WS_2与WSe_2单层膜中的A激子及其自旋动力学特性研究

单层过渡金属硫化物由于其特有的激子效应以及强自旋-谷耦合性质,在光电子学及谷电子学等方面有着很广阔的应用前景.利用超快时间分辨光谱,本文系统地比较了两类钨基单层硫化物(WS_2和WSe_2)的A-激子动力学和谷自旋弛豫特性.实验结果表明, WS_2单层膜的A-激子弛豫表现为双指数过程,而对于WSe_2,其A-激子衰减表现为三指数过程,且激子的寿命远长于前者. WS_2谷自旋极化弛豫表现为单指数衰减,其寿命约0.35 ps,主要由电子-空穴交换作用所主导.而对于WSe_2,谷自旋弛豫表现出双指数弛豫特性:一个寿命为0.5 ps的快过程和一个寿命为28 ps的慢过程.快过程的弛豫来源于电子-空穴交换作用,而慢过程则由于自旋晶格散射形成暗激子的过程.通过调谐抽运光波长,进一步证实WSe_2较WS_2更容易形成暗激子.

Dynamics of A-exciton and spin relaxation in WS2 and WSe2 monolayer

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.