Alice Kunin

Monolayer transition metal dichalcogenides (TMDs) are direct band gap semiconductors that have generated immense interest in recent years for their potential in a variety of optoelectronic applications due to their strong light absorption and emission and unique chiral optical selection rules. The remarkable properties of these materials are governed by strongly bound excitons that exhibit a variety of complex optically bright as well as optically dark exciton states that have been, until now, difficult to characterize with current experimental capabilities. In this talk, I will first show how time- and angle-resolved photoemission spectroscopy can be implemented as a powerful tool to directly image the ultrafast dynamics of both bright and dark excitons in micron-sized TMD monolayers in crystal momentum space. I will then present our recent experimental results imaging the formation and relaxation dynamics of different types of exciton states in monolayer WS2. With circularly polarized photoexcitation, we observe the initial chiral preparation of optically-allowed excitons and reveal the surprising conservation of both exciton binding energy as well as momentum in the depolarization process. I will show in detail how these findings compare with different proposed theoretical models for exciton valley depolarization in TMDs.