Directional Flow of Confined Polaritons in CrSBr.

Nanoscale control of energy transport is a central challenge in modern photonics. Utilization of exciton-polaritons-hybrid light-matter quasiparticles-is one viable approach, but it typically demands complex device engineering to enable directional transport. Here, it is demonstrated that the van der Waals magnet CrSBr offers an inherent avenue for steering polariton transport leveraging a unique combination of intrinsic optical anisotropy, high refractive index, and excitons dressed by photons. This combination enables low-loss guided modes that propagate over 50 µm along the crystal a-axis, while simultaneously inducing strong 1D confinement by forbidding transport along the orthogonal b-axis. By embedding CrSBr flakes in a microcavity, the confinement is further enhanced, as evidenced by energy modes that are discretized along the b axis but continuous along the a axis. Moreover, the magneto-exciton coupling characteristic of CrSBr allows unprecedented control over both unidirectional propagation and confinement. The results establish CrSBr as a versatile polaritonic platform for integrated optoelectronic device applications, including energy-efficient optical modulators and switches.