  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.
