We show that concept of parity-time (PT) symmetry can be expanded to include mixed photon-exciton modes by demonstrating that eigenmodes of active (pumped) strongly coupled cavity polaritons with population inversion exhibit characteristics that are remarkably akin to those of coupled photonic structures with parity-time symmetry. The exceptional point occurs when the Rabi splitting of polariton branches inherent in passive polaritonic systems decreases with increase in pumping, leading to population inversion, and eventually two polaritonic modes merge into a single mode, thus manifesting the frequency pulling effect inherent to all lasers. But, remarkably, this exceptional point occurs below the lasing threshold. Furthermore, unlike most manifestations of PT symmetry in optics, which are observed in the interaction between two analogous photonic modes in waveguides or cavities, in this work the exceptional point is found in interaction between two very dissimilar modes—one photonic and one material excitation (exciton). Aside from fundamentally noteworthy expansion of the concept of PT symmetry to new systems, there is a prospect of using the exceptional point in polaritons for practical applications, such as sensing.

中文翻译：

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极化腔和亚阈值Fabry-Perot激光器中的异常点

我们证明奇偶时间（PT）对称性的概念可以扩展为包括混合的光子-激子模式，这是通过证明具有种群反转的活动（泵浦）强耦合腔极化子的本征模式表现出与耦合光子结构非常相似的特征具有奇偶时间对称性。当无源极化系统中固有的极化子分支的拉比分裂随泵浦的增加而减小时，会导致种群反转，并且最终两个极化模式合并为一个单一模式，从而表现出所有激光器固有的拉频效应，这就是例外点。但是，值得注意的是，该例外点发生在激光阈值以下。此外，与光学中PT对称的大多数表现不同，在波导或空腔中的两个类似光子模式之间的相互作用中观察到了这一点，在这项工作中，发现了两个非常不同的模式（一个光子和一个材料激发（激子））之间的相互作用中的特殊点。除了将PT对称性的概念从根本上引人注目扩展到新系统之外，还有可能将极化子中的特殊点用于实际应用，例如传感。