Experimental Measurement of Ultrafast Carrier Dynamics in Mono- and Multi-layer Graphene Samples

The ultrafast dynamics of charge carriers in mono- and multi-layer graphene was investigated by femtosecond transient reflectivity measurements. The experiments were performed using 100-fs optical pump pulses at a wavelength of 400 nm and probe pulses at a wavelength of 800 nm. We observed a transient response on the time scale of several picoseconds. For bulk graphite, a decay time of $\sim $ 3 ps was found; for thin graphene multilayer samples, a reduced decay time was observed, dropping ultimately to $\sim $ 1 ps for a single graphene layer. The reflectivity transients can be understood in terms of coupling of the photo-generated electronic excitations to optical phonons, and the subsequent loss of energy from this sub-system. The possible role of graphene interactions with the quartz substrate and the effect of the graphene electronic specific heat on the decay rate will be discussed.