In our new work, we have demonstrated a new all-optical method for observing how electrons evolve in Dirac materials on femtosecond timescales. Using two-color high-harmonic spectroscopy, we investigated nonperturbative harmonic generation in highly oriented pyrolytic graphite (HOPG), a bulk material with graphene-like electronic properties. We discovered that intense laser fields rapidly saturate the population of Dirac electrons, suppressing harmonic emission during the latter part of the laser pulse and producing measurable emission time shifts of up to 17.5 femtoseconds. Combining precision experiments with semiconductor Bloch equation simulations, we show that these temporal shifts directly reveal ultrafast carrier population dynamics near the Dirac point. The work establishes high-harmonic generation as a sensitive probe of nonequilibrium electron dynamics in quantum materials and provides new insights into strong-field light-matter interactions. These findings open new opportunities for petahertz optoelectronics and ultrafast optical control of Dirac materials.

The work, led by Zhaopin Chen in collaboration with Prof. Ofer Neufeld and colleagues from the Technion and with Prof. Marcelo Ciappina from the Guangdong Technion – Israel Institute of Technology, has been published in Nature Communications. Read our paper here.