Field-Driven Multipolar Energy Transfer in Plasmonic Near Fields.

Plasmon-coupled resonance energy transfer (PC-RET) enables molecular excitations to be controlled by strongly confined electromagnetic fields. Here, we develop a plasmon-dressed, multipolar theory of PC-RET that explicitly incorporates both the local field and its gradients, as well as higher-order molecular multipolar transitions. Within this framework, the donor is treated as a field-driven multipolar emitter whose response is renormalized by the plasmonic near-field environment, giving rise to dipole-dipole, dipole-quadrupole, quadrupole-dipole, and quadrupole-quadrupole contributions. The donor-induced field and field gradient at the acceptor position are propagated to the acceptor via near-field interaction tensors, generating multiple energy-transfer channels that contribute both individually and through multipolar interference. Using a dipolar nanosphere model in which the radius controls the local field gradient, we show that plasmonic nanostructures can drive PC-RET far beyond the conventional dipole-dipole picture. Notably, although the total transfer rate retains a Förster-like d-6 distance dependence, the underlying mechanism can be dominated by gradient-enhanced quadrupole-quadrupole contributions within the dipole-allowed channel. Interference between multipolar pathways further modulates the dependence of the coupling on distance, direction, and molecular orientation. These results reveal a regime in which apparent Förster scaling masks fundamentally different microscopic mechanisms, and identify plasmon-induced field gradients and system geometry as key parameters for engineering nanoscale energy transfer with tunable pathways and directional selectivity.