Time-resolved ARPES of excitons in 2D semiconductors

在2022财年，我们成功地利用TR-ARPES收集了单层WS2中圆偏振光谷选择激发下激子动力学的数据。对数据的初步分析表明，不同温度下驱动谷退极化的机制不同。我们发现在90K时，脱极化的主要机制是交换相互作用，而在室温下，脱极化的主要机制是声子辅助的谷间散射。由于我们已经成功地制造出了能带宽度低至45 meV的高质量样品，我们现在可以解析自旋分裂的激子态，并直接观察激子态的动态演化。更重要的是，我们表明，在90K下，动量禁止激子保持谷偏振，保真度高于80%，寿命比明亮的光学活性激子长一个数量级。我们的发现开辟了一条将暗激子用于信息技术的新途径。关于这个题目的手稿正在准备中。

In FY2022, we have successfully gathered data using TR-ARPES on the dynamics of excitons upon valley selective excitation with circularly polarized light in monolayer WS2. The initial analysis of data show that different mechanisms drive the valley depolarization depending on temperature. We found that at 90K the main mechanisms for depolarization is exchange interaction whereas at room temperature it is phonon assisted intervalley scattering.Since we have managed to fabricate very high quality samples providing band linewidth as low as 45 meV, we can now resolve the spin-split excitonic states and directly observe the dynamical evolution of excitonic states.More importantly, we show that, at 90K, momentum forbidden excitons preserve the valley polarization with a robust fidelity higher than 80% and with an order of magnitude longer lifetime than bright optically active excitons. Our discovery opens a new route to use dark excitons for information technology.A manuscript is in preparation on this topic.