The ultrafast onset of exciton formation in 2D semiconductors

Chiara Trovatello, Florian Katsch, Nicholas J. Borys, Malte Selig, Kaiyuan Yao, Rocio Borrego-Varillas, Francesco Scotognella, Ilka Kriegel, Aiming Yan, Alex Zettl, P. James Schuck, Andreas Knorr (), Giulio Cerullo () and Stefano Dal Conte ()
Additional contact information
Chiara Trovatello: Politecnico di Milano
Florian Katsch: Technische Universität Berlin
Nicholas J. Borys: Lawrence Berkeley National Laboratory
Malte Selig: Technische Universität Berlin
Kaiyuan Yao: Lawrence Berkeley National Laboratory
Rocio Borrego-Varillas: Politecnico di Milano
Francesco Scotognella: Politecnico di Milano
Ilka Kriegel: Lawrence Berkeley National Laboratory
Aiming Yan: University of California at Berkeley
Alex Zettl: University of California at Berkeley
P. James Schuck: Lawrence Berkeley National Laboratory
Andreas Knorr: Technische Universität Berlin
Giulio Cerullo: Politecnico di Milano
Stefano Dal Conte: Politecnico di Milano

Nature Communications, 2020, vol. 11, issue 1, 1-8

Abstract: Abstract 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.

Date: 2020
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DOI: 10.1038/s41467-020-18835-5

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