Quadrupole-Driven Chiral Emission with Dipole-Quadrupole Interference Gating in an Achiral Nanocrescent.

Achiral plasmonic resonators are expected to emit helicity-symmetric radiation under reciprocal excitation. Here, a localized free-electron source is shown to deterministically open a chiral radiation channel in an achiral nanocrescent by activating a tensorial electric quadrupole resonance. Angle-resolved cathodoluminescence reveals switchable circular polarization with |CD| > 0.5 and a concomitant reversal of the emission direction when the electron beam is shifted between opposite tips. Moreover, a current-based multipolar analysis and far-field reconstruction framework identify a quadrupole-dominated band whose leading tensor elements split into diagonal and shear subspaces with near-quadrature phase relations, providing the phase resource for helicity, while directionality is set by electric dipole/electric quadrupole interference. These results establish free-electron excitation as a compact route to engineer chiral radiation without geometrically chiral architectures while also providing a transferable framework for analyzing and designing helicity-selective far-field emission in more general nanophotonic systems.