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 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.

CrSBr: An Air-Stable, Two-Dimensional Magnetic Semiconductor

“…Collectively, the mixed-dimensional properties of CrSBr’s crystalline and electronic structure position it as a semiconducting vdW platform for the discovery of new phenomena bridging 2D and 1D physics, as highlighted by the recent observation of hyperbolic exciton-polaritons in this material . CrSBr pairs these semiconducting properties with magnetic properties that are distinct from those of other vdW materials studied to date.…”

“…The discovery of exciton–magnon coupling, for example, has raised further questions about the launching and propagation of magnons in CrSBr, both of which affect the potential use of CrSBr in quantum transduction. Moreover, strong coupling between other quasiparticles in CrSBr has more recently been identified, including spin–phonon, magnon–magnon, , magnon–phonon, exciton–phonon, and exciton–polariton coupling. − , Developing a more complete understanding of the origin of such coupling may reveal many-body physics in CrSBr and will aid in the design of new materials with similar coupling.…”

“…Moreover, strong coupling between other quasiparticles in CrSBr has more recently been identified, including spin−phonon, 35 magnon−magnon, 29,43 magnon− phonon, 44 exciton−phonon, 59 and exciton−polariton coupling. [24][25][26]60 Developing a more complete understanding of the origin of such coupling may reveal many-body physics in CrSBr and will aid in the design of new materials with similar coupling.…”