Population inversion of photoexcited electrons and holes in graphene and its negative terahertz conductivity

We demonstrate that sufficiently strong optical excitation may result in the population inversion in graphene, so that the real part of the ac conductivity can be negative in the terahertz range of frequencies. We study also how the heating of the electron‐hole system influences the effect of negative ac conductivity. The effect of population inversion and negative ac conductivity might be used in graphene‐based coherent sources of terahertz electromagnetic radiation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)