We investigate theoretically the high-order harmonic generation (HHG) from the ZnO crystal by solving the semiconductor Bloch equations. It is shown that the harmonic intensities are enhanced and the individual harmonics are broadened by introducing a time delay between the two-color laser pulses. We find that a higher conduction band population does not lead to a higher emission efficiency of HHG, and we illustrate that the number of electron tunneling windows is the dominant mechanism causing the enhancement of HHG in the ZnO crystal. By analyzing crystal-momentum-resolved contributions, we demonstrate that ionization from a much wider range of initial momenta contributes to the enhancement of the harmonic spectrum. Moreover, we investigate the influence of the dynamical Bloch oscillations on HHG. Our results reveal the connection between the tunneling window and HHG, which is crucial for generating bright attosecond sources from solids.

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• Light-matter interaction
• Nonlinear optics
• Crystal optics