  Carrier dynamics in single-layer graphene grown by chemical vapor deposition
(CVD) is studied using spatially and temporally resolved pump-probe
spectroscopy by measuring both differential transmission and differential
reflection. By studying the expansion of a Gaussian spatial profile of carriers
excited by a 1500-nm pump pulse with a 1761-nm probe pulse, we observe a
diffusion of hot carriers of 5500 square centimeter per second. We also observe
that the expansion of the carrier density profile decreases to a slow rate
within 1 ps, which is unexpected. Furthermore, by using an 810-nm probe pulse
we observe that both the differential transmission and reflection change signs,
but also that this sign change can be permanently removed by exposure of the
graphene to femtosecond laser pulses of relatively high fluence. This indicates
that the differential transmission and reflection at later times may not be
directly caused by carriers, but may be from some residue material from the
sample fabrication or transfer process.
