Relaxation dynamics in graphene excited with low photon energies

The pump-induced transmission of graphene grown epitaxially on hexagonal SiC was studied in degenerate pump-probe experiments with photon energies in the range from 10 to 250 meV. Different relaxation times ranging from 0.5 ps to 300 ps were found. A strong increase of the relaxation time was observed when the photon energy was below the optical phonon energy (~200 meV). The experiments were complemented by microscopic modelling based on the graphene Bloch equations. This allowed us to identify the physical processes which are responsible for the carrier relaxation channels. We discuss the role of Auger-type processes, optical and acoustic phonons. The modelling, which is in good agreement with the experimental results, reveals that optical phonon scattering becomes significantly less efficient when the energy of the excitation photons is below the optical phonon energy. Surprisingly, however, scattering via optical phonons is still the predominant relaxation channel, even at lower energies. This is attributed to the presence of hot carriers, which can still fulfil energy conservation requirements [1].
For photon energies about twice the value of the Fermi energy Ef, which is about 10 meV for the quasi-intrinsic layers of the epitaxial graphene, an interesting effect is observed. As the photon energy is reduced below 2Ef, a transition from pump-induced transmission to pump-induced absorption occurs. While the induced transmission is associated with Pauli blocking of carriers which underwent interband transition, the induced absorption is caused by a change of the electron temperature due to intraband absorption.