Diversity of ultrafast hot-carrier-induced dynamics and striking sub-femtosecond hot-carrier scattering times in graphene

We study ultrafast dynamics in graphene grown by chemical vapor deposition, which presents a positive peak followed by a negative slow recovery process, and is different from the fully positive or negative dynamics reported. We discuss the diversity of ultrafast dynamics. A dynamic model of differential optical conductivity is developed to simulate ultrafast dynamics. It is found that the diversity of ultrafast dynamics originates from multiple parameter dependence. The appearance of ultrafast dynamics is mainly determined by the position of probed level with respect to static Fermi energy and/or momentum scattering time. The dynamic model is used to best fit the observed ultrafast dynamics to retrieve dynamic time constants. The thermalizing and cooling times of optical phonons are found consistent with the reported values, but a striking sub-femtosecond momentum scattering time and a sub-picosecond time of electron-hole recombination are obtained. The sub-femtosecond scattering time is much shorter than several to a few hundred femtoseconds accepted currently. (C) 2014 Elsevier Ltd. All rights reserved.