Ultrafast dynamics of highly-excited Dirac fermions in monolayer graphene*

One of the striking optical properties of single-layer graphene is the universal absorbance in the near-infrared-to-visible spectral range due to the Dirac spectrum of the low-energy electronic structure. High-fluence laser pump can produce superdense Dirac-fermionic excitations at the order of 10 femtoseconds so to reach the non-linear saturation of absorption. We construct a simple model for the transient state of the photo-excited graphene to explore the non-linear saturation of photoexcitations and the transport property of carries. The comparison of our model calculations with the experimental results shows good agreements.