Unveiling exciton formation: exploring the early stages in time, energy and momentum domain

TL;DR

This study combines advanced spectroscopy and simulations to reveal the ultrafast energy and momentum dynamics of exciton formation in bismuth tri-iodide, providing insights into early-stage exciton physics relevant for semiconductor devices.

Contribution

It introduces a combined experimental and theoretical approach to directly observe the initial exciton formation process in energy and momentum space.

Findings

Captured the timing of exciton formation in energy and momentum

Identified the progression from photoexcited electrons to trapped excitons

Provided a detailed characterization of early exciton dynamics in BiI3

Abstract

Resolving the early-stage dynamics of exciton formation following non-resonant photoexcitation in time, energy, and momentum is quite challenging due to their inherently fast timescales and the proximity of the excitonic state to the bottom of the conduction band. In this study, by combining time- and angle-resolved photoemission spectroscopy with \mathit{ab initio} numerical simulations, we capture the timing of the early-stage exciton dynamics in energy and momentum, starting from the photoexcited population in the conduction band, progressing through the formation of free excitons, and ultimately leading to their trapping in lattice deformations. The chosen material is bismuth tri-iodide ($BiI_3$), a layered semiconductor with a rich landscape of excitons in the electronic structure both in bulk and in monolayer form. The obtained results, providing a full characterization of the exciton formation, elucidate the early stages of the physical phenomena underlying the operation of the ultrafast semiconductor device.