Probing carrier dynamics in photo-excited graphene with time-resolved ARPES

The dynamics of photo-generated electron-hole pairs in solids are dictated by many-body interactions such as electron-electron and electron-phonon scattering. Hence, understanding and controlling these scattering channels is crucial for many optoelectronic applications, ranging from light harvesting to optical amplification. Here we measure the formation and relaxation of the photo-generated non-thermal carrier distribution in monolayer graphene with time- and angle-resolved photoemission spectroscopy. Using sub-10 fs pulses we identify impact ionization as the primary scattering channel, which dominates the dynamics for the first 25 fs after photo-excitation. Auger recombination is found to set in once the carriers have accumulated at the Dirac point with time scales between 100 and 250 fs, depending on the number of non-thermal carriers. Our observations help in gauging graphene's potential as a solar cell and TeraHertz lasing material.