Ultrafast terahertz conductivity in epitaxial graphene nanoribbons: an interplay between photoexcited and secondary hot carriers

TL;DR

This study investigates ultrafast charge carrier dynamics in graphene nanoribbons using terahertz spectroscopy, revealing how pump fluence influences photoconductivity, carrier mobility, and plasmonic resonance through hot carrier interactions.

Contribution

It provides new insights into the interplay between photoexcited and secondary hot carriers affecting ultrafast conductivity in graphene nanoribbons.

Findings

Negative THz photoconductivity at low fluences due to secondary hot carriers

Saturation of photoconductivity at high fluences from excess carriers

Pump fluence-dependent non-monotonic carrier mobility and plasmonic resonance

Abstract

Optical pump-terahertz probe spectroscopy has been used to investigate ultrafast photo-induced charge carrier transport in 3.4 $\mu$m wide graphene ribbons upon scaling the optical pump intensity. For low pump fluences, the deposited pump energy is rapidly redistributed through carrier-carrier scattering, producing secondary hot carriers: the picosecond THz photoconductivity then acquires a negative sign and scales linearly with an increasing pump fluence. At higher fluences, there are not enough equilibrium carriers able to accept the deposited energy, directly generated (excess) carriers start to contribute significantly to the photoconductivity with a positive sign leading to its saturation behavior. This leads to a non-monotonic variation of the carrier mobility and plasmonic resonance frequency as a function of the pump fluence and, at high fluences, to a balance between a decreasing carrier scattering time and an increasing Drude weight. In addition, a weak carrier localization observed for the polarization parallel to the ribbons at low pump fluences is progressively lifted upon increasing the pump fluence as a result of the rise of initial carrier temperature.