Spatial Coherence in Near-Field Raman Scattering

Inelastic light scattering in crystals has historically been treated as a spatially incoherent process, giving rise to incoherent optical radiation. Here we demonstrate that Raman scattering can be spatially coherent, in which case it depends on the dimensionality and symmetry of the scatterer. Using near-field spectroscopy, we measure a correlation length of $\ensuremath{\sim}30\text{ }\text{ }\mathrm{nm}$ for the optical phonons in graphene, the results varying with vibrational symmetries and spatial confinement of the phonons.