Ultrafast transient absorption microscopy studies of carrier dynamics in epitaxial graphene

Energy exchange between the electrons and phonons is particularly important to electron transport, and understanding this process will be vital for the realization of future graphene-based electronics. Epitaxial growth is a very promising approach for practical applications, as it has the ability to prepare graphene on a large scale and supported on a substrate. However, epitaxially grown graphene is highly inhomogeneous, with variations in the sample thickness occurring over length scale of a few micrometers. To pave the road for electronic devices based on epitaxial graphene, characterization methods with high spatial resolution are required. In this paper, we describe transient absorption microscopy as a novel tool to characterize graphene, and to interrogate the charge carrier dynamics. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate. This is attributed to differences in coupling between the graphene and the substrate. These results point to transient absorption microscopy as a potentially important tool for characterizing graphene.