Terahertz-Assisted Multiband High-Harmonic Spectroscopy

High-harmonic generation (HHG) is an extreme form of frequency upconversion that facilitates light-source engineering and ultrafast materials spectroscopy. Here, we broaden the spectroscopic scope of HHG, by demonstrating polarization manipulation of harmonic light in a dielectric, using a two-color field configuration that combines a midinfrared (MIR) driver with a terahertz (THz) perturbation. By varying the relative polarization axes of these fields, the emitted harmonics can be tuned to exhibit either linear or elliptical polarization. Supported by first-principles theory and semiclassical analysis, we show that our approach enables crystal-momentum-resolved dipole-vector spectroscopy across different bands. Crucially, we show that for certain field configurations, harmonic light emission will originate from electron-hole pairs created away from the minimum band gap. Furthermore, crossing MIR and THz polarization at oblique angles generates elliptically polarized harmonics whose microscopic origin is traced to the phase and amplitude imbalances of electron-hole trajectories released during adjacent half-cycles of the MIR field. Our Letter demonstrates new spectroscopy capabilities of HHG, deepens the contemporary microscopic understanding of the process, and paves the way for full polarization control of the harmonic light.