Recent advances in nanofabrication technology now enable unprecedented control over 2D heterostructures, in which single- or few-atom thick materials with synergetic opto-electronic properties can be combined to develop next-generation nanophotonic devices. Precise control of light can be achieved at the interface between 2D metal and dielectric layers, where surface plasmon polaritons strongly confine electromagnetic energy. Here we reveal quantum and finite-size effects in hybrid systems consisting of graphene and few-atomic-layer noble metals, based on a quantum description that captures the electronic band structure of these materials. These phenomena are found to play an important role in the metal screening of the plasmonic fields, determining the extent to which they propagate in the graphene layer. In particular, we find that a monoatomic metal layer is capable of pushing graphene plasmons toward the intraband transition region, rendering them acoustic, while the addition of more metal layers only produces minor changes in the dispersion but strongly affects the lifetime. We further find that a quantum approach is required to correctly account for the sizable Landau damping associated with single-particle excitations in the metal. We anticipate that these results will aid in the design of future platforms for extreme light-matter interaction on the nanoscale.
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