Microscopic theory of absorption and ultrafast many-particle kinetics in graphene

We investigate the relaxation kinetics of optically excited charge carriers in graphene focusing on the time-, momentum-, and angle-resolved interplay between carrier-carrier and carrier-phonon scattering channels. To benchmark the theoretical approach, we first discuss the linear absorption spectrum of graphene. In agreement with recent experimental results, our calculations reveal: (i) a pronounced excitonic effect at the saddle point, (ii) a constant absorbance in the visible region, and (iii) a drop-off for energies close to the Dirac point. After a nonlinear optical excitation, we observe that $\ensuremath{\Gamma}$-LO phonons efficiently and quickly redistribute the initially highly anisotropic nonequilibrium carrier distribution. In contrast, Coulomb-induced carrier relaxation is preferably carried out directly toward the Dirac point leading to an ultrafast thermalization of the carrier system. We evaluate the temporal dynamics of optical and acoustic phonons and discuss the energy dissipation arising from phonon-induced intra- and interband scattering. Furthermore, we investigate the influence of diagonal and off-diagonal many-particle dephasing on the ultrafast carrier relaxation dynamics. The gained insights contribute to a better microscopic understanding of optical and electronic properties of graphene.