Phonon-Coupled High-Harmonic Generation for Exploring Nonadiabatic Electron-Phonon Interactions

High harmonic generation (HHG) have received significant attention for the exploration of material properties and ultrafast dynamics. However, the lack of consideration for couplings between HHG and other quasiparticles, such as phonons, has been impeding the understanding of many-body interactions in HHG. Here, we reveal the many-body electron-phonon mechanism in the quasiparticle-coupled strong-field dynamics by investigating the nonadiabatic (NA) coherent-phonon-coupled HHG. Coherent phonons are revealed to effectively affect HHG via the adiabatic band modulation induced by phonon deformation effects and the NA and nonequilibrium distribution of photocarriers in multiple valleys. The adiabatic and NA mechanisms leave their fingerprint via influencing the phonon period and phase delay in the oscillation of HHG intensity, both of which are experimentally measurable. Investigation of these quantities enables the direct probing of the electron-phonon interaction in materials.