Long-lived anisotropic carrier populations in graphene excited by THz pulses

Calculations of non-equilibrium charge carrier distributions excited by polarized light indicate anisotropic occupations in momentum space. For most materials experimental insight in this anisotropy is obscured due to complex valence band structures [1]. Recently we have experimentally verified anisotropic carrier distributions and investigated their relaxation in graphene [2]. A near-infrared pump probe experiment with varying angle between pump and probe polarizations revealed anisotropic carrier populations on a 100 fs timescale as predicted by microscopic theory. An isotropic distribution is then reached by scattering via optical phonons. In graphene the Coulomb scattering, in first approximation, is restricted to collinear scattering along a line of the cone-like band structure and therefore preserves the angular orientation of the distribution.
By shifting the photon energies into the THz range (88 meV) and cooling the sample down to 20K we were now able to observe anisotropic charge carrier distributions with a lifetime as long as several picoseconds. Under these conditions scattering via optical phonons, with energies of about 200meV, is strongly suppressed. Note that the observed distribution has a very unusual character: It is completely thermalized for each k-direction pointing radially away from the Dirac point, but at the same time it is strongly anisotropic. The anisotropy is most pronounced for low pump fluences. Increasing the pump fluence from several nJ/cm² to µJ/cm² results in a transition from anisotropic to isotropic distributions. We suggest that this is associated with optical phonon scattering that is enabled at high electron temperatures. The experimental results are compared with microscopic theory that takes into account combined effects of Coulomb and carrier-phonon scattering.

References
[1] J. Rioux, J.E. Sipe, Physica E 45, 1-15 (2012)
[2] M. Mittendorff, T. Winzer, E. Malic, A. Knorr, C. Berger, W. A. de Heer, H. Schneider, M. Helm and S. Winnerl, Nano Lett. 14, 1504-1507 (2014)