Time resolved spectroscopy on quantum dots and graphene at the FELBE free-electron laser

The free-electron laser FELBE at the Forschungszentrum Dresden-Rossendorf enables experiments with spectral, temporal, and, by means of near-field microscopy, also high spatial resolution. FELBE delivers picosecond IR and THz pulses in a wavelength range from 5 µm to 280 µm. Operation in a continous pulsing regime and synchronization to various tabletop laser sources allows for two-color pump-probe experiments. Here we review the potential of the laser and focus on two highlight pump-probe experiments. In the first experiments, the relaxation dynamics in self assembled InGaAs quantum dots at energies below the Reststrahlen band is studied. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to 2 ps at 30 meV (7 THz). The results are in very good agreement with our microscopic theory of the carrier relaxation process, taking into account polaron decay via acoustic phonons [1]. In the second experiment, the relaxation dynamics in graphene is investigated at photon energies E = 20 – 250 meV. For excitations below the phonon energy of the G-mode, long relaxation times (20 – 40 ps) are observed. While the pump-induced transmission is positive for E > 30 meV, pump-induced absorption occurred for E < 20 meV. The increased transmission for E > 30 meV is caused by bleaching of the interband transition. We attribute the induced absorption to heating of carriers by intraband free-carrier absorption for E < 2E_F (E_F: Fermi energy).

[1] E. A. Zibik et al. Nature Materials 8, 803 - 807 (2009).