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The ultrafast onset of exciton formation in 2D semiconductors

Author

Listed:
  • Chiara Trovatello

    (Politecnico di Milano)

  • Florian Katsch

    (Technische Universität Berlin)

  • Nicholas J. Borys

    (Lawrence Berkeley National Laboratory
    Montana State University)

  • Malte Selig

    (Technische Universität Berlin)

  • Kaiyuan Yao

    (Lawrence Berkeley National Laboratory
    University of California
    Columbia University)

  • Rocio Borrego-Varillas

    (Politecnico di Milano
    IFN-CNR)

  • Francesco Scotognella

    (Politecnico di Milano
    Lawrence Berkeley National Laboratory)

  • Ilka Kriegel

    (Lawrence Berkeley National Laboratory
    Istituto Italiano di Tecnologia (IIT))

  • Aiming Yan

    (University of California at Berkeley
    Lawrence Berkeley National Laboratory
    Berkeley and the Lawrence Berkeley National Laboratory
    University of California at Santa Cruz)

  • Alex Zettl

    (University of California at Berkeley
    Lawrence Berkeley National Laboratory
    Berkeley and the Lawrence Berkeley National Laboratory)

  • P. James Schuck

    (Lawrence Berkeley National Laboratory
    University of California
    Columbia University)

  • Andreas Knorr

    (Technische Universität Berlin)

  • Giulio Cerullo

    (Politecnico di Milano
    IFN-CNR)

  • Stefano Dal Conte

    (Politecnico di Milano)

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.

Suggested Citation

  • 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, 2020. "The ultrafast onset of exciton formation in 2D semiconductors," Nature Communications, Nature, vol. 11(1), pages 1-8, December.
    • Handle: RePEc:nat:natcom:v:11:y:2020:i:1:d:10.1038_s41467-020-18835-5
    DOI: 10.1038/s41467-020-18835-5
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    Cited by:

    1. Angela Montanaro & Francesca Giusti & Matteo Zanfrognini & Paola Pietro & Filippo Glerean & Giacomo Jarc & Enrico Maria Rigoni & Shahla Y. Mathengattil & Daniele Varsano & Massimo Rontani & Andrea Per, 2022. "Anomalous non-equilibrium response in black phosphorus to sub-gap mid-infrared excitation," Nature Communications, Nature, vol. 13(1), pages 1-7, December.

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