Strong-Field Perspective on High-Harmonic Radiation from Bulk Solids
  Mechanisms of high-harmonic generation from crystals are described by treating the electric field of a laser as a quasi-static strong field. Under the quasi-static electric field, electrons in periodic potentials form dressed states, known as Wannier-Stark states. The energy differences between the dressed states determine the frequencies of the radiation. The radiation yield is determined by the magnitudes of the inter-band and intra-band current matrix elements between the dressed states. The generation of attosecond pulses from solids is predicted. Ramifications for strong-field physics are discussed. 
Takuya Higuchi, Mark I. Stockman, and Peter Hommelhoff
physics.optics cond-mat.mtrl-sci physics.atom-ph