Panoramic Observation of Hyperbolic Exciton Polaritons in CrSBr.

Hyperbolic polaritons exhibit nonclosed hyperbolic isofrequency contours and hold great promise for integrated photonics due to their ultralarge momentum and directional propagation characteristics. However, their direct far-field observation remains challenging due to the large momentum mismatch. Here, we report a far-field, momentum-space imaging technique for hyperbolic exciton polaritons in the van der Waals magnet CrSBr. By employing one-dimensional gold gratings in conjunction with polarization- and angle-resolved reflectance spectroscopy, we achieve panoramic detection of multiple hyperbolic polariton mode families with momentum up to 5.6 times the vacuum wave vector. The experimental dispersion relations and isofrequency contours agree excellently with theoretical calculations, validating the method. Furthermore, leveraging the strong exciton-magnon coupling inherent to CrSBr, we demonstrate active tuning of the energy dispersion of the hyperbolic modes by an external magnetic field. Our Letter provides a comprehensive platform for investigating hyperbolic polaritons and establishes CrSBr hyperbolic polaritons as a magnetically tunable system for tailored light-matter interactions in the near-infrared range.