Evolution of anisotropic-to-isotropic photoexcited carrier distribution in graphene

Femtosecond degenerate and nondegenerate pump-probe spectroscopy in graphene reveals the evolution of photoexcited carrier distribution in the energy band during relaxation: the initial occupation of photoexcited carriers centered at the excitation state is anisotropic in momentum space; this anisotropic distribution rapidly relaxes through an intermediate state argued to be fully isotropic in the energy band due to phonon-involved cascade scattering. In addition, the experiment suggests that graphene optical absorbances for in-plane and out-of-plane optical fields are identical.