Intensity dependence of high-order harmonic generation in ZnO

The laser intensity dependence of high-order harmonic generation (HHG) in ZnO is investigated theoretically. We find that the intensities of lower-order harmonics mainly contributed by the intraband electron oscillation increase with light intensity monotonously. However, the intensities of higher-order harmonics where the interband electron-hole recollision dominates oscillate with light intensity. We build models of intraband and interband electronic dynamics to study the effect of laser intensity on every single-order harmonic that is far from the band gap. The model results are in good agreement with the numerical solution of semiconductor Bloch equations. The nonzero initial k positions play an important role in HHG, especially in higher-order harmonics. With the saddle-point method, the critical distance for electron-hole recombination in the current material and laser parameters is found to be one lattice constant in the dephasing time of a quarter of laser cycle. A unified description of channel closing effects is proposed to explain the laser intensity dependence of above threshold ionization and HHG in gases and solids including HHG not far from the band gap.