AI 中文总结
本研究通过低温共振拉曼光谱，揭示单层WSe₂中亮激子X_KK与动量暗激子X_d由谷间声子散射耦合，为解释其异常明亮的发射光谱提供了微观框架。
AI 中文摘要
原子级薄的过渡金属二硫化物中的激子-声子耦合控制着激子热化、谷间散射等关键过程，且难以通过光学光谱直接获取。本研究在低温下采用共振拉曼光谱探测hBN封装的WSe₂单层中的激子-声子耦合，通过调谐激发激光至亮激子X_KK共振，观测到丰富的拉曼光谱，并聚焦于简并A₁'/E'光学声子模式的共振轮廓。该轮廓呈现两个非对称共振峰，其能量差显著超过声子能量，这一特征无法仅用一阶拉曼散射解释。研究表明，引入三阶拉曼散射可解决该差异：有限动量声子介导的谷间散射将亮激子X_KK与动量暗激子X_d耦合。采用包含相干一阶与三阶散射的模型对两个独立样品的实验共振轮廓进行拟合，得到一致的激子能量与线宽，其中动量暗激子X_d比亮激子X_KK低约45meV至55meV。结果表明亮激子与暗激子间存在高效耦合，本研究为理解WSe₂单层尽管存在自旋禁阻的最低激子跃迁却呈现异常明亮的发射光谱提供了微观框架，并凸显了动量暗激子在共振光-物质相互作用中的作用。
英文摘要
Exciton-phonon coupling in atomically thin transition metal dichalcogenides governs key processes such as exciton thermalization and intervalley scattering and remains challenging to access directly by optical spectroscopy. Here, we employ resonance Raman spectroscopy at cryogenic temperatures to probe exciton-phonon coupling in hBN-encapsulated WSe_2 monolayers. Tuning the excitation laser across the bright exciton X_KK resonance, we observe rich Raman spectra and focus on the resonance profile of the degenerate A_1'/E' optical phonon mode. The profile exhibits two asymmetric resonance peaks whose energetic separation significantly exceeds the phonon energy - a feature that cannot be explained by first-order Raman scattering alone. We demonstrate that this discrepancy is resolved by including third-order Raman scattering, in which intervalley scattering enabled by a finite-momentum phonon couples the bright exciton X_KK to a momentum-dark exciton X_d. Fitting the experimental resonance profiles of two independent samples with a model comprising coherent first- and third-order scattering yields consistent exciton energies and linewidths, with a momentum-dark exciton X_d approximately 45meV to 55meV below the bright exciton X_KK. The results indicate efficient bright-to-dark exciton coupling. Our findings provide a microscopic framework for understanding the anomalously bright emission spectra of WSe_2 monolayers despite its spin-forbidden lowest exciton transition and highlight the role of momentum-dark excitons in resonant light-matter interaction.
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