Relaxation dynamics of graphene investigated in the mid-infrared and THz spectral range

The unique band structure of graphene results in optical properties, which are promising with respect to graphene applications in detectors, saturable absorbers and frequency mixers. For these applications the knowledge of the carrier dynamics is essential. However, while the relaxation dynamics in graphene is extensively studied in pump-probe experiments involving pumping with near-infrared photons, only few studies exist for pumping in the mid-infrared and THz spectral range [1,2]. Here we present results of single-color pump probe experiments on epitaxial graphene on SiC studied in a wide range of photon energies (8 – 245 meV). A significant slowing down of the carrier relaxation is observed when the photon energy is reduced to values below the optical phonon energy of ~200 meV. For photon energies below twice the value of the Fermi energy (Ef ≈ -13 meV) negative pump-probe signals, i.e. induced absorption is observed. This is associated with intraband absorption contributions at energies, where interband absorption is initially not possible. Our experimental results are compared to microscopic calculations based on the density matrix formalism. Comparison of experiment and theory reveals the role of scattering via optical and acoustic phonons as well as contributions from Auger scattering processes. Applying a magnetic field perpendicular to the graphene layers leads to a not equidistant Landau-level splitting. Using circularly polarized radiation for pump-probe experiments allows one to selectively measure individual Landau-level transitions. The pump-probe signals exhibit a strong dependence on the polarization state of both pump and probe radiation.
[1] S. Winnerl et al. Phys. Rev. Lett. 107, 237401 (2011).
[2] S. Tani, F. Blanchard, and K. Tanaka, Phys. Rev. Lett. 109, 166603 (2012).