Two-color harmonic spectroscopy of ultrafast Dirac electron dynamics

High-harmonic generation (HHG), a hallmark of attosecond science, is a nonperturbative nonlinear process in gases and solids. In gases, strong driving fields can deplete the ground state and suppress harmonic emission on the trailing edge of the pulse. Here we report an analogous effect in a gapless Dirac semimetal, highly oriented pyrolytic graphite, where ultrafast carrier saturation suppresses harmonic emission during nonperturbative harmonic generation (NPHG). Using two-color spectroscopy, we track the excitation dynamics of Dirac electron-hole pairs during the driving pulse and show that the buildup of out-of-equilibrium hot carriers near the Dirac points suppresses interband harmonics and induces measurable femtosecond-scale temporal shifts. Our results reveal that field-driven carrier saturation critically shapes the interplay of interband and intraband currents in Dirac materials. These findings demonstrate the potential of NPHG and HHG as a sensitive, all-optical probe of ultrafast carrier dynamics, offering novel opportunities for ultrafast optoelectronics in Dirac materials. Despite recent advances in nonperturbative harmonic generation in condensed matter, characterization of ultrafast carrier saturation dynamics remained elusive. Here, the authors provide evidence of such physics in highly oriented pyrolytic graphite, a Dirac material, opening up applications in ultrafast optoelectronics.