The importance of spatial coherence in optics is well established, but so far it has been neglected in Raman spectroscopy targeting vibrational modes. Raman scattering, which is an inelastic process, has been broadly treated in papers and classical textbooks as spatially incoherent, but we show that inelastic light scattering in the near-field regime is a partially coherent process that can be used to measure nanoscale correlation lengths in various material systems.
We theoretically investigate the Raman modes of pristine monolayer graphene to determine how the scattered signal depends on the distance between the sample and a laser-irradiated gold tip; the tip acts as a broadband optical antenna to transmit information from the near field to the far field. As the correlation length increases, we find increasingly different behaviors for the strengths of various bands present in the Raman spectrum of graphene. We note that the characteristic correlation lengths are nearly an order of magnitude smaller than optical wavelengths. As a result of coherence, we find that the Raman intensities on the nanoscale depend strongly on phonon symmetry and spatial confinement.
Our work presents a theoretical breakthrough in our understanding of inelastic scattering and defines a new paradigm for studying correlation properties in emerging material systems.
