Phonon anomalies in Graphene induced by highly excited charge carriers

Summary form only given. Electron-phonon scattering and anharmonicity are the dominant mechanisms, that enable to describe the equilibrium phonon properties in graphene and Raman scattering is the main tool for their characterization. In the first tens fs after the photoexcitation, an out of equilibrium distribution of (hot) electron is induced with respect to the (cold) phonon bath. Within a few picoseconds, the fast electron-electron and electron-phonon non radiative recombination channels determine the equilibrium between the electronic distribution and the lattice. Therefore, on the laboratory timescale, continuous wave laser sources, commonly used for high resolution spontaneous Raman scattering, examine already equilibrated carrier-phonon distributions. A way to impulsively localize energy into graphene's electronic subsystem is provided by sub picosecond photoexcitation. While the behaviour of hot charge carriers to such ultrafast perturbation has been thoroughly elucidated unraveling the nature of optical phonons under a strongly out of equilibrium regime is a challenge. We perform spontaneous Raman measurements in graphene by using a 3-ps laser excitation, which is revealed to be a good agreement between impulsive stimulation and the necessity of spectral resolution. Furthermore, we show how the Raman response of graphene can be detected in presence of an electronic subsystem temperature largely exceeding that of the phonon bath. We find a peculiar behaviour of the period and lifetime of both G and 2D phonons as function of the carriers' temperature in the range 1700-3100 K, which is strongly suggestive of a smearing out of the Dirac cones. We rationalize such behaviour by accordingly revisiting the traditional theoretical modeling of the electron-phonon coupling in this highly excited transient scenario, which is critical in the emerging field of graphene-based nanophotonic and optoelectronic devices operating at THz rates.