A holistic view of the dynamics of long-lived valley polarized dark excitonic states in monolayer WS2

Xing Zhu; David R. Bacon; Vivek Pareek; Julien Madéo; Takashi Taniguchi; Kenji Watanabe; Michael K. L. Man; Keshav M. Dani

doi:10.1038/s41467-025-61677-2

Journal article A holistic view of the dynamics of long-lived valley polarized dark excitonic states in monolayer WS2

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Xing Zhu, David R. Bacon, Vivek Pareek, Julien Madéo, Takashi Taniguchi, Kenji Watanabe, Michael K. L. Man, Keshav M. Dani. A holistic view of the dynamics of long-lived valley polarized dark excitonic states in monolayer WS2. Nature Communications. 2025, 16 (1), 6385. https://doi.org/10.1038/s41467-025-61677-2

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With their long lifetime and protection against decoherence, dark excitons in monolayer semiconductors offer a promising route for quantum technology applications. Optical techniques have previously been used to show that dark excitons can sustain a long-lived valley polarization. However, several important aspects remain unknown regarding the fundamental nature of valley polarized dark excitons, such as the relative contributions of the different types of dark excitons to the degree of valley polarization, how these relative contributions evolve over time, and the role of excitation conditions therein. Here, using time- and angle-resolved photoemission spectroscopy with high energy resolution, we obtain a holistic view of the dynamics of valley-polarized bright excitons into a variety of states over the entire Brillouin zone. By varying experimental conditions, we reconcile between reports from two different experimental platforms – we switch between the rapid valley depolarization reported previously in TR-ARPES experiments, and the ability to see long-lived valley polarized dark excitons observed previously in optical studies. For the latter, we find that the intervalley momentum-dark exciton dominates at early times, measuring 85% of the overall excitonic population at 1 ps time-delay, with a 40% degree of valley polarization, while our data indicates that valley polarized spin-dark states dominate the excited state population at much longer time delay (> 10ps). Our measurements provide important information regarding the timescales and degrees to which different species of dark excitons dominate and contribute to the previously observed long-lived valley polarization in optics.

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