Ultrafast Time-Resolved Spectroscopy of Graphene after Mid-Infrared Excitation

In this time-resolved photoconductivity experiment we photo-excite graphene with femtosecond pulses of mid-infrared light and measure the resulting change in transmission of terahertz frequency radiation. In single-layer CVD graphene films we observe an increase in terahertz transmission following photoexcitation, reflecting either a light-induced decrease in conductivity or gain at terahertz frequencies. Previous timeresolved terahertz transmission experiments in CVD graphene have reported either gain or a decrease in loss. Gain has been attributed to photo-excitation of electrons from the valence band of graphene to the conduction band leading to a population inversion, while decrease in loss has been attributed to carrier heating due to photo-excitation. Our work reproduces earlier reports of increased terahertz transmission following photoexcitation by near-infrared and mid-infrared light, but we also observe this phenomenon after photo-excitation with long-wavelength infrared light where interband excitation is not possible, nullifying the hypothesis of interband gain, and instead supporting the model of decreased conductivity due to heating of the electron gas