Zhu Xing, Bacon David R, Pareek Vivek, Madéo Julien, Taniguchi Takashi, Watanabe Kenji, Man Michael K L, Dani Keshav M
Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology, Okinawa, Japan.
Department of Chemistry, University College London, London, United Kingdom.
Nat Commun. 2025 Jul 10;16(1):6385. doi: 10.1038/s41467-025-61677-2.
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.
凭借其长寿命和抗退相干特性，单层半导体中的暗激子为量子技术提供了一条有前景的途径。光学技术此前已观测到具有长寿命谷极化的暗激子。然而，仍有几个方面尚不清楚，例如不同谷极化暗激子的数量和时间演化以及激发条件的作用。在此，我们使用时间分辨和角分辨光电子能谱，获得了谷选择性光激发后动力学的整体视图。通过改变实验条件，我们调和了此前在时间分辨角分辨光电子能谱中报道的快速谷去极化现象，以及光学研究中对长寿命谷极化暗激子的观测结果。对于后者，我们发现动量暗激子在早期占主导地位，维持着40%的谷极化程度，而谷极化自旋暗态在较长时间占主导地位。我们的测量提供了时间尺度以及不同暗激子如何对光学中此前观测到的长寿命谷极化做出贡献。
