  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 non-selectivity of light chirality or
a strong contribution to the background. Here, we theoretically propose an
achiral plasmonic resonator, based on a rectangular nanoslit in a thin gold
layer behaving as a magnetic dipole, to generate a tunable nanosource of purely
superchiral light. This nanosource is free of any background, and the sign of
its chirality is externally tunable in wavelength and polarization. These
properties result from the coupling between the incident wave and the magnetic
dipolar character of our nano-antenna. Thus, our results propose a platform
with deep subwavelength detection volumes for chiral molecules in particular,
in the visible, and a roadmap for optimizing the signal-to-noise ratios in
circular dichroism measurements to reach single-molecule sensitivity.
