Mass-Singularity-Induced Phase Jump in High-Harmonic Generation from Symmetry-Breaking Crystals.

Symmetry plays a central role in fundamental physics. Here we identify a phase-jump phenomenon in high-order harmonic generation in symmetry-breaking crystals. This phenomenon originates from the singularity in the effective mass of electron-hole pairs at nonzero-distance collisions, a factor usually ignored in the literature. However, it determines the basic properties of harmonic emissions. This phase-jump phenomenon results in a suppression of the harmonic spectrum by orders, which is strongly orientation dependent. It is universal and not limited to a specific crystal. Clues to this can be found in published ab initio simulations and experimental results, but not interpreted. Our findings pave the way for a better understanding of the ultrafast dynamics of electron-hole pairs in XUV photon emission.