Pulsed high harmonic generation of light due to pumped Bloch   oscillations in noninteracting metals
  We derive a simple theory for high-order harmonic generation due to pumping a noninteracting metal with a large amplitude oscillating electric field. The model assumes that the radiated light field arises from the acceleration of electrons due to the time-varying current generated by the pump, and also assumes that the system has a constant density of photoexcited carriers, hence it ignores the dipole excitation between bands (which would create carriers in semiconductors). We examine the circumstances under which odd harmonic frequencies would be expected to dominate the spectrum of radiated light, and we also apply the model to real materials like ZnO, for which high-order harmonic generation has already been demonstrated in experiments. 
J. K. Freericks, A. Y. Liu, A. F. Kemper, and T. P. Devereaux
physics.optics cond-mat.str-el