Directional Flow of Confined Polaritons in CrSBr

Abstract: 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, we demonstrate 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 tens of microns along the crystal $a$-axis, while simultaneously inducing strong one-dimensional confinement along the orthogonal $b$-axis. By embedding CrSBr flakes in a microcavity, we further enhance the confinement, 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. Our results establish CrSBr as a versatile polaritonic platform for integrated optoelectronic device applications, including energy-efficient optical modulators and switches.