Ambipolar charge-transfer graphene plasmonic cavities

Plasmon polaritons in van der Waals materials hold promise for various photonics applications^1-4. The deterministic imprinting of spatial patterns of high carrier density in plasmonic cavities and nanoscale circuitry can enable the realization of advanced nonlinear nanophotonic⁵ and strong light-matter interaction platforms⁶. Here we demonstrate an oxidation-activated charge transfer strategy to program ambipolar low-loss graphene plasmonic structures. By covering graphene with transition-metal dichalcogenides and subsequently oxidizing the transition-metal dichalcogenides into transition-metal oxides, we activate charge transfer rooted in the dissimilar work functions between transition-metal oxides and graphene. Nano-infrared imaging reveals ambipolar low-loss plasmon polaritons at the transition-metal-oxide/graphene interfaces. Further, by inserting dielectric van der Waals spacers, we can precisely control the electron and hole densities induced by oxidation-activated charge transfer and achieve plasmons with a near-intrinsic quality factor. Using this strategy, we imprint plasmonic cavities with laterally abrupt doping profiles with nanoscale precision and demonstrate plasmonic whispering-gallery resonators based on suspended graphene encapsulated in transition-metal oxides.