Probing lattice fluctuations using solid-state high-harmonic spectroscopy

TL;DR

This study reveals that thermal lattice fluctuations significantly affect high-harmonic generation in solids, with a strong temperature dependence linked to lattice vibration suppression.

Contribution

It quantifies the impact of thermal lattice fluctuations on HHG in a superatomic semiconductor, highlighting their role in phase dispersion and harmonic suppression.

Findings

High-harmonic yield increases as temperature decreases.

Abrupt increase in HHG below 50 K correlates with suppressed lattice vibrations.

Thermal fluctuations induce phase dispersion, reducing harmonic coherence.

Abstract

Solid-state high-harmonic spectroscopy allows the study of strongly driven ultrafast electron dynamics. Microscopically, high harmonics are generated by strong-laser-field acceleration of electron-hole pairs through the lattice. At finite temperatures, atomic-scale structural fluctuations are ubiquitous and are expected to influence the electron-hole trajectories. Yet, the effect of thermal lattice fluctuations on solid-state high-harmonic generation (HHG) has not been quantified. Here, we demonstrate a profound sensitivity of HHG to thermal lattice fluctuations, by characterizing the temperature dependence of HHG in Re6Se8Cl2, a superatomic semiconductor. As the sample temperature is decreased, the high-harmonic yield exhibits a slow increase, followed by an abrupt increase below 50 K, consistent with the temperature at which lattice vibrations are strongly suppressed. Our calculations show that thermal lattice fluctuations both weaken the harmonic response from individual distorted configurations and induce phase dispersion across the ensemble, leading to a pronounced suppression of the coherently emitted harmonics. We show that this effect can be interpreted in terms of an effective electronic dephasing time that varies with temperature. Our results are relevant to dephasing in broad strong-field phenomena, including lightwave electronics and Floquet engineering. The wide tunability of superatomic crystals further enables materials-controlled strong-field physics.