Observation of 2D semiconductor P-type dark-exciton lifetime using two-photon ultrafast spectroscopy

We report direct measurements of intrinsic lifetimes of P-type dark-excitons in MoS₂ monolayers. Using sub-gap excitation, we demonstrate two-photon excited direct population of P-type dark excitons, observe their scattering to bright states and decay with femtosecond resolution. In contrast to one-photon excitation schemes, non-monotonic density variation in bright exciton population observed under two-photon excitation shows the indirect nature of its population and competing decay pathways. Detailed modeling of different recombination pathways of bright and dark excitons allows experimental measurement of 2P dark → 1S bright exciton scattering rates. These insights into the dark states in a MoS₂ monolayer pave the way for novel devices such as quantum memories and computing.