Anisotropic Nanoscale Coherent Polariton Transport in CrSBr

In a combined experimental and theoretical study, we demonstrate anisotropic polariton transport on the nanoscale in the van der Waals antiferromagnet CrSBr. While effective cavity-polariton formation emerges via the self-hybridization of ultrahigh oscillator strength excitons with a thin slab photonic mode, the absence of external mirrors facilitates spectroscopic investigation of these polaritons via cathodoluminescence (CL) on length scales inaccessible to optical excitation. This direct access allows us to precisely chart the polariton landscape with nanometric resolution, and probe polariton interference phenomena. Our finding highlights that coherent polariton transport follows the C2v symmetry of CrSBr, highly directional transport along the a-axis, while no coherent features are found along the b-axis. By extending CL spectroscopy to strongly anisotropic cavity polaritons, our work establishes this technique as a powerful tool to probe highly directional expansion and relaxation phenomena on the nanoscale, paving the way for investigating complex polaritonic landscapes in asymmetric systems.