Momentum resolved optical pump-probe spectroscopy in monolayer graphene: An analytical model and measurements

Further development in graphene based photonics and optoelectronics requires fundamental information on the evolution of the strongly non-equilibrium charge carrier distribution created by the light-carrier interaction. Here, we report polarization and fluence dependent ultrafast optical pump-probe spectroscopy of high quality CVD-grown monolayer graphene. An analytical model has been developed and employed to describe the experiments. Graphene offers a unique opportunity to probe the photocarrier occupation, not only at different energies using a two-color setup, but also in different directions in momentum space applying linearly polarized beams. The latter approach is possible due to the pseudospin-momentum coupling which results in an optical pseudospin-selection rule. Our method allows us to quantify and control the relative contributions of both the strongly non-equilibrium anisotropic occupation and the hot Fermi-Dirac photocarrier distribution to the total differential transmission measured. We provide a conclusive and quantitative evidence for an anisotropic photocarrier occupation with a life-time of about $100\,\mathrm{fs}$ and claim that its relaxation towards the isotropic distribution occurs mostly due to optical phonon emission.