Hyperbolic exciton polaritons in a van der Waals magnet

Francesco L. Ruta, Shuai Zhang, Yinming Shao, Samuel L. Moore, Swagata Acharya, Zhiyuan Sun, Siyuan Qiu, Johannes Geurs, Brian S. Y. Kim, Matthew Fu, Daniel G. Chica, Dimitar Pashov, Xiaodong Xu, Di Xiao, Milan Delor, X-Y. Zhu, Andrew J. Millis, Xavier Roy, James C. Hone, Cory R. Dean, Mikhail I. Katsnelson, Mark van Schilfgaarde and D. N. Basov

Nature Communications 14, 8261 (2023)
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 inside the anti-crossing called hyperbolic exciton polaritons (HEPs), which confine light subdiffractionally with enhanced density of states. Here, we show observational evidence of steady-state HEPs in the van der Waals magnet chromium sulfide bromide (CrSBr) using a cryogenic near-infrared near-field microscope. At low temperatures, in the magnetically-ordered state, anisotropic exciton resonances sharpen, driving the permittivity negative along one crystal axis and enabling HEP propagation. We characterize HEP momentum and losses in CrSBr, also demonstrating coupling to excitonic sidebands and enhancement by magnetic order: which boosts exciton spectral weight via wavefunction delocalization. Our findings open new pathways to nanoscale manipulation of excitons and light, including routes to magnetic, nonlocal, and quantum polaritonics.