Observation of coherent G-mode phonon oscillations in graphene films

We have observed coherent G-mode lattice vibrations in three stacked-mono layer and multi-layer graphene films by using ultrashort pulses from a Ti:Sapphire laser. The degenerated $E_{2g} $modes were excited through the impulsive stimulated Raman scattering process, and detected through induced reflectivity modulations. The G-mode frequency from the stacked-mono layer graphene is shifted toward higher energy compared with that of the multi-layer graphene. A dephasing time of abound 0.6 ps for the stacked-mono layer graphene was found to be shorter than that of semiconducting single-walled carbon nanotubes (1.48 ps) and slower than that of metallic single-walled carbon nanotubes, due to stronger electron-phonon interactions, where the phonon energy can be dissipated by exciting electrons between a linear bands of graphene. Through the strong polarization dependence of coherent G-mode lattice vibrations, we confirmed that the $E_{2g}^{(2)} $ symmetry is dominant.