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 and Keshav M. Dani ()
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Xing Zhu: Okinawa Institute of Science and Technology
David R. Bacon: Okinawa Institute of Science and Technology
Vivek Pareek: Okinawa Institute of Science and Technology
Julien Madéo: Okinawa Institute of Science and Technology
Takashi Taniguchi: 1-1 Namiki
Kenji Watanabe: 1-1 Namiki
Michael K. L. Man: Okinawa Institute of Science and Technology
Keshav M. Dani: Okinawa Institute of Science and Technology

Nature Communications, 2025, vol. 16, issue 1, 1-8

Abstract: Abstract With their long lifetime and protection against decoherence, dark excitons in monolayer semiconductors offer a promising route for quantum technologies. Optical techniques have previously observed dark excitons with a long-lived valley polarization. However, several aspects remain unknown, such as the populations and time evolution of the different valley-polarized dark excitons and the role of excitation conditions. Here, using time- and angle-resolved photoemission spectroscopy, we obtain a holistic view of the dynamics after valley-selective photoexcitation. By varying experimental conditions, we reconcile between the rapid valley depolarization previously reported in TR-ARPES, and the observation of long-lived valley polarized dark excitons in optical studies. For the latter, we find that momentum-dark excitons largely dominate at early times sustaining a 40% degree of valley polarization, while valley-polarized spin-dark states dominate at longer times. Our measurements provide the timescales and how the different dark excitons contribute to the previously observed long-lived valley polarization in optics.

Date: 2025
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DOI: 10.1038/s41467-025-61677-2

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