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

摘要

The free-electron laser FELBE at the Helmholtz-Zentrum 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 4 um to 280 um. Here we review the potential of the laser and focus on two highlight pump-probe experiments. In the first experiment, 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. In the second experiment, the relaxation dynamics in graphene is investigated at photon energies E = 20 - 250 meV. For excitations below the energy of the optical phonon (G mode), the relaxation times are more than one order of magnitude longer as compared to the relaxation times observed for near infrared excitation.