Direct imaging of valley-polarized excitons in 2D semiconductors*

The monolayer transition-metal dichalcogenides (TMDCs) have been attractive for the applications of valleytronics since excitonic states with valley degree of freedom can be manipulated through circularly polarized light. However, with the complex landscape of exciton species in TMDCs involving spin- or momentum- forbidden dark excitons, the valley depolarization dynamics still lack clear understanding. Such measurements require direct access to the momentum and energy coordinate of constituent electrons and holes, but few experimental techniques provide such information. Meanwhile, time- and angle- resolved photoemission spectroscopy (TR-ARPES) has become a powerful tool to study excitons of 2D semiconductors directly in energy-momentum space [1–4]. In this talk, we will discuss our latest momentum-resolved study on the valley-polarized excitons in monolayer WS₂ using TR-ARPES.

[1]    J. Madéo et al., Directly Visualizing the Momentum-Forbidden Dark Excitons and Their Dynamics in Atomically Thin Semiconductors, Science 370, 1199 (2020).

[2]    M. K. L. Man et al., Experimental Measurement of the Intrinsic Excitonic Wave Function, Science Advances 7, eabg0192 (2021).

[3]    O. Karni et al., Structure of the Moiré Exciton Captured by Imaging Its Electron and Hole, Nature 603, 247 (2022).

[4]    D. Schmitt et al., Formation of Moiré Interlayer Excitons in Space and Time, Nature 608, 499 (2022).