Role of ultrafast electron-thermal-phonon interactions in high harmonic generation and dephasing from graphene

High harmonic generation (HHG) is a commonly explored process across material systems, where intense lasers drive attosecond-to-femtosecond electron dynamics within solid bands, causing high-energy emission. The main physical players in HHG are the electrons and photons, which are commonly thought to dictate the HHG spectral properties. However, solids also host ubiquitous phonons that are usually relevant on longer timescales, and are therefore largely assumed negligible in HHG. In general, it is unclear if/how phonons partake in HHG and in dephasing of the electron dynamics, which has been very recently proposed in different contexts.

I will present my recent theoretical study (https://arxiv.org/abs/2604.23294) of HHG in graphene with a formalism that semi-classically models the contribution of the gamma optical phonon. I will show that optical phonons dephase interband coherences in the single femtosecond timescale I the strong field regime. This timescale is substantially faster compared to electron-electron scattering, suggesting that thermal phonons dominate electronic decoherence in strong-fields. In particular optical-phonons strongly suppress HHG yields by coupling to interband currents, this could potentially explain the lack of experimental observation of HHG above 3 eV from graphene. In addition HHG yields become temperature-dependent due to phonon occupations, though in graphene this dependence is weak since phonon energy scales are dominated by zero-point motion.

Our results shed light on the dephasing time problem in HHG and role of phonons on attosecond timescales, and should be transferable to other systems and processes as well, motivating novel spectroscopies of phonon dynamics.