Theoretical analysis of high-order harmonic generation in liquids by a semiclassical method

The study of high-order harmonic generation in liquid-state materials is still in its infancy. Quantitative models are highly required to understand the mechanisms. Here we develop a semiclassical method to investigate the ultrafast electron dynamics in liquids under monochromatic and two-color laser fields in momentum space. By assuming that different $k$ values are decoupled, we construct a quasienergy band for the liquid model and give the group velocity of the ionized electrons under the driving field. It successfully predicts the caustic enhancement of the liquid harmonic spectrum. We also find that the cutoff of the liquid harmonic spectrum is determined by the $k$-dependent band gap. Our results provide a different perspective for analyzing the nonlinear and nonperturbative liquid-state harmonic generation.