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 mid-infrared 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.S. Cha, S. Sim, J. Park and H. Choi were supported by the National Research Foundation of Korea (NRF) through the government of Korea (MSIP) (Grants No. NRF-2011-0013255, NRF-2009-0083512, NRF-2015R1A2A1A10052520), Global Frontier Program (2014M3A6B3063709), the Yonsei University Yonsei-SNU Collaborative Research Fund of 2014 and the Yonsei University Future-leading Research Initiative of 2014. J.H.S., H.H. and M.-H.J. were supported by Institute for Basic Science (IBS), Korea under the contract number of IBS-R014-G1-2016-a00