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Biexcitonic optical Stark effects in monolayer molybdenum diselenide

Abstract

Floquet states, where a periodic optical field coherently drives electrons in solids1,2,3, can enable novel quantum states of matter4,5,6. A prominent approach to realize Floquet states is based on the optical Stark effect. Previous studies on the optical Stark effect often treated the excited state in solids as free quasi-particles3,7,8,9,10,11,12. However, exciton–exciton interactions can be sizeably enhanced in low-dimensional systems and may lead to light–matter interactions that are qualitatively different from those in the non-interacting picture. Here we use monolayer molybdenum diselenide (MoSe2) as a model system to demonstrate that the driving optical field can couple a hierarchy of excitonic states, and the many-body inter-valley biexciton state plays a dominant role in the optical Stark effect. Specifically, the exciton–biexciton coupling in monolayer MoSe2 breaks down the valley selection rules based on the non-interacting exciton picture. The photon-dressed excitonic states exhibit an energy redshift, splitting or blueshift as the driving photon frequency varies below the exciton transition. We determine a binding energy of 21 meV for the inter-valley biexciton and a transition dipole moment of 9.3 debye for the exciton–biexciton transition. Our study reveals the crucial role of many-body effects in coherent light–matter interaction in atomically thin two-dimensional materials.

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Fig. 1: Schematic diagrams of optical transition in MoSe2.
Fig. 2: Valley-dependent optical Stark effects.
Fig. 3: Biexcitonic coherent optical Stark effects.
Fig. 4: Anomalous optical Stark shift in the Kʹ valley.

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Acknowledgements

This work was primarily supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract no. DE-AC02-05-CH11231 (Van der Waals Heterostructures program KCWF16). Preparation of the hBN-encapsulated monolayer MoSe2 is supported by the National Science Foundation EFRI programme (EFMA-1542741). Growth of hBN crystals was supported by the Elemental Strategy Initiative conducted by the MEXT, Japan and JSPS KAKENHI grant numbers JP15K21722 and JP25106006. S.T. acknowledges the support from NSF DMR 1552220 NSF CAREER award.

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C.-K.Y. and F.W. conceived the research. C.-K.Y. carried out optical measurements, assisted by J.H. and C.-S.Y. C.-K.Y., F.W. and J.H. analysed the data and performed theoretical analysis. J.H. fabricated the devices, assisted by A.W., C.-K.L. and S.Z. Y.S, H.C. and S.T. synthesized MoSe2 crystals. K.W. and T.T. synthesized hBN crystals. C.-K.Y. and F.W. wrote the manuscript, with input from all authors.

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Correspondence to Chaw-Keong Yong or Feng Wang.

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Yong, CK., Horng, J., Shen, Y. et al. Biexcitonic optical Stark effects in monolayer molybdenum diselenide. Nature Phys 14, 1092–1096 (2018). https://doi.org/10.1038/s41567-018-0216-7

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