Trovatello, ChiaraKatsch, FlorianBorys, Nicholas J.Selig, MalteYao, KaiyuanBorrego-Varillas, RocioScotognella, FrancescoKriegel, IlkaYan, AimingZettl, AlexSchuck, P. JamesKnorr, AndreasCerullo, GiulioDal Conte, Stefano2022-06-212022-06-212020-10Trovatello, C., Katsch, F., Borys, N. J., Selig, M., Yao, K., Borrego-Varillas, R., ... & Conte, S. D. (2020). The ultrafast onset of exciton formation in 2D semiconductors. Nature communications, 11(1), 1-8.2041-1723https://scholarworks.montana.edu/handle/1/16845The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process.enhttp://creativecommons.org/licenses/by/4.0/Article