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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.
Stein, Jacob, "Ultrafast Time-Resolved Spectroscopy of Graphene after Mid-Infrared Excitation" (2013). Physics and Astronomy Honors Projects. 15.
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