  We couple photoluminescent semiconducting 7-atom wide armchair edge graphene
nanoribbons to plasmonic nanoantenna arrays and demonstrate an enhancement of
the photoluminescence and Raman scattering intensity of the nanoribbons by more
than one order of magnitude. The increase in signal allows us to study Raman
spectra with high signal-to-noise ratio. Using plasmonic enhancement we are
able to detect the off-resonant Raman signals from the modified radial
breathing-like mode (RBLM) due to physisorbed molecules, the 3rd order RBLM,
and C-H vibrations. We find excellent agreement between data and simulations
describing the spectral dependence of the enhancement and modifications of the
polarization anisotropy. The strong field gradients in the optical near-field
further allow us to probe the subwavelength coherence properties of the phonon
modes in the nanoribbons. We theoretically model this considering a finite
coherence length along the GNR direction. Our results allow estimating the
coherence length in graphene nanoribbons.
