Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible Region

We investigated the ultrafast carrier dynamics and phonon relaxation of CVD-grown monolayer and 9-layer graphene on a quartz substrate. Excitation was performed at 400 and 800 nm. The normalized change in optical density Delta OD was probed over the range 260-640 nm (1.94-4.77 eV), reaching down into the region of graphene's Fano resonance, previously not investigated in femtosecond broadband pump-probe experiments. Time constants of 160 fs and 4 Ps were found and assigned to carrier-optical phonon scattering and slower phonon relaxation processes, respectively. The carrier distribution at early times was clearly hotter for 400 nm excitation than for 800 nm excitation. A pronounced spectral bleach feature was observed below 300 nm. It immediately formed after photoexcitation and recovered slowly, with a time constant of 35 ps for monolayer and time constants of 120 and 970 ps for 9-layer graphene. The same dynamics were found for weak transient absorption features above 300 nm, which emerged after ca. 0.5 Ps. The slow dynamics were assigned to interfacial heat flow from graphene to the quartz substrate. The bleach and absorption features were well described by a simple model assuming a red-shift of the Fano resonance. This red-shift disappeared with progressive cooling of graphene. We therefore suggest that the red-shift is induced by shrinking of the band separation due to lattice heating.