Photoconductivity of an intrinsic graphen
  We examine the photoconductivity of an intrinsic graphene associated with far- and mid-infrared irradiation at low temperatures. The model under consideration accounts for the excitation of the electron-hole pairs by incident radiation, the interband generation-recombination transitions due to thermal radiation, and the intraband energy relaxation due to acoustic phonon scattering. The momentum relaxation is assumed to be caused by elastic scattering. The pertinent collision integrals are adapted for the case of the massless energy spectrum of carriers that interact with the longitudinal acoustic mode and the thermal radiation. It is found that the photoconductivity is determined by an interplay between weak energy relaxation and generation-recombination processes. Due to this the threshold of nonlinear response is fairly low. 
F. T. Vasko V. Ryzhii