Plasmonic Nanocavity-Assisted Long-Range Dipole–Dipole Interactions for Rare-Earth Ions

) exhibit sharp 4f-4f transitions that are highly sensitive to dipole-dipole interactions (DDIs), yet the intrinsic short-range of DDIs restricts their application in long-distance quantum coupling. Here, we demonstrate a plasmonic nanocavity pair that simultaneously couples to upconversion nanoparticles (UCNPs) and quantum dots (QDs), enabling enhanced emission and inter-emitter interaction. The combined effects of Purcell enhancement and surface plasmon polariton (SPP) propagation extend the DDI-mediated energy transfer over a record distance exceeding 7.5 μm, 2 orders of magnitude longer than conventional Förster resonance systems. Moreover, the polarization-controlled SPP propagation directs the in-plane energy redistribution, achieving anisotropic and tunable coupling. This work establishes a viable route toward controllable, long-range DDIs among rare-earth emitters, advancing integrated quantum photonic architectures.