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Quantum mechanical effects of single particles can affect the collective plasmon behaviors substantially. In this work, the quantum control of plasmon excitation and propagation in graphene is demonstrated by adopting the variable quantum transmission of carriers at Heaviside potential steps as a tuning knob. First, the plasmon reflection is revealed to be tunable within a broad range by varying the ratio γ between the carrier energy and potential height, which originates from the quantum mechanical effect of carrier propagation at potential steps. Moreover, the plasmon excitation by free-space photos can be regulated from fully suppressed to fully launched in graphene potential wells also through adjusting γ, which defines the degrees of the carrier confinement in the potential wells. These discovered quantum plasmon effects offer a unified quantum-mechanical solution toward ultimate control of both plasmon launching and propagating, which are indispensable processes in building plasmon circuitry.

The idea of squeezing optical field intensity into nanoscopic dimensions can be achieved through plasmon polaritons, where the prerequisite is to bridge the unmatched momentum of plasmons and free-space photons. Conventionally, complicated subwavelength structures or artificial dipole nanostructures are adopted to impart the necessary momentum for the plasmon excitation. In this work, we show that by using the near-field imaging technique, the plasmon can be launched directly from the edge of graphene lying on the high-κ oxide substrates when illuminated by an infrared light. In addition, we show that such an edge-excited mode can be remarkably tailored by changing the angle between the graphene edge and the incident light field. Further theoretical analysis reveals the strength of the edge-excited mode and its superposition with a tip-excited mode and an edge localized mode. We attribute our findings to the reduced Coulomb scattering and phonon scattering in graphene, allowing the edge-excited mode to be identified. The conceptual edge "antenna" is found to be a very convenient approach to initiate plasmons in two-dimensional systems, which opens up a compelling route for realising nanophotonic applications.

Graphene superlattices have been extensively investigated and exhibit various emergent single-particle properties in addition to those of pristine graphene, yet to date, the collective plasmonic behaviors of the Dirac electrons in such systems remain largely unexplored. Here we use a microscopic description to explore the quantum plasmonic properties of one-dimensional graphene superlattices under physically realistic conditions. The emerging additional Dirac points at nonzero electron energies caused by the chiral nature of graphene carriers lead to interband excitations between the induced minibands. Such segments of the electron-hole pair continuum are strongly coupled to the plasmon modes along the superlattice direction. As a result, there exist a spectrum of emergent quantum plasmonic effects, including enhanced damping, multiple plasmon modes at the same wave vector, and oscillatory plasmon energies with respect to the sweeping Fermi level. In contrast, the coupling along the perpendicular direction is much weaker, and the plasmonic dispersion and damping remain largely intact as those of pristine graphene. These findings of fundamental nature may also offer additional technological potentials in graphene optoelectronics, such as plasmonic waveguides.