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Abstract
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.
Highlights
The 1s exciton—the ground state of a bound electron-hole pair—is central to understanding the photoresponse of monolayer transition metal dichalcogenides
Since the optoelectronic response is governed by the light-induced dynamic behaviour of this elementary ground state, knowledge of the 1s exciton response to the optical stimuli has been a crucial issue in many optoelectronic applications, such as phototransistors[1,2], photovoltaics[3], light-emitting diodes[4,5], van der Waals heterostructure-based optoelectronics[6,7,8] and valleytronic device applications[5,9,10,11,12]
The fundamental 1s exciton, theories predicted the presence of densely spaced exciton states in monolayer MoS2 with 1s exciton Ebind of 0.4–0.54 eV, whose (s-like) bright and (p-like) dark exciton characters were later confirmed by a series of seminal experiments via linear one-photon absorption[21,22], two-photon photoluminescence excitation (PLE)[22,23,24,32] and nonlinear wave-mixing spectroscopy[25,32], whereby Ebind was experimentally measured to be between 0.22 and 0.44 eV; the reported Ebind, shows somewhat discrepancy depending on the measurement methods and is varied from samples to samples[23,26,27]
Summary
The 1s exciton—the ground state of a bound electron-hole pair—is central to understanding the photoresponse of monolayer transition metal dichalcogenides. A natural question is how the internal excitonic transitions are interrelated on photoexcitation Accessing these intraexcitonic transitions, demands a fundamentally different experimental tool capable of probing optical transitions from 1s ‘bright’ to np ‘dark’ states. 26) and 0.44 eV (refs 23,26); the reported Ebind, shows somewhat discrepancy depending on the measurement methods and is varied from samples to samples[23,26,27] These experimental techniques, they are appropriate to clarify the optical state of the excitons, may address indirectly the dynamic transient information between the 1s ‘bright’ and the excited np ‘dark’ exciton (n is the principle quantum number); we denoted the exciton states in analogy to the hydrogen series[21]. The time-dependent IR absorption rapidly subdues over broad probe–photon energies, representing the transient absorption from the 1s to the quasi-continuum states after pump excitation
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