Title: Tuning quantum non-local effects in graphene plasmonics,
Abstract: The response of an electron system to electromagnetic fields with sharp
spatial variations is strongly dependent on quantum electronic properties, even
in ambient conditions, but difficult to access experimentally. We use
propagating graphene plasmons, together with an engineered dielectric-metallic
environment, to probe the graphene electron liquid and unveil its detailed
electronic response at short wavelengths.The near-field imaging experiments
reveal a parameter-free match with the full theoretical quantum description of
the massless Dirac electron gas, in which we identify three types of quantum
effects as keys to understanding the experimental response of graphene to
short-ranged terahertz electric fields. The first type is of single-particle
nature and is related to shape deformations of the Fermi surface during a
plasmon oscillations. The second and third types are a many-body effect
controlled by the inertia and compressibility of the interacting electron
liquid in graphene. We demonstrate how, in principle, our experimental approach
can determine the full spatiotemporal response of an electron system.