Ladungsträgerdynamik in Graphen

This work reports on investigations of carrier dynamics in graphene after optical excitation by an ultrashort laser pulse. Optical pump-probe-experiments create an excited nonequilibrium carrier distribution and its subsequent relaxation is monitored at a temporal resolution of a few femtoseconds. In graphene, the role of extremly efficient electron-electron- and electron-phonon-scattering play a significant role in the thermalisation, cooling and recombination processes and the nature of the underlying scattering processes and the occurance of Pauli-blocking and the hot-phonon-effect and are identified by extensive variations of excitation density. Moreover, it is reported on the first direct experimental observation of carrier multiplication in graphene reaching a multiplication factor of up to 2 and persisting on a picoseconds time scale. Exploiting multicolor pump−probe measurement techniques, the excited nonequilibrium carrier distribution is retrieved on an ultrafast time scale. This provides access to the temporal evolution of the optically excited carrier density and thus allows quantitative conclusions on possible carrier multiplication. The experimental results confirm independent microscopic time- and momentum-resolved calculations on the ultrafast relaxation dynamics of optically excited carriers at extraordinary precision