▎ 摘　　要

We study the dynamic ac conductivity of a nonequilibrium two-dimensional electron-hole system in optically pumped graphene. Considering the contribution of both interband and intraband transitions, we demonstrate that at sufficiently strong pumping the population inversion in graphene can lead to the negative net ac conductivity in the terahertz range of frequencies. This effect might be used in graphene-based coherent sources of terahertz radiation. (c) 2007 American Institute of Physics.