Monolayer transition metal dichalcogenides such as MoS2, WS2, MoSe2, WSe2, and MoTe2 received a lot of attention recently due to their atomic thickness in combination with an optical band gap in the visible or infrared. These properties render them a promising material class for new opto-electronic devices. Strong spin-orbit coupling together with the absence of inversion symmetry leads to an emission of polarized photoluminescence after excitation with circularly polarized light. Therefore, the K and K’ valley of the semiconductor can be selectively addressed by left and right handed circularly polarized light, which is interesting for valleytronic applications. To understand the mechanisms governing the creation and destruction of valley polarization, time-resolved experiments are necessary. While stationary photoluminescence experiments show nearly perfect valley polarization, indicating very slow intervalley scattering processes, first valley-selective pump-probe experiments yielded a strong signal immediately after optical excitation in both the pumped and unpumped valley, suggesting a small valley polarization. To understand this behavior, we performed a joint experiment-theory study on the time-resolved valley dynamics in atomically thin WS2. We find strong intervalley Coulomb coupling governing the dynamics in the atomically thin semiconductor. Our results are also applicable to the other transition metal dichalcogenides MoS2, MoSe2, and WSe2, where strong intervalley Coulomb coupling is expected.
Rudolf Bratschitsch, "Ultrafast valley dynamics in atomically thin transition metal dichalcogenides
(Conference Presentation)," Proc. SPIE 9931, Spintronics IX, 99313D (Presented at SPIE Nanoscience + Engineering: August 31, 2016; Published: 4 November 2016); https://doi.org/10.1117/12.2239329.5166910684001.
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