Tracing ultrafast separation and coalescence of carrier distributions in graphene with time-resolved photoemission

Graphene, a recently discovered two-dimensional form of carbon, is a strong candidate for many future electronic devices. There is, however, still much debate over how the electronic properties of graphene behave on ultrashort time scales. Here by employing the technique of time-resolved photoemission, we obtain the evolving quantum distributions of the electrons and holes: on an ultrashort 500 fs time scale, the electron and hole populations can be described by two separate Fermi-Dirac distributions, whereas on longer time scales the populations coalesce to form a single Fermi-Dirac distribution at an elevated temperature. These studies represent the first direct measure of carrier distribution dynamics in monolayer graphene after ultrafast photoexcitation.