High-harmonic generation in zinc oxide subjected to intense mid-infrared femtosecond laser pulse

Abstract

We theoretically investigate photo-excitation of electron-hole pairs and high harmonic generation in the bulk of zinc oxide (ZnO) subjected to intense femto-second laser pulses with mid-infrared wavelength. The main microscopic mechanism of solid-state HHG is identified by separating resonant from non-resonant non-linear optical responses in the photo-excited solid. It allows us to obtain an effective description of the light-matter interaction in which electrons become subject to weak atto-second pulse train with the second harmonic of the drive laser frequency being the repetition frequency in the train. Under a condition of constructive interference between electronic transitions at each half-cycle of the drive laser pulse, resonant-like excitation of electron-hole pairs occurs, analogously to above threshold ionization in a gas phase. The inter-band motion of charge carriers creates rapidly oscillating electric dipole moment, which emits radiation in the form of high-order harmonics of the drive laser frequency. We also discuss the importance of the pulse envelope for producing clean frequency combs as observed in experiments. Good semi-quantitative agreement with the experimental data is found: clean and well defined odd-order harmonic peaks extending well beyond the band edge of ZnO are exhibited for laser linearly polarized at right angles to the optical axis of the crystal.