Hot Dirac Fermion Dynamics and Coherently Controlled Photocurrent Generation in Epitaxial Graphene.

Abstract

We investigate the ultrafast relaxation dynamics of hot Dirac Fermionic quasiparticles in multilayer epitaxial graphene using ultrafast optical differential transmission (DT) spectroscopy. We observe DT spectra which are well described by interband transitions with no electron-hole interaction. Following the initial thermalization and emission of high-energy phonons, electron cooling is determined by electron-acoustic phonon scattering. The spectra also provide strong evidence for the multilayer structure and a measure of the doping profile, thus giving insight into the screening length in thermally grown epitaxial graphene on SiC. From the zero crossings of the differential transmission (DT) signal tails, we can resolve 4 heavily doped layers with Fermi levels of 361meV, 214meV, 140meV, 93meV above the Dirac point in the sample, respectively. The screening length is determined to be 2-3 layers in carbon face grown epitaxial graphene. The measured DT spectrum can be well explained by a dynamic conductivity simulation incorporating the in plane disorder and an elevated lattice temperature. We observed evidence for thermal coupling of hot carriers between graphene layers by ultrafast degenerate pump-probe spectroscopy and determined the interlayer thermal coupling time to be below the time resolution of the experiment (100fs). A second series of experiments focuses on the generation of ballistic electric currents in unbiased epitaxial graphene at 300 K via quantum interference between phase-controlled cross-polarized fundamental and second harmonic 220- fs pulses. The transient ballistic currents are detected via the emitted terahertz radiation. Due to graphene’s special structural symmetry, the injected current direction can be well controlled by the polarization of the pump beam in epitaxial graphene. The results match theoretical calculations showing that the current direction can be controlled through changing the relative phase between two pump beams. By pre-injecting background hot carriers into the system, we study the enhancement of hot carriers in phase breaking scattering due to hot carriers and the results show that this scattering rate increased monotonically with the hot electron temperature. This all-optical current injection provides not only a non-contact way of injecting directional current into graphene, but also new insight into optical and transport processes in epitaxial graphene.