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

原子薄的半导体过渡金属二卤化物(TMD)结合了电荷载流子的近乎理想的二维限制和强烈减少了环境中的介电屏蔽。这两者的结合导致了电子-空穴对之间强烈的吸引库仑相互作用，这些电子-空穴对形成了各种亮激子和暗激子以及紧密束缚的带电激子(三子)和束缚双激子态(双激子)。这些基本激发对原子薄TMD的光学性质起着主导作用，为研究二维新的激子物理提供了理想的前提条件。特别是，原子薄TMD的超快非线性光学响应受多体激子-激子、激子-电子和激子-声子相互作用的支配。因此，通过外加磁场操纵多体相互作用，改变激子激发通道及其动力学，对于深入理解决定光学响应的潜在微观过程以及原子薄TMD的未来应用具有特别重要的意义。该计划的主要目的是在理论和实验的共同努力中阐明磁场控制的原子薄(TMD)中的谷内和谷间TMD激子、三子和双激子之间的超快动力学和驰豫机制。在实验上，我们将测量时间分辨光致发光和泵浦探测实验中不同磁场几何形状和可控掺杂下的动力学。在理论部分，我们计划将我们的泵浦-探测光谱的微观形式扩展到包括非相干的、随时间演化的效应，如弛豫和退相，以及在有限延迟时间内主导实验的暗激子的形成。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。对这些原子薄的二维纳米结构中控制超快动力学的基本多粒子过程的深入了解将对设计和设计基于TMD的新型光电子器件具有至关重要的意义。

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.