Characterization of early-stage exciton dynamics in bulk semiconductor BiI₃ via tr-ARPES

Exciton physics dominates the optical properties of semiconductors, so a detailed and comprehensive description of the origin of excitons as well as their relaxation pathways is crucial for the development of this field. When the pump energy exceeds the fundamental bandgap, a population of quasi-free charge carriers is excited in the conduction band and then excitons emerge from this population. The following cascade process that accounts for the exciton relaxation have remained elusive until now.

In this work, we track the exciton formation and the consequent cascade mechanism reconstructing theoretically the time-resolved angle-resolved photoemission spectroscopy (tr-ARPES) signal. To this end, we use a DFT+GW+BSE scheme as a starting point and we compute a broad excitonic spectrum to capture the signal due to both bound and unbound excited states. In order to model the tr-ARPES signal, we define a time-dependent distribution function that is at the beginning dominated by a Gaussian wave-packet initially centered (t = 0) at the pumping energy, which evolves into a Boltzmann distribution as the system progressively relaxes toward the minimum of the exciton dispersion.

Using the bulk semiconductor BiI₃ as a playground, we explore the exciton dynamics at different energy ranges and we compare the results with experimental measurements. In such a way, combining experimental and theoretical efforts, we track the exciton formation and the subsequent relaxation, which is essential for a complete understanding of the entire exciton relaxation dynamics and their application in ultrafast semiconductor devices.