▎ 摘　　要

Graphene, a truly two-dimensional material with a unique linear energy-momentum dispersion, demonstrates novel photonic properties such as universal absorption and conductivity, with applications including terahertz lasing, broadband midinfrared detectors, and tunable ultrafast lasers. Understanding the ultrafast nonequilibrium dynamics of photocarriers in graphene's unique relativistic band structure is important for the development of such high-speed, graphene-based photonic devices, as well as from a fundamental point of view. Here, our experiments indicate the relativistic nature of a nonequilibrium gas of electrons and holes photogenerated in a graphene monolayer as early as 100 fs after photoexcitation. We observe a nonlinear scaling in the Drude-like optical conductivity of the photocarriers with respect to their density, in striking contrast to the linear scaling expected from conventional materials with parabolic dispersion relations. Our measurements also indicate that hot photocarriers cool on a sub-100-fs time scale via interactions with optical phonons. These results elucidate the unique nature of the ultrafast dynamics of photocarriers in a relativistic material, in contrast to conventional materials, and provide a way to manipulate graphene's optical conductivity for applications in photonics and plasmonics.