Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

As a gapless material with linear dispersion graphene is of great interest for the infrared spectral range. In this study we investigate the carrier relaxation dynamics of graphene in a wide spectral range from the near to the far-infrared, covering more than two orders of magnitude in photon energy.
The samples for this study are epitaxially grown graphene layers on the carbon-terminated face of SiC, which behave essentially like a stack of electronically uncoupled layers. In the near infrared (NIR) spectral range (photon energy 0.41 eV – 1.5 eV) degenerate and two-color experiments were performed. In the mid (MIR) and far infrared (FIR) range (photon energy 10 meV – 250 meV) degenerate pump-experiments were carried out employing the free-electron laser FELBE as a source.
In the NIR range pump-induced transmission was observed in two-color experiments with both red and blue shifted probe radiation. The signals in case of the blue shifted probe are evidence for a hot carrier distribution. The thermalization process is beyond the temporal resolution of our experiment (~ 100 fs in the near infrared). The observed decay times are in the range of 2 – 4 ps. In the MIR range we observe a significant increase of the relaxation time as the photon energy is decreased to values below the optical phonon energy (~200 meV). These experiments are complemented by microscopic theory based on the density matrix formalism [1]. The theory reflects the trends seen in the experiment well. It reveals the contribution of Coulomb scattering as well as the role of both optical and acoustic phonons in the observed dynamics.
In the FIR range an unexpected change from enhanced transmission to enhanced absorption is found (cf. Fig. 1). It is caused by an interplay of interband and intraband processes. For photon energies above twice the value of the Fermi energy, bleaching of interband transitions results in pump-induced transmission. For smaller photon energies, however, interband transitions are not possible. Here intraband transitions cause a heating of the carrier distribution, which is responsible for the intraband absorption.