Center for Functional Nanomaterials Seminar

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 reciprocal 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. Alice Kunin is currently a Senior Postdoctoral Associate at Stony Brook University in the group of Thomas Allison and an incoming Assistant Professor in the Department of Chemistry at Princeton University. As a postdoc, she has worked to pioneer a new approach to ultrafast time- and angle-resolved photoemission spectroscopy by combining frequency comb laser-based high harmonic generation with new surface imaging techniques. Her independent research will focus on advancing these state-of-the-art spectroscopic tools to characterize the ultrafast dynamics of optically excited states in novel quantum materials, organic/inorganic interfaces, liquids, and in situ surface photocatalytic reactions. Alice completed her PhD in 2019 at the University of California, Berkeley as a DoD National Defense Science and Engineering Graduate Fellow in the group of Daniel Neumark.