The Effect of Multiple Conduction Bands on High Harmonic Emission from   Dielectrics
  We find that, for sufficiently strong mid-IR fields, transitions between different conduction bands play an important role in the generation of high-order harmonics in a dielectric. The transitions make a significant contribution to the harmonic signal, and they can create a single effective band for the motion of an electron wave packet. We show how high harmonic spectra produced during the interaction of ultrashort laser pulses with periodic solids provide a spectroscopic tool for understanding the effective band structure that controls electron dynamics in these media. 
Peter G. Hawkins, Misha Yu. Ivanov, Vladislav S. Yakovlev
cond-mat.other physics.optics quant-ph