High‐harmonic generation (HHG) in graphene heterostructures, consisting of metallic
nanoribbons separated from a graphene sheet by either a few‐nanometer layer of aluminum
oxide or an atomic monolayer of hexagonal boron nitride, is investigated. The nanoribbons
amplify the near‐field at the graphene layer relative to the externally applied pumping, thus
allowing the observation of third‐and fifth‐harmonic generation in the carbon monolayer at
modest pump powers in the mid‐infrared. The dependence of the nonlinear signals on the …