Direct imaging of valley-polarized excitons in 2D semiconductors

The monolayer transition-metal dichalcogenides (TMDCs), lacking inversion symmetry, allow the manipulation of excitonic states with valley degree of freedom through circularly polarized light [1-3]. However, with the complex landscape of exciton species in TMDCs, such as spin- or momentum- forbidden dark excitons, the valley depolarization mechanisms, which are of great importance for valleytronic applications, 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 in energy-momentum space [4-7]. In this talk, we focus on our momentum-resolved study on the valley-polarized excitons in monolayer WS 2