Tang Yanhao, Mak Kin Fai, Shan Jie
School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA.
Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY, USA.
Nat Commun. 2019 Sep 6;10(1):4047. doi: 10.1038/s41467-019-12129-1.
Single-layer transition metal dichalcogenides provide a promising material system to explore the electron's valley degree of freedom as a quantum information carrier. The valley degree of freedom can be directly accessed by means of optical excitation. However, rapid valley relaxation of optically excited electron-hole pairs (excitons) through the exchange interaction has been a major roadblock. Theoretically such valley relaxation is suppressed in dark excitons, suggesting a potential route for long valley lifetimes. Here we develop a waveguide-based method to detect time-resolved and energy-resolved dark exciton emission in single-layer WSe, which involves spin-forbidden optical transitions with an out-of-plane dipole moment. The valley degree of freedom of dark excitons is accessed through the valley-dependent Zeeman effect under an out-of-plane magnetic field. We find a short valley lifetime for the dark neutral exciton, likely due to the short-range electron-hole exchange, but long valley lifetimes exceeding several nanoseconds for the dark charged excitons.
单层过渡金属二硫属化物为探索电子的谷自由度作为一种量子信息载体提供了一个很有前景的材料体系。谷自由度可以通过光激发直接获取。然而，通过交换相互作用，光激发的电子 - 空穴对（激子）的快速谷弛豫一直是一个主要障碍。理论上，这种谷弛豫在暗激子中受到抑制，这表明了实现长谷寿命的潜在途径。在这里，我们开发了一种基于波导的方法来检测单层WSe₂中时间分辨和能量分辨的暗激子发射，这涉及具有面外偶极矩的自旋禁戒光学跃迁。通过面外磁场下与谷相关的塞曼效应来获取暗激子的谷自由度。我们发现暗中性激子的谷寿命较短，这可能是由于短程电子 - 空穴交换，但暗带电激子的谷寿命超过几纳秒，较长。
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