Coherence length (Lc) of the Raman scattering process in graphene as a function of Fermi energy is obtained with spatially coherent tip-enhanced Raman spectroscopy. Lc decreases when the Fermi energy is moved into the neutrality point, consistent with the concept of the Kohn anomaly within a ballistic transport regime. Since the Raman scattering involves electrons and phonons, the observed results can be rationalized either as due to unusually large variation of the longitudinal optical phonon group velocity vg, reaching twice the value for the longitudinal acoustic phonon, or due to changes in the electron energy uncertainty, both properties being important for optical and transport phenomena that might not be observable by any other technique.
Keywords: Coherence Length.; Graphene; Kohn anomaly; TERS.