Hyperbolic Phonon Polaritons in van der Waals magnet CrSBr

Hyperbolic polaritons represent strong light-matter interactions in extremely anisotropic materials, which have opposite signs of the dielectric functions along orthogonal directions. These hybrid quasiparticles dominate the optical response at the nanoscale, and exhibit various exotic waveguiding properties originating from the hyperbolic frequency surfaces. In particular, hyperbolic exciton polaritons (HEP) have been demonstrated in the van der Waals magnet chromium sulfide bromide (CrSBr) with near-field nano-imaging. The HEPs result from the strong magnetically confined excitons in CrSBr, and therefore only appear at cryogenic temperatures with moderate quality factor. Here, we report the observation of hyperbolic phonon polaritons (HPP) in CrSBr. The hyperbolic phonon polaritons in bulk CrSBr exists at room temperature, with high quality factors that allow real-space propagation visualized using synchrotron infrared nanospectroscopy. These findings highlight the exceptional properties of CrSBr and pave the way for on-chip communication and sensing applications across distinct hyperbolic spectrum windows.