Propagation and localization of quantum dot emission along a gap-plasmonic transmission line

Plasmonic transmission lines have great potential to serve as direct interconnects between nanoscale light spots. The guiding of gap plasmons in the slot between adjacent nanowire pairs provides improved propagation of surface plasmon polaritons while keeping strong light confinement. Yet propagation is fundamentally limited by losses in the metal. Here we show a workaround operation of the gap-plasmon transmission line, exploiting both gap and external modes present in the structure. Interference between these modes allows us to take advantage of the larger propagation distance of the external mode while preserving the high confinement of the gap mode, resulting in nanoscale confinement of the optical field over a longer distance. The performance of the gap-plasmon transmission line is probed experimentally by recording the propagation of quantum dots luminescence over distances of more than 4 μm. We observe a 35% increase in the effective propagation length of this multimode system compared to the theoretical limit for a pure gap mode. The applicability of this simple method to nanofabricated structures is theoretically confirmed and offers a realistic way to combine longer propagation distances with lateral plasmon confinement for far field nanoscale interconnects.