Hyperbolic exciton polaritons in a van der Waals magnet

Abstract Exciton polaritons are quasiparticles of photons coupled strongly to bound electron-hole pairs, manifesting as an anti-crossing light dispersion near an exciton resonance. Highly-anisotropic semiconductors with opposite-signed permittivities along different crystal axes are predicted to host exotic modes called hyperbolic exciton polaritons (HEPs), which permit spatial confinement beyond the material light cone and enhanced density of states. Here, we show the first observational evidence of steady-state HEPs using a cryogenic near-field microscope in the van der Waals magnet chromium sulfide bromide. At low temperature, in the magnetically-ordered state, anisotropic exciton resonances sharpen, driving the permittivity negative along one crystal axis and enabling HEP propagation. Further, we demonstrate coupling of HEPs to excitonic sidebands and increasing exciton spectral weight near the magnetic transitions corresponding to exciton wavefunction delocalization. Our findings open new pathways to nanoscale manipulation of excitons and light, including routes to magnetic, nonlocal, and quantum polaritonics in a light-emitting material.