Multipolar Coupling Engineered by Normalized Geometric Torsion for Enhanced Chiral Plasmonic Responses

ABSTRACT Plasmonic chiral nanoantennas offer exceptional opportunities for chiral sensing and polarization control, yet rational design is still limited by unclear higher‐order mode coupling and the absence of intuitive geometric guidance. Here, we investigate multipolar mode coupling in single gold helical nanoantennas via full polarizability tensor retrieval. We demonstrate that optical activity arises from electric dipole–magnetic dipole coupling at the fundamental resonance and electric dipole–electric quadrupole coupling at the higher‐order resonance. We introduce a normalized geometric torsion factor that quantitatively links structural twist to chiroptical response and mode‐dependent far‐field scattering, enabling direct optimization without exhaustive parameter scanning. This work establishes a unified framework for understanding and engineering strong optical chirality, providing a general strategy for high‐performance chiral nanophotonic devices.