Selective pump-probe measurements in Landau quantized graphene

In two dimensional electron gases with parabolic band structure, the Landau level (LL) system is equidistant, hence bleaching of the absorption is strongly suppressed. In contrast to quantum wells in conventional semiconductors the dispersion in graphene is linear for low energies. Due to this linearity the Landau levels are not equidistant and pump-probe measurements are feasible. Additionally a Landau level at zero energy appears. We performed pump-probe measurements on quasi-neutral sheets of rotated multilayer epitaxial graphene in magnetic fields. The free-electron laser FELBE at Dresden-Rossendorf served as radiation source at a constant wavelength of 16.5 µm (photon energy of ~75 meV). The magnetic field was applied perpendicularly to the graphene sheets by a magneto-optical superconducting magnet system, which is able to generate a magnetic field of up to 7T. This combination of photon energy and variable magnetic field allowed us to perform resonant pump-probe measurements at three different Landau level transitions (see inset of Fig: 1): For the transitions LL-1(-2) -> LL2(1) and LL-2(-3) -> LL3(2) we could observe a slight increase of the pump-probe signal while the relaxation time stayed constant. For the transition LL-1(0) -> LL0(1) the amplitude of the pump-probe signal increased by a factor of 2.5, the relaxation time decreased from 20 ps to 5 ps. In fig.1 the signal amplitude is plotted as a function of the square root of the magnetic field. The faster relaxation was contrary to our expectations: Due to the reduced phase space we expected the relaxation time to increase. For further insights into the carrier dynamics we performed measurements with circularly polarized radiation. Therewith it was possible to distinguish between the transitions LL-1->LL0 and LL0->LL1. These measurements revealed complex dynamics involving positive and negative signals with a very fast component and a slow component with an increased relaxation time. We present a model that takes into account Auger-type processes as well as carrier cooling. The model explains the main signatures of the measured signals.