Time-resolved ARPES of excitons in 2D semiconductors

在FY 2022中，我们成功地使用TR-ARPES收集了单层WS 2中圆偏振光谷选择性激发激子动力学的数据。对数据的初步分析表明，不同的机制驱动谷去极化取决于温度。我们发现在90 K时退极化的主要机制是交换相互作用，而在室温时是声子辅助的谷间散射。由于我们已经成功地制备了非常高质量的样品，其带宽低至45 meV，我们现在可以分辨自旋分裂激子态，并直接观察激子态的动力学演化。更重要的是，我们表明，在90 K时，动量禁戒激子以高于80%的鲁棒保真度和比明亮的光学活性激子长一个数量级的寿命保持谷偏振。我们的发现为暗激子在信息技术中的应用开辟了一条新的途径。

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.