Direct Time- and Momentum-Resolved Imaging of Exciton Dynamics in Monolayer WS₂

Monolayer transition metal dichalcogenides (TMDs) have generated immense interest in recent years for their potential in a variety of optoelectronic applications due to their unique spin-valley optical selection rules. The remarkable properties of these materials are governed by strongly bound excitons that exhibit a variety of complex radiative and non-radiative relaxation pathways. In this talk, I will first show how time- and angle-resolved photoemission spectroscopy can be implemented to efficiently image, in parallel, the ultrafast dynamics of the full Brillouin zone of micron-sized TMD heterostructures. With a repetition rate of 61 MHz, we are able to employ low, few μJ/cm² pump fluences to maintain low exciton densities and determine the lifetimes and relaxation pathways of both optically bright and momentum- and/or spin-forbidden dark excitons with 225 fs time resolution. I will then present our recent experimental results directly imaging the formation and relaxation dynamics of A and B excitons in monolayer WS₂. Following linearly polarized pump excitation, we observe the relaxation of hot excitons within the K valleys to form a long-lived dark exciton state, as well as population of the Sigma valleys yielding long-lived momentum-forbidden excitons. With circularly polarized photoexcitation, we observe the initial preparation of excitons in the K valleys and a delayed rise for signal in the K’ valleys, indicative of valley depolarization due to intervalley scattering.