Optics, dynamics and transport of exciton polaritons in atomically thin semiconductors

当原子薄的过渡金属二卤化物(TMD)集成到合适的光学微腔中时，其紧密束缚的激子可以与腔光子杂化形成激子极化子。这些准粒子继承了它们组成部分的性质，潜在地将光子的空间相干性和长传播长度与基于材料的激发的可调性和非线性结合在一起。TMD激子的大振子强度导致大量的拉比分裂，而大的结合能允许室温激子-极化电子。对极化激子弛豫的微观理解对于解释光谱测量以及理解诸如玻色爱因斯坦凝聚和谷极化保持等现象是必不可少的。到目前为止，TMD极化子的大部分理论都依赖于简单的唯象速率方程来描述粒子动力学和弛豫。此外，动量间接激子的影响也被忽略了。在这个项目中，我们建议发展一个复杂的基于密度矩阵形式的多粒子理论，结合Hopfield方法，在微观基础上处理TMD单分子层以及扭曲的TMD同质和异质双分子膜中光-物质相互作用的强耦合和极强耦合区域。我们的研究计划包括两个主要目标：获得TMD单层和扭曲双层的强耦合区域内激子、光子和声子的相互作用的微观洞察，以及学习如何通过改变扭转角度、腔长、温度和镜面反射率来控制激子-声子-偏振子物理。我们将使用Wannier-Hopfield相结合的方法来发展一个考虑激子能量全景的极化激子弛豫模型。这将使我们能够揭示层内和层间激子极化子形成、热化和衰变背后的时间和能量分辨的多粒子过程。了解TMD中极强耦合机制对激子光学、动力学和输运的影响。在这里，光-激子相互作用强到足以将激子耦合到电子-空穴连续体。由于其巨大的振子强度，TMD为研究这一广泛探索的体系提供了一个理想的平台。我们将发展一个新的微观模型，关键是包括束缚激子态和非束缚电子-空穴跃迁，来描述光诱导的内部激子波函数的修改，并寻找这一新物理的实验特征。我们在模拟多粒子物理和光-物质相互作用方面的长期专业知识使我们在应对这一提议的挑战方面处于有利地位。我们的研究将为微观理解原子薄纳米材料中的强耦合和极强耦合机制提供重大进展。

When atomically thin transition metal dichalcogenides (TMD) are integrated into suitable optical microcavities, their tightly-bound excitons can hybridize with cavity photons to form exciton polaritons. These quasi-particles inherit properties from their constituent parts, potentially combining the spatial coherence and long propagation lengths of photons with the tunability and nonlinearity of material-based excitations. The large oscillator strength of TMD excitons leads to a massive Rabi splitting, and the large binding energy allows for room-temperature exciton-polaritonics. A microscopic understanding of polariton relaxation is essential for interpreting optical spectroscopy measurements, as well as understanding phenomena, such as Bose Einstein condensation and valley polarization retention. To date, much of the theory of TMD polaritons has relied on simple phenomenological rate equations for population dynamics and relaxation. Furthermore, the impact of momentum-indirect excitons has been neglected. In this project, we propose to develop a sophisticated many-particle theory based on the density matrix formalism, combined with a Hopfield approach, to treat on microscopic footing the strong and very-strong coupling regimes of light-matter interaction in TMD monolayers as well as twisted TMD homo- and heterobilayers. Our research plan consists of two main goals: Gain microscopic insights into the interplay of excitons, photons, and phonons within the strong coupling regime for TMD monolayers and twisted bilayers, and learn how to control the exciton-phonon-polariton physics by changing the twist angle, cavity length, temperature, and mirror reflectivity. We will use a combined Wannier-Hopfield approach to develop a model of polariton relaxation that considers the full exciton energy landscape. This will allow us to reveal the time- and energy-resolved many-particle processes behind the formation, thermalization and decay of both intra- and interlayer exciton polaritons. Understand the impact of the very-strong coupling regime on exciton optics, dynamics, and transport in TMDs. Here, the light-exciton interaction is strong enough to couple excitons to the electron-hole continuum. Due to their large oscillator strength, TMDs offer an ideal platform to study this widely unexplored regime. We will develop a new microscopic model, crucially including both bound exciton states and the unbound electron-hole transitions, to describe the light-induced modification of the internal exciton wavefunction, and seek experimental signatures of this novel physics. Our long-standing expertise in modelling many-particle physics and light-matter interactions places us in a strong position to tackle the challenges of this proposal. Our study will provide a major advance for microscopic understanding of the strong and very-strong coupling regime in the technologically promising class of atomically thin nanomaterials.