High-harmonic generation in alpha-quartz by the electron-hole   recombination
  A calculation of the high-harmonic generation (HHG) in $\alpha$-quartz using the time-dependent density functional theory is reported. The inter-band process is attributed to the dominant in HHG above the band gap. The photon energy is set to 1.55 eV, and the cutoff energy of the plateau region is found to be limited at the 19th harmonic (30 eV). The dependence of the HHG efficiency at the cutoff energy region on laser intensity is consistent with that of the hole density in the lowest-lying valence band. Numerical results indicate that electron-hole recombination plays a crucial role in HHG in $\alpha$-quartz. It is found that a 200 attosecond pulse train is produced using HHG around the plateau cutoff energy. 
T. Otobe
physics.optics physics.atom-ph