Transient response of photoexcited electrons: negative and oscillating   current
  Time-dependent current of the electrons excited in the conduction band after ultrafast interband photogeneration is studied theoretically. The transient photocurrent is calculated for the nonlinear regime of response to a stationary electric field. The response demonstrates transient absolute negative conductivity when the electrons are excited slightly below the optical phonon energy, while the periodic oscillations of the electric current appear after formation of the streaming distribution. The quenching of these peculiarities by the elastic scattering of electrons is also considered. 
O.E. Raichev and F.T. Vasko