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Population inversion and giant bandgap renormalization in atomically thin WS2 layers

Nature Photonics volume 9, pages 466–470 (2015)Cite this article

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Abstract

Control of the optical properties of matter on ultrashort timescales is of both fundamental interest and central importance for applications in photonics. It is desirable to achieve pronounced changes over a broad spectral range using the least possible amount of material. Here, we demonstrate a dramatic change over a spectral range of hundreds of meV on the femtosecond timescale in the optical response of atomically thin two-dimensional crystals of the transition-metal dichalcogenide WS2 following excitation by intense optical pump pulses. Our findings reveal the role of extremely strong Coulomb interactions. At the direct gap, we observe a Mott transition from excitonic states to free carriers, accompanied by a giant bandgap renormalization of approximately 500 meV and the development of population inversion.

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Figure 1: Photoinduced optical response of WS2 bilayers.
Figure 2: Optical absorption of a two-dimensional semiconductor in the presence of high electron–hole densities.
Figure 3: Evolution of the population inversion with pump fluence.
Figure 4: Photoinduced optical response of WS2 monolayers.

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Acknowledgements

This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, with funding at Columbia University through the Energy Frontier Research Center under Grant DE-SC0001085 and at SLAC National Accelerator Laboratory through the AMOS programme within the Chemical Sciences, Geosciences and Biosciences Division, by the Keck Foundation, and by the Air Force Office of Scientific Research (grant no. FA9550-14-1-0268). C.R. and A.C. acknowledge partial funding from the Alexander von Humboldt Foundation within the Feodor Lynen Research Fellowship programme. H.M.H. and A.F.R. were supported, respectively, by the NSF through an IGERT Fellowship (grant no. DGE-1069240) and by a Graduate Research Fellowship (DGE-1144155).

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Author notes

  1. Alexey Chernikov and Claudia Ruppert: These authors contributed equally to this work

Authors and Affiliations

  1. Departments of Physics and Electrical Engineering, Columbia University, 538 West 120th Street, New York, 10027, New York, USA

    Alexey Chernikov, Claudia Ruppert, Heather M. Hill, Albert F. Rigosi & Tony F. Heinz

  2. Department of Applied Physics, Stanford University, Stanford, 94305, California, USA

    Tony F. Heinz

  3. SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, 94025, California, USA

    Tony F. Heinz

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  1. Alexey Chernikov
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Contributions

A.C. and C.R., contributing equally to this work, designed the experiment, carried out the measurements and analysed the data. H.M.H. and A.F.R. prepared and characterized the samples. A.C., C.R. and T.F.H. wrote the manuscript. All authors contributed to discussions.

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Correspondence to Alexey Chernikov or Tony F. Heinz.

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Chernikov, A., Ruppert, C., Hill, H. et al. Population inversion and giant bandgap renormalization in atomically thin WS2 layers. Nature Photon 9, 466–470 (2015). https://doi.org/10.1038/nphoton.2015.104

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