  Optical pump-terahertz probe spectroscopy has been used to investigate
ultrafast photo-induced charge carrier transport in epitaxial graphene
nanoribbons. The picosecond THz photoconductivity first increases with an
increasing pump fluence and then it saturates at high fluences. This behavior
is due to an interplay between contributions of the directly photoexcited
carriers and the secondary carriers, which are equilibrium conduction band
carriers strongly heated by the carrier-carrier scattering process after the
photoexcitation. This phenomenon leads to a non-monotonic variation of the
carrier mobility and plasmonic resonance frequency as a function of the pump
fluence and, at higher fluences, to a balance between a decreasing carrier
scattering time and an increasing Drude weight. In addition, a weak carrier
localization occurring at low pump fluences is progressively lifted as a result
of increasing initial carrier temperature.
