Ultrafast spectro-microscopy of highly excited low dimensional materials

Born-Oppenheimer approximation (ABO) is the assumption that the motion of atomic nuclei and electrons in molecules can be separated and independently treated. In solids, ABO is well justified when the energy gap between ground and excited electronic states is larger than the energy scale of the nuclear motion. Graphene represents a notable exception of this acceptantance. In particular, here we unravel the key role of the gapless linear Dirac dispersion in the vibrational Raman response of the system in the case of impulsively photoexcited graphene. First, we unambiguously describe Four-Wave Mixing (FWM) processes in graphene, which depend on the resonant nature of the electronic interactions. Indeed, the overall spectral response is described in terms of a third order diagrammatic description of the light-matter interaction. We disclose that the interference between Coherent anti-Stokes Raman Scattering (CARS) and Non-Vibrationally Resonant Background (NVRB) generates Lorentzian dip spectral profiles. Actually, by introducing an experimental time delayed FWM scheme, able to modify the relative strength of the two contributions, we observe the first evidence of CARS peak equivalent to the Raman spectrum in graphene. Second, we adopt sub picosecond photoexcitation which impulsively localize energy into graphene electronic subsystem. While the response of hot charge carriers is well-characterized, unraveling the behavior of optical phonons under strongly out-of- equilibrium conditions remains a challenge. Using a 3-ps laser excitation, which trades off between impulsive stimulation and spectral resolution, 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 the G and 2D phonons as function of the carriers temperature in the range 1700-3100 K, suggesting a broadening of the Dirac cones. Accordingly, we reconsider the traditional scenario of the electron-phonon scattering in a highly excited transient regime