Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The peculiar properties of graphene, most importantly its gapless, linear band structure, give rise to exciting phenomena such as constant optical absorption over a wide range of photon energies and an unusual quantum Hall effect. Furthermore graphene is considered as an interesting material for electronic and optoelectronic applications. A good understanding of the relaxation dynamics is essential both for fundamental research and applications.
After an introduction into graphene research, I will present our results of pump-probe experiments in the mid- and far-infrared spectral range (photon energy 245 meV – 10 meV). We identify the role of optical phonons in the relaxation dynamics and discuss contributions from Auger-type processes and acoustic phonons. Furthermore an interesting change in sign of the pump-probe signal is observed at low photon energies (20 – 30 meV), which can be explained by an interplay of intraband and interband absorption processes [1]. In the second part of the talk I will focus on the relaxation dynamics in Landau-quantized graphene. Since the Landau levels (LLs) in graphene are not equidistant but scale with both the square root of the LL index and the square root of the magnetic field, optical pumping of individual LL-transitions is possible. Here we focus on the LL-1 → LL0 and LL0 → LL1 transitions, which exhibit equal transition energies but can be distinguished by applying either right-handed or left-handed circularly polarized light. Employing all four combinations of pumping and probing with right-handed and left-handed circularly polarized light, respectively, results in complex pump-probe signals: They involve fast and slow components, which exhibit either positive or negative signs. The relaxation dynamics in this regime can be understood by attributing the fast components to Auger-type processes and the slower components to phonon-mediated energy relaxation.
[1] S. Winnerl, M. Orlita, P. Plochocka, P. Kossacki, M. Potemski, T. Winzer, E. Malic, A. Knorr, M. Sprinkle, C. Berger, W. A. de Heer, H. Schneider, and M. Helm, Phys. Rev. Lett. 107, 237401 (2011).