Rashba-induced intersubband interference in high-harmonic generation

High-harmonic spectroscopy is exquisitely sensitive to the electronic structure and dynamics of quantum materials, yet quantum interference between spin-orbit-split bands has remained experimentally elusive. We observe this interference between Rashba-split bands in bismuth tellurium iodide (BiTeI) driven by strong mid-infrared laser fields. The sixth harmonic exhibits intensity-scaling kinks and an M-shaped ellipticity dependence. Theoretical simulations attribute these features to spin-orbit–mediated multichannel quantum interference, which arises from Landau-Zener-Stückelberg–type intersubband tunneling between Rashba-split bands. The suppression of harmonic yield under linearly polarized pulses arises from destructive interference among electron-hole recombination pathways, whereas increasing ellipticity modifies their relative phase and induces a distinct spectral redshift. These results establish high-harmonic spectroscopy as a probe of spin-textured band dynamics and ultrafast spin-polarized currents.