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 that progress toward the compactness limit—footprints < λ/10 with annular features ~λ/40. By developing a perturbative framework, we reveal the governing mechanism: a flattened and gap-tunable cavity plasmon dispersion that extremely compresses the surface plasmon polariton (SPP) wavelength. Besides, cavity PVs also exhibit quantized and resonance-enhanced spectral bands together with Bessel-type near field envelopes, originating from cavity SPP reflection at the rim and the ensuing standing wave formation. Full-wave simulations corroborate the framework, revealing discrete cavity resonances, clean phase singularities, and predictable hot-spot radii within deep-subwavelength footprints. The progression from lens-based vortices to cavity-based vortices provides an ultra-compact OAM platform toward the compactness limit, advancing chip-scale integration.