等离激元与激子耦合体系中发光现象的理论研究

In recent years, coupled systems involving metal plasmons and molecular/semiconductor excitons have attracted much attention due to many specific and controllable optical properties they exhibit. At present, the research on the luminescence of the coupling system mainly focuses on the mechanism of exciton emission enhancement and quenching, as well as the corresponding coupling regimes. In theory, excitons are usually treated as a simple two-level system while ignoring the complex excitonic states in a real material and their interactions. In this project, we focus on several types of special excitonic states related to the luminescence of coupled systems. We will study the effect of the dark exciton state on the exciton luminescence under the plasmon field and the superradiation phenomenon of an excitonic collective excited state modulated by the plasmon. The dark exciton state is ubiquitous in the material and can be effectively excited and de-excited under the strongly localized plasmonic field, and then affects the system luminescence indirectly. In addition, a new coupling channel can be established between different excitons due to the presence of plasmons, so that a corresponding collective excitation mode may appear. These exciton states will affect the coupling system in real materials. Therefore, the implementation of this project is expected to reveal the special physical phenomena of excitonic systems under high intensity and highly localized electric fields, and is thus instructive for the application of such coupling systems in future optoelectronic devices.

近年来，金属等离激元和分子/半导体激子的耦合体系因其表现出的许多特异而可控的光学性能引起了人们的广泛关注。目前对耦合体系发光现象的研究主要集中于激子发光增强及淬灭出现的机制和对应的耦合条件。理论上通常将激子作为简单二能级系统处理，而忽略掉真实材料中复杂的激子态以及它们的相互作用。本项目中，我们重点关注与耦合体系发光相关的几类特殊激子态。我们将研究暗激子态在等离激元作用下对激子发光的影响，以及由等离激元调制的激子集体激发态的超辐射现象。暗激子态在材料中普遍存在，在等离激元的强局域场作用下可以被有效的激发和退激发，并且间接影响系统发光。另外，多个激子间由于等离激元的出现可以建立新的耦合通道，从而可能出现对应的集体激发模式。这些激子态在真实材料中都将对耦合系统放光造成影响。因此，本项目的实施将有望揭示激子体系在高强度高局域外场下的特殊物理现象，对这类耦合体系在未来光电器件中的应用具有指导意义。

金属等离激元和分子/半导体激子的耦合体系中存在许多特异的光电子学现象。这些现象本身和其背后的物理近年来引起了人们的广泛关注。有别于传统的理论近似，其通常将激子体系视为简单二能级体系，在本项目的研究中，我们关注实际体系中更为复杂的能级结构，从而需要考虑多种激子态在等离激元外场下的相互关联和受到的调制。在此背景下，本项目重点研究了耦合体系中复杂的激子态在等离激元场作用下被调制的激发和退激发过程。在由扫描隧道显微镜的针尖和衬底形成的金属纳腔中，我们研究了分子激子激发和退激发的微观过程，研究了自旋三重态这一暗激子态在上转换发光中起到的中间态作用。针对量子点激子态，我们研究了其在受到等离激元场作用下的退激发速率，特别是那些在传统光学选择定则主导下不发光的激子态的退激发增强。而对于一般的激子和等离激元耦合体系，我们给出了一个描述耦合体系发光行为的唯象模型，同时给出了一个通过不同激子态退激发通道间的竞争，来实现不同发光光谱的理论设计模型。本项目的实施加深了我们对于激子体系在高强度高局域的等离激元场下的物理现象的理解，对耦合体系在未来光电子器件的应用具有重要的指导意义。