Browsing by Author "Yan, Aiming"

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The ultrafast onset of exciton formation in 2D semiconductors
(Springer Science and Business Media LLC, 2020-10) Trovatello, Chiara; Katsch, Florian; Borys, Nicholas J.; Selig, Malte; Yao, Kaiyuan; Borrego-Varillas, Rocio; Scotognella, Francesco; Kriegel, Ilka; Yan, Aiming; Zettl, Alex; Schuck, P. James; Knorr, Andreas; Cerullo, Giulio; Dal Conte, Stefano
The 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.