Raman spectroscopy of graphene under ultrafast laser excitation.
The equilibrium optical phonons of graphene are well characterized in terms of anharmonicity and electron-phonon interactions; however, their non-equilibrium properties in the presence of hot charge carriers are still not fully explored. Here we study the Raman spectrum of graphene under ultrafast laser excitation with 3 ps pulses, which trade off between impulsive stimulation and spectral resolution. We localize energy into hot carriers, generating non-equilibrium temperatures in the ~1700-3100 K range, far exceeding that of the phonon bath, while simultaneously detecting the Raman response. The linewidths of both G and 2D peaks show an increase as function of the electronic temperature. We explain this as a result of the Dirac cones' broadening and electron-phonon scattering in the highly excited transient regime, important for the emerging field of graphene-based photonics and optoelectronics.
Engineering and Physical Sciences Research Council (EP/K017144/1)
Engineering and Physical Sciences Research Council (EP/L016087/1)
EPSRC (via University of Manchester) (R119256)
European Commission Horizon 2020 (H2020) Future and Emerging Technologies (FET) (696656)