AI 中文总结
本研究开发基于GW+BSE的从头算方案，利用调谐至激子-激子跃迁范围的瞬态吸收实验，检测MoS₂单层的暗激子并揭示其光谱特征，为暗激子表征提供了有效方法。
AI 中文摘要
暗激子在二维半导体的非平衡动力学中发挥核心作用，但仍难以表征。本研究表明，调谐至激子-激子跃迁能量范围（exc-tr-abs）的探测脉冲的瞬态吸收实验，可用于检测任何布居的暗激子态的激发，包括对称性禁阻、自旋禁阻及有限动量的暗激子。我们开发了基于GW+BSE的从头算方案，用于计算任意布居激子分布的exc-tr-abs光谱，通过在局域光滑规范下评估激子-激子偶极矩阵元，将不同类别的暗激子置于同等地位，该矩阵元同时考虑带内和带间贡献。作为测试案例，我们采用已有实验数据的MoS₂单层，其exc-tr-abs信号来自激子色散四个不同谷（Γ、K、M、Q谷）的贡献之和，对最终信号的解释与仅基于q=Γ处跃迁的解释显著不同；各谷的激子-激子偶极强度相近，相对权重主要由初始激子布居决定。态与自旋分辨分析显示，主峰可描述为A、B激子的1s→2p和1s→3p跃迁，但包含谷间自旋翻转与自旋守恒激子的跃迁叠加，且为单个谷内不同动量的平均贡献。
英文摘要
Dark excitons play a central role in the nonequilibrium dynamics of two-dimensional semiconductors, but remain difficult to characterize. Transient-absorption experiments, with probes tuned in exciton-exciton transitions energy range (exc-tr-abs), can detect excitations from any populated dark excitons, including symmetry-forbidden, spin-forbidden, and finite-momentum ones. In this work, we develop a $GW$+BSE scheme for computing exc-tr-abs spectra from arbitrary populated exciton distributions. These dark excitons are included equally by evaluating exciton-exciton dipoles in a locally smooth gauge, including intra- and inter-band contributions. For monolayer MoS$_2$, the exc-tr-abs signal arises from the $Γ$, $K$, $M$, and $Q$ valleys, differing substantially from the $Γ$-only interpretation. Exciton-exciton dipoles show similar intensities across these valleys, while their weights are dictated by initial excitonic populations. State- and spin-resolved analyses assign the peaks to $1s \rightarrow 2p$ and $1s \rightarrow 3p$ transitions from both spin-flip and spin-conserving A and B excitons across valleys and momenta.
