Ultrafast dynamics in atomically thin transition metal dichalcogenides in a magnetic field

原子薄的半导体过渡金属二硫属化物（TMD）结合了几乎理想的二维限制的电荷载流子和强烈减少介电屏蔽环境。两者的结合导致电子-空穴对之间的强吸引库仑相互作用，其形成各种亮激子和暗激子以及紧密束缚的带电激子（trions）和束缚双激子态（biexcitons）。这些基本激发态在原子薄TMD的光学性质中起着主导作用，并为研究二维激子物理提供了理想的先决条件。特别是，原子薄的TMD的超快非线性光学响应由多体激子-激子、激子-电子和激子-声子相互作用控制。因此，操纵多体相互作用的外部磁场，修改激子激发通道和它们的动力学，是特别感兴趣的深入了解底层的微观过程，这决定了光学响应，并为未来的应用原子薄TMD。拟议项目的主要目标是阐明在一个联合的理论和实验努力的磁场控制的内和谷间TMD激子，trions，和双激子在原子薄（TMD）之间的超快动力学和弛豫机制。在实验上，我们将在不同的磁场几何形状和受控掺杂下测量时间分辨光致发光和泵浦探测实验中的动态。在理论部分，我们计划扩展我们的微观形式主义的泵浦-探测光谱，包括不相干的，在时间上不断变化的影响，如弛豫和退相，和暗激子的形成，在实验中占主导地位，在有限的延迟时间。这些过程同时发生在动量和自旋亮激子以及动量和自旋暗激子的时间动力学中。在这些原子级薄的二维纳米结构中，对控制超快动力学的基本多粒子过程的深入了解对于设计和工程新型TMD光电器件至关重要。

Atomically thin semiconducting transition metal dichalcogenides (TMDs) unite nearly ideal two-dimensional confinement of charge carriers and strongly reduced dielectric screening from the environment. The combination of both results in strong attractive Coulomb interactions between electron-hole pairs which form a variety of bright and dark excitons as well as tightly bound charged excitons (trions) and bound two-exciton states (biexcitons). These fundamental excitations play a dominant role in the optical properties of atomically thin TMDs and provide ideal prerequisites to investigate new exciton physics in two dimensions. In particular, the ultrafast nonlinear optical response of atomically thin TMDs is governed by many-body exciton-exciton, exciton-electron, and exciton-phonon interactions. Consequently, the manipulation of many-body interactions by external magnetic fields, modifying excitonic excitation channels and their dynamics, is of particular interest for an in-depth understanding of underlying microscopic processes, which determine the optical response, and for future applications of atomically thin TMDs. The main goal of the proposed project is to elucidate in a joint theory and experiment effort the ultrafast dynamics and relaxation mechanisms between magnetic-field-controlled intra- and intervalley TMD excitons, trions, and biexcitons in atomically thin (TMDs). Experimentally, we will measure the dynamics in time-resolved photoluminescence and pump-probe experiments in different magnetic field geometries and with controlled doping. In the theory part, we plan to extend our microscopic formalism of pump-probe spectroscopy to include incoherent, in time evolving effects such as relaxation and dephasing, and dark exciton formation, dominating in experiments at finite delay time. These processes occur simultaneously in the temporal dynamics of momentum- and spin-bright as well as momentum- and spin-dark excitons. The gained insights into the fundamental many-particle processes governing the ultrafast dynamics in these atomically thin two-dimensional nanostructures will be of crucial importance for designing and engineering novel TMD-based optoelectronic devices.