  High-harmonic generation by a laser-driven solid slab is simulated using
time-dependent density functional theory. Multiple harmonic plateaus up to very
high harmonic orders are observed already at surprisingly low field strengths.
The full all-electron harmonic spectra are, in general, very different from
those of any individual Kohn-Sham orbital. Freezing the Kohn-Sham potential
instead is found to be a good approximation for the laser intensities and
harmonic orders considered. The origins of the plateau cutoffs are explained in
terms of band gaps that can be reached by Kohn-Sham electrons and holes moving
through the band structure.
