Asymmetric Propagation of Exciton Polaritons in a Wedged Fabry–Pérot Cavity with a Monolayer WS ₂

Exciton polaritons are promising candidates for next-generation photonic devices, and their controllable propagation is crucial for developing functional polaritonic circuits. Here, by controlling a small inclination angle between two distributed Bragg reflectors, we create a spatial gradient in cavity photon energy within a wedge-shaped Fabry–Pérot microcavity, leading to an effective tuning on the polariton potential landscape. Through characterizations of angle-resolved photoluminescence and reflectivity spectra, we unambiguously demonstrate the asymmetric propagation of exciton polaritons of monolayer WS 2 encapsulated in this spatial gradient cavity. This achievement was enabled by our controlled angle-transfer mechanism, which permits precise tuning of the cavity wedge angle resolution down to 0.02°. More importantly, we reveal distinctly different behaviors of polaritons on the upper polariton branch (UPB) and lower polariton branch (LPB). While the UPB polaritons dominantly propagate toward the lower-potential direction, the LPB polaritons display bidirectional propagation features. This behavior is controlled both by the ballistic transport mechanism and the potential landscape. Our findings provide fundamental insights into polariton propagation and enrich design principles for polaritonic devices with engineered directional properties.