirality is crucial for many fields, from biology and chemistry to the pharmaceutical industry. By generating superchiral light, nanophotonics has brought innovative solutions to reduce the detection volume and increase sensitivity at the cost of a nonselectivity of light chirality or a strong contribution to the background. Here, we theoretically propose a simple achiral plasmonic resonator based on a rectangular nanoslit in a thin metallic layer behaving as a magnetic dipole to generate a tunable nanosource of purely chiral light working from the UV to the infrared. This nanosource is free of any background, and the sign of its chirality is externally tunable in wavelength and polarization. These unique properties, resulting from the coupling between the incident wave and the magnetic dipolar character of our nanoantenna, coupled with a method of Fluorescent Detected Circular Dichroism (FDCD), shown to be 2 orders of magnitude more sensitive than classical circular dichroism measurements, thus provide a platform with deep subwavelength detection volumes for chiral molecules and a roadmap for optimizing the signal-to-noise ratios in circular dichroism measurements to reach single-molecule sensitivity.