Ultrafast nonequilibrium carrier dynamics in a single graphene layer

Abstract

Nonequilibrium carrier dynamics in single exfoliated graphene layers on muscovite substrates are studied by ultrafast optical pump-probe spectroscopy and compared with microscopic theory. The very high 10-fs-time resolution allows for mapping the ultrafast carrier equilibration into a quasi-Fermi distribution and the subsequent slower relaxation stages. Coulomb-mediated carrier-carrier and carrier-optical phonon scattering are essential for forming hot separate Fermi distributions of electrons and holes which cool by intraband optical phonon emission. Carrier cooling and recombination are influenced by hot phonon effects.

Faraday effect in graphene enclosed in an optical cavity and the equation of motion method for the study of magneto-optical transport in solids

“…Our assumption is consistent with the values found in pump-probe experiments performed in exitaxial and exfoliated graphene samples 49,50 , and on infrared spectroscopy studies of the Drude conductivity of graphene. 51 Figure 11: Faraday rotation angle (given in degrees), normalized transmittance, and ellipticity of electromagnetic radiation passing through graphene subjected to a perpendicular magnetic field.…”

Relaxation Dynamics in Graphene

“…Common to all studies is a bi-exponential decay of the pump-induced di erential transmission (DT) spectrum. The fast decay component in the range of few tens of femtoseconds is assigned to an ultrafast Coulomb-dominated carrier redistribution towards a hot FermiDirac distribution, whereas the slower decay component in the range of a picosecond re ects the equilibration between the electron and the phonon system [22]. However, while some of the studies report on a purely positive transient DT spectrum [18,20,83], others exhibit a zero-crossing after the initial decay [19, 22, 23, 81, 84 86].…”

“…In graphene, a linearly polarized ultrafast optical pulse excitation gives rise to an initially anisotropic distribution of carriers at high energy [28,29]. The e cient carrier-carrier and carrier-phonon scattering processes quickly relax the hot carriers into an isotropic thermal distribution, which is then followed by carrier cooling due to carrier-phonon processes [17,22,23]. A variety of dynamical phenomena, such as the appearance of a signi cant carrier multiplication and transient optical gain, have been theoretically predicted [30 33] and experimentally con rmed [32,34,35].…”

“…However, for all other cases, where negative DT components occur for photon energies much larger than the Fermi energy, the underlying mechanism has been controversially discussed in literature. In particular, pump-induced bandstructure renormalization for transient [22] or in the long time limit [87] at the M -point as well as intraband absorption [24] have been suggested as explanations for the observed neg-ative DT signals. In this chapter a microscopic explanation for the occurrence of transient negative di erential transmission in graphene is presented, where the detailed interplay of intra-and interband absorption processes on the transient DT in graphene is investigated.…”

“…Typically, the carrier relaxation has been accessed via high-resolution pump-probe experiments [22,23,25,81,82]. Common to all studies is a bi-exponential decay of the pump-induced di erential transmission (DT) spectrum.…”