Sub-10 fs Intervalley Exciton Coupling in Monolayer MoS 2 Revealed by Helicity-Resolved Two-Dimensional Electronic Spectroscopy
Sub-10 fs Intervalley Exciton Coupling in Monolayer MoS 2 Revealed by Helicity-Resolved Two-Dimensional Electronic Spectroscopy
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螺旋分辨二维电子光谱揭示单层 MoS 2 中的亚 10 fs 谷间激子耦合
DOI：
10.1021/acsnano.1c02381
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发表时间：
2021
期刊：
影响因子：
17.1
通讯作者：
Engel, Gregory S.
中科院分区：
文献类型：
--
作者：
Lloyd, Lawson T.;Wood, Ryan E.;Mujid, Fauzia;Sohoni, Siddhartha;Ji, Karen L.;Ting, Po-Chieh;Higgins, Jacob S.;Park, Jiwoong;Engel, Gregory S.
过渡金属双硫族化合物（TMDs）单层膜中K和K′高对称点处的谷赝自旋在下一代光电子学中具有潜在的光寻址自由度。然而，谷间散射和电荷载流子的弛豫导致谷去极化，限制了实际应用。此外，增强的库仑相互作用导致显着的激子效应，占主导地位的光学响应和初始谷去极化动力学，但复杂的超快光谱实验在短时间延迟的解释。采用宽带螺旋分辨二维电子光谱（2DES），我们观察到所有A和B谷激子态之间的超快（~ 10 fs）谷间耦合导致大面积单层MoS 2薄膜中谷指数的完全击穿.这些耦合和随后的动力学表现出最小的激发通量或温度依赖性，并对样品晶粒尺寸和固有应变的变化是鲁棒的。我们的观察强烈表明，这种直接的谷间耦合的时间尺度上的光激发是一个固有的属性大面积MoS 2不同的动态载流子或激子散射，声子驱动的过程，和多激子效应。这种超快的谷间耦合对单层TMD中基于激子的谷电子器件提出了根本性的挑战，必须克服才能完全实现大面积谷电子器件。
The valley pseudospin at the K and K′ high-symmetry points in monolayer transition metal dichalcogenides (TMDs) has potential as an optically addressable degree of freedom in next-generation optoelectronics. However, intervalley scattering and relaxation of charge carriers leads to valley depolarization and limits practical applications. In addition, enhanced Coulomb interactions lead to pronounced excitonic effects that dominate the optical response and initial valley depolarization dynamics but complicate the interpretation of ultrafast spectroscopic experiments at short time delays. Employing broadband helicity-resolved two-dimensional electronic spectroscopy (2DES), we observe ultrafast (∼10 fs) intervalley coupling between all A and B valley exciton states that results in a complete breakdown of the valley index in large-area monolayer MoS2films. These couplings and subsequent dynamics exhibit minimal excitation fluence or temperature dependence and are robust toward changes in sample grain size and inherent strain. Our observations strongly suggest that this direct intervalley coupling on the time scale of optical excitation is an inherent property of large-area MoS2distinct from dynamic carrier or exciton scattering, phonon-driven processes, and multiexciton effects. This ultrafast intervalley coupling poses a fundamental challenge for exciton-based valleytronics in monolayer TMDs and must be overcome to fully realize large-area valleytronic devices.