Abstract Benefiting from strong photon–exciton and phonon–exciton interactions in atomic thickness, transition metal dichalcogenides (TMDCs) are viewed as one promising platform for exploring elementary excitonic photoluminescence (PL) and intrinsic spin–valley properties at the monolayer limit. Despite well-studied Stokes downconversion (DC) PL, the anti-Stokes upconversion (UC) PL has been recently reported in TMDC monolayers, which mainly focus on UC mechanisms while detailed valley-related dynamical processes are unwittingly less concerned. Here, we carry out an in-depth investigation on both DC and UC emission features of monolayer WS2 at room temperature, where UC PL persists with energy gain up to 190 meV. The PL excitation and power-dependent experiments clearly distinguish the origins of DC PL and UC PL, which refer to saturated absorption and phonon-assisted transition from charged trions to neutral A-excitons. And contrast valley properties are observed in DC and UC scenarios with polarization-resolved PL and pump–probe measurements. According to the experimental facts, phenomenological dynamical DC and UC scenarios are modeled with intervalley depolarization taken into consideration, in which intermediates from spontaneous intervalley depolarization account for the observed emission and valley properties. This work can help understand the light–matter interactions and valley properties in monolayer TMDCs.

中文翻译：

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室温下单层WS2下转换和上转换发射的谷去极化

摘要 受益于原子厚度中强的光子-激子和声子-激子相互作用，过渡金属二硫属化物（TMDC）被​​视为探索单层极限的基本激子光致发光（PL）和本征自旋谷特性的一个有前途的平台。尽管对斯托克斯下变频 (DC) PL 进行了深入研究，但最近在 TMDC 单层中报道了反斯托克斯上变频 (UC) PL，其主要关注 UC 机制，而详细的谷相关动力学过程在不知不觉中较少受到关注。在这里，我们对室温下单层 WS2 的 DC 和 UC 发射特征进行了深入研究，其中 UC PL 持续能量增益高达 190 meV。PL 激励和功率相关实验清楚地区分 DC PL 和 UC PL 的起源，它指的是饱和吸收和声子辅助从带电三元到中性 A 激子的跃迁。在 DC 和 UC 场景中，通过偏振分辨 PL 和泵浦探针测量观察到对比谷特性。根据实验事实，现象学动力学 DC 和 UC 场景建模时考虑了间隔去极化，其中来自自发间隔去极化的中间体解释了观察到的发射和谷特性。这项工作有助于理解单层 TMDC 中的光-物质相互作用和谷特性。现象学动力学 DC 和 UC 场景建模时考虑了间隔去极化，其中来自自发间隔去极化的中间体解释了观察到的发射和谷特性。这项工作有助于理解单层 TMDC 中的光-物质相互作用和谷特性。现象学动力学 DC 和 UC 场景建模时考虑了间隔去极化，其中来自自发间隔去极化的中间体解释了观察到的发射和谷特性。这项工作有助于理解单层 TMDC 中的光-物质相互作用和谷特性。