Understanding the Nature of Polariton‐Like Emission in 2D Materials Coupled to Open Optical Cavities

The interaction of an exciton and cavity can result in the formation of hybrid states called polaritons, which are identified by the observation of a spectral Rabi splitting that represents the polariton eigenenergies. Recent research has, however, shown that spectroscopic measurements of Rabi splitting may not always correspond to the actual splitting energy of polaritonic states. This work reports a unique case where polaritonic behavior is observed without any spectral Rabi splitting. The system under study is an exciton‐cavity consisting of a 2D exciton monolayer (tungsten disulfide, WS 2 ) coupled to an open optical cavity. The exciton‐cavity exhibits polariton‐like dispersive emission observed through back focal plane imaging by varying the coupling strength and cavity losses of the open cavity. A theoretical analysis of the quasi‐normal modes of the system reveals that this behavior occurs in the vicinity of an exceptional point, due to the emergence of polaritonic states. It is shown that the corresponding energy splitting cannot be resolved in the absorption spectrum, placing the system in the so‐called dark‐strong coupling regime, where polaritonic states exist without spectral Rabi splitting. These results provide evidence that polaritonic effects can be experimentally observed when the conventional strong coupling condition is not satisfied.