Direct Observation of Room‐Temperature Intravalley Coherent Coupling Processes in Monolayer MoS₂

Abstract

Comprehensive understanding of the interactions between excited states with spin-valley polarizations is crucial for the applications of monolayer 2D transition metal dichalcogenides (TMDs) in spin-valleytronics. Here, by broadband femtosecond transient absorption spectroscopy with circularly polarized pump/probe lights, a systematic investigation on the many-body interactions of band-edge valley excitons in monolayer MoS 2 is performed, where intravalley coherent coupling processes are directly observed at room temperature. Namely, when the intravalley mixing states in the superposition region of A-exciton and B-exciton states are excited, there is an intravalley coherent coupling between A-exciton and B-exciton in K valley (or A′-exciton and B′-exciton in K′ valley) with an oscillation frequency of ≈7-8 THz, rather than the intervalley excitonic coupling in general. The findings elucidate intravalley mixing and coherent oscillations in valley excitons of monolayer TMDs at room temperature, which can give insight into the many-body coupling mechanisms of excited valley excitons in 2D TMDs.

Determining Band Splitting and Spin-Flip Dynamics in Monolayer MoS2

“…As shown in Figures c and d, the A exciton polarization exhibits a fast decay of ∼110 fs and displays no apparent dependence on pump fluence and temperature. Because of the limitation of the IRF (∼150 fs), the valley polarization degree (∼40%) we obtained is less than 100%, which is consistent with the previously reported value of 17–50% for TMDCs materials. − The same behavior was also observed for B excitons (Figure S3).…”

Optically Addressing Exciton Spin and Pseudospin in Nanomaterials for Spintronics Applications

“…Using TRCD can provide substantial insights into the competing many-body mechanisms within TMDCs, namely the optical Stark effect, the direct Coulomb driven Dexter-like intervalley interaction and the intravalley exciton exchange interactions. 96 Although it is relatively simple to implement and requires no magnetic field, TRCD is a high-background experiment, which may limit S/N compared with other measures of spin-valley relaxation. For example, if the transient signals have noise or drift during the measurement, these artifacts will map onto the TRCD signal, complicating analysis.…”

“…Using TRCD can provide substantial insights into the competing many-body mechanisms within TMDCs, namely the optical Stark effect, the direct Coulomb driven Dexter-like intervalley interaction and the intravalley exciton exchange interactions. 96 …”

Resonance Photoluminescence Enhancement of Monolayer MoS2 via a Plasmonic Nanowire Dimer Optical Antenna

“…Atomically thin transition-metal dichalcogenides (TMDs) such as MoS 2 , − direct-band-gap two-dimensional materials, have attracted great interest and exhibited promising applications in field-effect transistors, , solar cells, photodetectors, − light-emitting devices, strong-coupling regimes, plasmon–exciton coupling and surface-enhanced Raman scattering (SERS), , due to their extraordinary physical properties. Nevertheless, their low quantum efficiency and weak light absorption have been bottlenecks in these applications.…”