1s-intraexcitonic dynamics in monolayer MoS2 probed by ultrafast mid-infrared spectroscopy

The 1s exciton-the ground state of a bound electron-hole pair-is central to understanding the photoresponse of monolayer transition metal dichalcogenides. Above the 1s exciton, recent visible and near-infrared investigations have revealed that the excited excitons are much richer, exhibiting a series of Rydberg-like states. A natural question is then how the internal excitonic transitions are interrelated on photoexcitation. Accessing these intraexcitonic transitions, however, demands a fundamentally different experimental tool capable of probing optical transitions from 1s 'bright' to np 'dark' states. Here we employ ultrafast midinfrared spectroscopy to explore the 1s intraexcitonic transitions in monolayer MoS2. We observed twofold 1s-3p intraexcitonic transitions within the A and B excitons and 1s-2p transition between the A and B excitons. Our results revealed that it takes about 0.7 ps for the 1s A exciton to reach quasi-equilibrium; a characteristic time that is associated with a rapid population transfer from the 1s B exciton, providing rich characteristics of many-body exciton dynamics in two-dimensional materials.