Dynamics of A-exciton and spin relaxation in WS2 and WSe2 monolayer
Dynamics of A-exciton and spin relaxation in WS2 and WSe2 monolayer
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WS2 和 WSe2 单层中 A 激子和自旋弛豫的动力学
DOI：
10.7498/aps.68.20181769
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发表时间：
2019
影响因子：
1
通讯作者：
Ma Guo-Hong
中科院分区：
文献类型：
--
作者：
Yu Yang;Zhang Wen-Jie;Zhao Wan-Ying;Lin Xian;Jin Zuan-Ming;Liu Wei-Min;Ma Guo-Hong
二维过渡金属二硫属化物（2D TMD）由于其特殊的激子效应和强自旋谷锁定而成为光电子学和谷电子学中良好的候选材料。由于量子限域效应的增强和介质屏蔽效应的减弱，电子空穴对的光激发大大增强，使得TMD激子结合能很大，使得激子在室温甚至更高的温度下很容易被观察到。室温下二维TMD的光学响应以激子为主，这为研究激子或三重子的产生、弛豫和相互作用提供了理想的介质。通过采用超快时间分辨光谱，我们通过实验研究了两种类型的 TMD（即 WS2 和 WSe2 单层）的 A 激子和自旋弛豫的动态行为。通过调节简并泵浦和探测激光束的激发波长，WS2单层和WSe2单层在其A激子共振跃迁位置或A激子共振跃迁位置附近被激发，以便比较两种相似的WS2和WSe2单层结构的能带结构和激子偏振的动态演化。我们的实验结果表明，WS2 中 A 激子的弛豫表现出双指数衰减，而 WSe2 中 A 激子的弛豫表现出三指数衰减，并且 WSe2 中 A 激子的寿命比 WS2 对应物长得多。 WS2中A激子的自旋弛豫表现出单指数特征，寿命为0.35 ps，主要由电子-空穴交换相互作用主导。对于WSe2的情况，自旋弛豫可以很好地用双指数函数拟合，快部分的寿命为0.5 ps，慢部分的寿命为28 ps。快速弛豫由电子-空穴交换相互作用主导，而缓慢弛豫则来自自旋晶格耦合形成暗激子。通过调节 A 激子跃迁附近的激发波长，WSe2 中暗激子的形成被证明比 WS2 单层中暗激子的形成更有效。我们的实验结果为深入理解激子与TMD结构之间的关系提供了定性的物理图像，也为进一步设计和调控基于TMD的光电器件提供了参考。
Two-dimensional transitional metal dichalcogenide (2D TMD) emerges as a good candidate material in optoelectronics and valleytronics due to its particular exciton effect and strong spin-valley locking. Owing to the enhancement of quantum confinement effect and the decline of dielectric shielding effect, the optical excitation of electron-hole pair is enhanced substantially, which makes large TMD exciton binding energy and makes excitons observed easily at room temperature or even higher temperature. Optical response of 2D TMD is dominated by excitons at room temperature, which provides an ideal medium for studying the generation, relaxation and interaction of excitons or trions. By employing ultrafast time resolved spectroscopy, we investigate experimentally the dynamic behaviors of A-exciton and spin relaxations for two types of TMDs, i.e. WS2 and WSe2 monolayers, respectively. By tuning the excitation wavelength of the degenerate pump and probe laser beam, the WS2 monolayer and WSe2 monolayer are excited at their A-exciton resonance transition position or near their A-exciton resonance transition position in order to compare the dynamical evolutions of band structure and exciton polarization of the two similar WS2 and WSe2 monolayer structures. Our experimental results reveal that the relaxation of A exciton in WS2 shows biexponential decay, while that of WSe2 shows triexponential decay, and the A-exciton life time in WSe2 is much longer than that of WS2 counterpart. The spin relaxation of A exciton in WS2 shows a monoexponential feature with a lifetime of 0.35 ps, which is dominated by the electron-hole exchange interaction. For the case of WSe2, the spin relaxation can be well fitted with biexponential function, the fast component has a lifetime of 0.5 ps and the slow one has a lifetime of 28 ps. The fast relaxation is dominated by the electron-hole exchange interaction, and the slow one comes from the formation of dark exciton via spin-lattice coupling. By tuning the excitation wavelength around A-exciton transition, the formation of dark exciton in WSe2 is demonstrated to be much more effective than that in WS2 monolayer. Our experimental results provide qualitative physical images for an in-depth understanding of the relationship between exciton and TMD structure, and also provide reference for further designing and regulating the TMDs based optoelectronic devices.