Abstract
The inherent valley-contrasting optical selection rules for interband transitions at the K and K′ valleys in monolayer MoS2 have attracted extensive interest. Carriers in these two valleys can be selectively excited by circularly polarized optical fields. The comprehensive dynamics of spin valley coupled polarization and polarized exciton are completely resolved in this work. Here, we present a systematic study of the ultrafast dynamics of monolayer MoS2 including spin randomization, exciton dissociation, free carrier relaxation, and electron-hole recombination by helicity- and photon energy-resolved transient spectroscopy. The time constants for these processes are 60 fs, 1 ps, 25 ps, and ~300 ps, respectively. The ultrafast dynamics of spin polarization, valley population, and exciton dissociation provides the desired information about the mechanism of radiationless transitions in various applications of 2D transition metal dichalcogenides. For example, spin valley coupled polarization provides a promising way to build optically selective-driven ultrafast valleytronics at room temperature. Therefore, a full understanding of the ultrafast dynamics in MoS2 is expected to provide important fundamental and technological perspectives.
Highlights
The inherent valley-contrasting optical selection rules for interband transitions at the K and K9 valleys in monolayer MoS2 have attracted extensive interest
The comprehensive dynamics of spin valley coupled polarization and polarized exciton are completely resolved in this work
We present a systematic study of the ultrafast dynamics of monolayer MoS2 including spin randomization, exciton dissociation, free carrier relaxation, and electron-hole recombination by helicity- and photon energy-resolved transient spectroscopy
Summary
Correspondence and requests for materials should be addressed to C.-W.L. The absorption spectrum of a monolayer MoS2 clearly presents A (,1.89 eV) and B (,2.04 eV) excitonic transitions, which indicates the splitting of the valence band at the K valley due to spin-orbit coupling[14,15]. The difference absorbance, DAexciton, probed by s1 and s2 circular polarizations have the same photobleaching phenomena, i.e., the blue shift of exciton A in the time range of ,10 ps This implies that the excess population of excitons is created in both the K and K9 valleys through intervalley scattering, which further reveals the relaxation of hole spin polarization. A complete understanding of spin-valley coupled polarization anisotropy and the carrier dynamics of atomic layer MoS2, which can further help us to develop the ultrafast multi-level logic gates at room temperature.
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