We characterize plasmonic enhancement in a hotspot between two Au nanodisks using Raman scattering of graphene. Single layer graphene is suspended across the dimer cavity and provides an ideal two-dimensional test material for the local near-field distribution. We detect a Raman enhancement of the order of 10(3) originating from the cavity. Spatially resolved Raman measurements reveal a near-field localization one order of magnitude smaller than the wavelength of the excitation, which can be turned off by rotating the polarization of the excitation. The suspended graphene is under tensile strain. The resulting phonon mode softening allows for a clear identification of the enhanced signal compared to unperturbed graphene.

We report calculated Raman spectra of the D and 2D modes in graphene and graphite. Evaluating the Raman amplitude in the two-dimensional Brillouin zone, we reproduce the splitting of the modes when going from single-layer graphene to graphite. The energy dependence of the D mode in graphene is 24%-32% smaller than in graphite. We discuss the intensity of the D line and show that the double resonant phonons originate from the low-symmetry parts of the Brillouin zone.

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