Near-Perfect Chirality and Giant Spin-Orbit Conversion in a Single Plasmonic Cavity.

To overcome the difficulty of single nanostructures in approaching the theoretical limit of chiroptical performance, we design a single plasmonic twisted dimer cavity whose magnetic gap plasmon mode enables magnetic polarization near-field engineering for high chirality. The structure exhibits strong extinction under circularly polarized excitation with one handedness, while its response to the orthogonally circularly polarized light is almost perfectly suppressed, yielding a chiral g-factor as high as 1.94. Meanwhile, the structure demonstrates strong chiral-selective spin-orbit angular momentum conversion with ∼95% efficiency under one handedness and only ∼1% under the other. By tuning geometric parameters, the g-factor can be continuously adjusted from 0 to 1.94. Without relying on periodic coupling or collective effects, this work achieves near-perfect chirality and highly efficient angular momentum manipulation solely through intrinsic near-field matching, providing a new design strategy for highly selective, ultracompact integrated chiral photonic devices.