The chiral coupling of an emitter to waveguide mode, i.e., the propagation direction of the excited waveguide mode is locked to the transverse spin (T-spin) of a circularly polarized emitter, has exhibited unprecedented applications in nanophotonics and quantum information processing. This chiral coupling can be largely enhanced in terms of unidirectivity, efficiency, and spontaneous emission rate by introducing resonant modes as coupling interfaces. However, this indirect chiral coupling still undergoes limitations in flexibility and miniaturization, and the underlying physical mechanisms are to be clarified. Here, we present an intuitive and rigorous approach for analyzing the direct/indirect chiral coupling, and thereout, derive some general relations between the chiral-coupling directionality and the T-spin of the field or emitter. Based on the theories, we propose an indirect chiral-coupling system on the platform of surface plasmon polariton (SPP), with a nanocavity supporting Fabry–Perot (FP) resonance of dual SPP modes serving as a novel coupling interface. The FP resonance provides flexible design freedoms which can modulate the chirality of the T-spin (and the resultant chiral-coupling directionality) to flip or disappear. A unidirectivity up to 99.9% along with a high coupling efficiency and enhancement of spontaneous emission rate is achieved. Two first-principles-based SPP models for the reciprocal and original problems are built up to verify the decisive role of the FP resonance in achieving the chiral coupling. The proposed theories and novel chiral-coupling interface will be beneficial to the design of more compact and flexible chiral-coupling systems for diverse applications.

中文翻译：

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间接手性耦合理论和法布里-珀罗共振作为柔性手性耦合界面的提议

发射器与波导模式的手性耦合，即激发波导模式的传播方向被锁定到圆偏振发射器的横向自旋（T-spin），在纳米光子学和量子信息处理中表现出前所未有的应用。通过引入共振模式作为耦合界面，这种手性耦合可以在单向性、效率和自发发射率方面大大提高。然而，这种间接手性耦合在灵活性和小型化方面仍然受到限制，并且潜在的物理机制有待阐明。在这里，我们提出了一种直观而严谨的方法来分析直接/间接手性耦合，并由此推导出手性耦合方向性与场或发射器的 T 自旋之间的一些一般关系。基于这些理论，我们提出了一种在表面等离子体激元（SPP）平台上的间接手性耦合系统，其纳米腔支持双 SPP 模式的法布里-珀罗（FP）共振作为一种新型耦合界面。FP 共振提供了灵活的设计自由度，可以调节 T 自旋的手性（以及由此产生的手性耦合方向性）以翻转或消失。实现了高达 99.9% 的单向性以及高耦合效率和自发发射率的提高。建立了两个基于第一性原理的互反问题和原问题的 SPP 模型，以验证 FP 共振在实现手性耦合中的决定性作用。