Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70-120 fs range is followed by a slower relaxation process in the 0.4-1.7 ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene.

The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling occurs on sub-picosecond time scales and that interband recombination times are carrier density dependent.Graphene is a 2D lattice of carbon atoms arranged in a honeycomb crystal structure with a zero (or nearzero) bandgap and a linear energy-momentum dispersion relation for both electrons and holes [1,2]. The unique electronic and optical properties of graphene make it a promising material for the development of high-speed electron devices, including field-effect transistors, pn-diodes, Terahertz oscillators, and electronic and optical sensors [2,3,4,5,6,7]. The realization of graphene-based devices requires understanding the nonequilibrium carrier dynamics as well as the rate at which electron-hole recombination occurs.Measurements of the ultrafast intraband relaxation dynamics of photogenerated electrons and holes in epitaxial graphene using both degenerate [8] and non-degenerate [9] optical-pump optical-probe spectroscopy have been previously reported. Similar measurements for exfoliated graphene mono-and multi-layers have also been carried out [10]. These measurements were sensitive to the interband conductivity of graphene and probed the time evolution of the carrier occupation at specific energies in the bands. Consequently, they were not able to directly measure the time scales associated with carrier recombination. At room temperature, the optical response of graphene in the THz frequency range is described by the intraband conductivity -the free carrier responsewhich depends not only on the total carrier concentration but also on the carrier distribution in energy [11]. Therefore, THz radiation can be used to study the carrier relaxation and recombination dynamics in graphene. In this paper, we present results obtained from opticalpump THz-probe spectroscopy of epitaxial graphene in which the time-dependent conductivity of graphene that has been excited with an optical pump pulse is probed with a few-cycle THz pulse. We observe cooling of the photogenerated carrier distribution as well as electronhole recombination in graphene in real time. Our results indicate that the recombination times in graphene depend on the carrier density and material disorder.The epitaxial graphene samples used in this work were grown on the carbon face of semi-insulating 6H-SiC wafers using techniques that have been reported previously [12]. As discussed in [8,11], X-ray photoemission, Raman, and optical/IR/THz transmission spectroscopy ...

Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multi-layer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent carrier cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5-2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions.

We present experimental results on the optical absorption spectra of epitaxial graphene from the visible to the terahertz (THz) frequency range. In the THz range, the absorption is dominated by intraband processes with a frequency dependence similar to the Drude model. In the near IR range, the absorption is due to interband processes and the measured optical conductivity is close to the theoretical value of e 2 /4h. We extract values for the carrier densities, the number of carbon atom layers, and the intraband scattering times from the measurements.

Electron-hole generation and recombination rates for intravalley and intervalley phonon scattering in Graphene are presented. The transverse and the longitudinal optical phonon modes (E2g-modes) near the zone center (Γ-point) contribute to intravalley interband carrier scattering. At the zone edge (K(K ′ )-point), only the transverse optical phonon mode (A ′ 1 -mode) contributes significantly to intervalley interband scattering with recombination rates faster than those due to zone center phonons. The calculated recombination times range from less than a picosecond to more than hundreds of picoseconds and are strong functions of temperature and electron and hole densities. The theoretical calculations agree well with experimental measurements of the recombination rates of photoexcited carriers in graphene.