Transient absorption signatures of asymmetric carrier cooling in semiconductors

Carrier dynamics in semiconductors is central to devices for electronics, optoelectronics, energy, and quantum technologies. Following photoexcitation, electrons and holes relax on ultrafast timescales, yet resolving their individual dynamics remains challenging because widely used pump-probe spectroscopies, such as transient absorption (TA), probe the combined carrier response. Here, we disentangle the cooling dynamics of excited electrons and holes through a joint experiment-theory approach that combines high time-resolution femtosecond TA spectroscopy with quantitative first-principles simulations. We show a pronounced asymmetry in electron and hole cooling in both wurtzite and zinc-blende semiconductors, exemplified by gallium nitride (GaN) and gallium arsenide (GaAs). Guided by theory, we unravel signatures of electron and hole cooling in our femtosecond-resolved TA spectra and show that the TA response is dominated by electron cooling, which occurs on characteristic timescales on the order of 100 fs and is governed by scattering with optical phonons. In contrast, our simulations predict that hole cooling is one order of magnitude faster and produces a comparatively weak contribution to the TA response. Our study reveals the microscopic origin of this asymmetric carrier cooling, establishing it as a general feature of direct-gap wurtzite and zinc-blende semiconductors. Our work shows a predictive experiment-theory framework for understanding the individual dynamics of excited electron and hole carriers, with broad implications for conventional and quantum materials.