Ultrafast nonequilibrium carrier dynamics in a single graphene layer

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.

Figure 1

(a) Structure of the graphene layer on mica. The incident probe light hits the surface at an angle $\alpha ={35}^{°}$. (b) Schematic linear band structure of graphene, the arrow indicating the pumped and probed optical transitions. (c) Spectrally integrated transmission change (points) as a function of pump-probe delay. Solid line: biexponential fit. Gray line: cross correlation of pump and probe pulses. Inset: linear dependence of the maximum transmission change on pump fluence.

Figure 2

Spectrally resolved transmission change as a function of probe photon energy for different delay times [black (thick) lines]. Dotted line: spectrum of the pump pulses. Red (thin) lines: corresponding simulated carrier distribution changes $\Delta f$.

Figure 3

(a) Calculated equilibration of carrier distribution [$f(\hslash \omega$)] due to carrier-carrier and carrier-phonon scattering. (b) Change of the carrier distribution $\Delta f$ for the first picosecond integrated over the experiment’s relevant energy window (0.6–0.9 eV). (c) Time evolution of the carrier temperature $T$${}_c$ as determined by fitting a Fermi-Dirac distribution to the simulation results.