Highly anisotropic hybridization, dispersion, damping, and propagation of quantum plasmons in graphene superlattices

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.