  We report plasmon-enhanced Raman scattering in graphene coupled to a single
plasmonic hotspot measured as a function of laser energy. The enhancement
profiles of the G peak show strong enhancement (up to $10^5$) and narrow
resonances (30 meV) that are induced by the localized surface plasmon of a gold
nanodimer. We observe the evolution of defect-mode scattering in a defect-free
graphene lattice in resonance with the plasmon. We propose a quantum theory of
plasmon-enhanced Raman scattering, where the plasmon forms an integral part of
the excitation process. Quantum interferences between scattering channels
explain the experimentally observed resonance profiles, in particular, the
marked difference in enhancement factors for incoming and outgoing resonance
and the appearance of the defect-type modes.
