Probing Thermally Driven Lattice Dynamics via High-harmonic spectroscopy

Solid-state high-harmonic generation (HHG) probes ultrafast electron–hole dynamics in crystals driven by strong laser fields, where carriers are accelerated over multiple unit cells. At finite temperature, thermal lattice fluctuations induce dephasing along these trajectories, yet their impact on high-harmonic emission remains largely unexplored. Here, we explore this effect by considering the temperature-dependence of HHG in the van-der-Waals superatomic semiconductor Re₆Se₈Cl₂. In both experiments and large-scale calculations solving the semi-conductor Bloch equations, we observe a monotonic enhancement of the harmonic yield with decreasing temperature, accompanied by a pronounced increase in emission below approximately 50 K, coinciding with the vanishing of lattice dynamics. This behavior indicates reduced carrier dephasing due to diminished lattice-induced scattering. By quantitatively matching experimental spectra with semiconductor Bloch equation simulations, we extract the temperature-dependent dephasing time relevant to high-harmonic emission. Our results identify lattice-induced dephasing as a fundamental theoretical mechanism governing solid-state HHG and establish HHG as a sensitive nonlinear probe of electron–phonon coupling and coherence in strongly driven quantum materials.