Electron-wave-packet-deformation–induced attosecond emission time shift in gas high-order harmonic generation

The laser-induced recollision of electron trajectories is usually used to describe high-order harmonic generation (HHG), which also lays the foundation for high-order harmonic spectroscopy. Electron trajectory models based on the saddle-point approximation are conceptually similar to the light ray model in optics. Considering that ray optics is not complete to explain all wave optics phenomena, we revisit the validity of trajectory models when incorporating the Coulomb potential. Results show that, under commonly used gas HHG experimental conditions, both the classical three-step model and the quantum orbit model have dozens or hundreds of attoseconds of deviation in emission time. By using the Gaussian wave-packet method, we reveal that the time shift can be attributed to the deformation of the recollision electron wave packet. Wave-packet deformation is an inherent wavelike phenomenon caused by group delay dispersion and higher-order dispersion, and thereby is difficult to modify by particlelike trajectories. In addition, we reveal the notable influence of wave-packet deformation on observables in two-color field HHG experiments.