  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.
