Plasmonic vortex cavities toward the ultra-compact limit

Compact control of optical orbital angular momentum (OAM) is central to chip-scale photonic integration. Plasmonic vortex lenses have therefore emerged as a key on-chip platform for encoding and manipulating OAM through tunable topological-charge engineering. Advancing beyond plasmonic vortex lenses, here we propose plasmonic vortex cavities as a gap-plasmon-based cavity concept for vortex confinement, in which the plasmonic vortex is supported by a metal-insulator-metal (MIM) cavity with footprints below λ /10 and annular features smaller than λ /60. A unified physical framework is established and reveals the governing physics: the strongly flattened gap-SPP dispersion controlled by the gap width compresses the SPP wavelength and shrinks the vortex size toward the ultra-compact limit set by quantum effects; meanwhile, intrinsic cavity reflection yields strong standing-wave enhancement. Full-wave simulations corroborate the framework, revealing clear and strong PV fields within deep-subwavelength footprints. The paradigm transition from lens-type to cavity-type vortices provides an ultra-compact OAM platform, advancing chip-scale integration.