Resonant-state-induced interference minima in high-order harmonic generation from liquid systems

High-order harmonic generation (HHG) from liquid systems is investigated by a random atomic chain model. It is found that the interference minimum appears in the harmonic spectrum, and the position of the spectral minimum increases monotonically with increasing laser wavelength in the range of 1300, 1700 nm. The paths contributing to the harmonics have been defined to explain the cause of the harmonic minimum. For the case of laser wavelength in the range of 1300, 1600 nm, the harmonic minimum results from the interference cancellation between path IA and path IB. For the case of a longer wavelength, such as 1700 nm, the harmonic minimum is produced by the interference cancellation between path I and path II. It is stated that lasers with longer wavelengths have a higher probability of driving electrons to more distant areas, and path II plays a crucial role in harmonic radiation. Time-dependent population of quasi-free states and the time-dependent evolution of electrons in the coordinate space are also simulated to analyze the physical processes. A new perspective for understanding the source of harmonic order interference cancellation is provided, and this might be used for further research on the mechanism of liquid harmonic radiation.