Probing the quantum surface-response of materials using ultraconfined graphene plasmons

When graphene is placed in the near vicinity of a metal substrate, graphene plasmons are screened by the metal thereby giving rise to acoustic graphene plasmons. These exhibit record-high field confinement, squeezing light down to nanometer dimensions. Hence, plasmons in such heterostructures make ideal candidates to probe the quantum nonlocal electrodynamic response of the nearby metal. We treat graphene at the level of the RPA and describe the nonclassical optical response of the metal using a framework of mesoscopic electrodynamics based on microscopic surface-response functions, known as Feibelman d-parameters, which embody quantum corrections in the metal’s response. We show that the graphene plasmons’ resonances exhibit quantum shifts due to the quantum surface-response of the metal, and show how these spectral shifts can be used to interrogate the quantum surface-response of metals, thus provide a theoretical basis for experimentally inferring the said quantum surface-response.