In this talk I will first recall briefly the general properties of 2D excitons in Transition Metal Dichalcogenides (TMD) monolayers: giant binding energy, oscillator strength, exchange interactions, spin/valley locking ...¹.

Encapsulation of TMD monolayers in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogeneous exciton linewidth^2,3. We have demonstrated that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect ⁴.

We also measured the exciton fine structure by magneto-photoluminescence spectroscopy in magnetic fields up to 30 T ^5,6. I will show that the bright-dark exciton splitting can be tuned by a few meV, as a result of a significant Lamb shift of the optically active exciton which arises from emission and absorption of virtual photons triggered by the vacuum fluctuations of the electromagnetic field⁷ .

Finally I will present recent experimental results on spin/valley pumping of resident electrons in WSe₂ and WS₂ monolayers⁸. Using a spatially-resolved optical pump-probe experiment, we measure the lateral transport of spin/valley polarized electrons over very long distances (tens of micrometers)⁹. These results highlight the key role played by the spin-valley locking effect in TMD monolayers on the pumping efficiency and the polarized electron transport.

¹ G. Wang et al, Rev. Mod. Phys. 90, 021001 (2018)

² F. Cadiz et al, Phys. Rev. X 7, 021026 (2017)

³ G. Wang et al, Phys. Rev. Lett. 119, 047401 (2017)

⁴ H. Fang et al, Phys. Rev. Lett. 123, 067401 (2019)

⁵ C. Robert et al, Phys. Rev. Lett . 126, 067403 (2021)

⁶ C. Robert et al, Nature Com. 11, 4037 (2020)

⁷ L. Ren et al, submitted (2023)

⁸ C. Robert et al, Nature Com. 12, 5455 (2021)

⁹ C. Robert et al, Phys. Rev. Lett. 129, 027402 (2022)