Effect of the interference between interband currents on the crystal harmonic spectra

By numerically solving the semiconductor Bloch equation, we study the high-order harmonic emission spectrum of a MgO crystal under the interaction of a linearly polarized laser pulse. Based on two-band and three-band models, by comparing the variation of the harmonic spectrum with the laser peak intensity, it is found that when the laser intensity is lower, the harmonic spectra from both cases are generally consistent above the minimum band gap. However, some harmonic intensities in the three-band model are significantly enhanced or weakened compared to the two-band case when the laser intensity is higher. By analysis of the electron population and the harmonic phase, it is demonstrated that the harmonic enhancement or reduction is caused by the interference between two interband polarization sources, one of which originates from the current between the first conduction band and the valence band and the other of which is from the polarization between the second and the first conduction band. The harmonic interference between different interband currents contains rich dynamic information and has a significant impact on the harmonic structure and yield, which is helpful for further understanding the generation mechanism of crystal harmonics.