Electron-phonon scattering, along with anharmonicity, has a pivotal role to enable the description of the equilibrium phonon properties in graphene  and Raman scattering is the main avenue to their characterization . The interaction with light initially generates an out of equilibrium distribution of (hot) electrons with respect to the (cold) phonon bath. Relaxation to thermal equilibrium occurs within a few picoseconds through the fast electron-electron and electron-phonon non radiative recombination channels. Hence, on the laboratory timescale, continuous wave laser sources commonly used for high resolution spontaneous Raman scattering probe a charge carriers-lattice system already thermally equilibrated.
Sub picosecond photoexcitation represents a way to impulsively localize energy into graphene’s electronic subsystem. While the response of hot charge carriers to such ultrafast perturbation has been thoroughly investigated [3,4], disclosing the behaviour of optical phonons under strongly out of equilibrium conditions remains a challenge. We perform spontaneous Raman measurements in graphene by using a 3-ps laser pulses, which is revealed to trade off 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 . The energy and lifetime of both the G and 2D phonons as function of the carriers’ temperature in the range 1700-3100 K indicates a broadening of the Dirac cones. This is explained by modelling the electron-phonon scattering  in a highly excited transient regime, which is critical for the emerging field of graphene-based nanophotonic and optoelectronic devices operating at THz rates.
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