Ultrafast carrier kinetics in exfoliated graphene and thin graphite films

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Abstract
Time-resolved transmissivity and reflectivity of exfoliated graphene and thin graphite films on a 295 K SiO2/Si substrate are measured at 1300 nm following excitation by 150 fs, 800 nm pump pulses. From the extracted transient optical conductivity we identify a fast recovery time constant which increases from ~200 to 300 fs and a longer one which increases from 2.5 to 5 ps as the number of atomic layers increases from 1 to ~260. We attribute the temporal recovery to carrier cooling and recombination with the layer dependence related to substrate coupling. Results are compared with related measurements for epitaxial, multilayer graphene.