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.

Optically Addressing Exciton Spin and Pseudospin in Nanomaterials for Spintronics Applications

“…Also, the impact of the magnetic order−disorder transition on excitons is not very well understood. In a recent work, Ruta et al 130 shows that the oscillator strength of the ground state excitons in bulk CrBrS enhances significantly due to magnetic ordering. Their combined theory and experimental work shows that the quantitative nature of the enhancement can be described by QSGW ̂theory where magnetic ordering favors the electron part of the excitonic wave function to delocalize and reduce onsite d-d contributions to the exciton spectral weight, leading to enhancement in excitonic oscillator strength.…”

Planar hyperbolic polaritons in 2D van der Waals materials

“…The first experimental observation of in-plane hyperbolic excitonpolaritons (HEPs) was reported in the vdW semiconductor chromium sulfide bromide (CrSBr) 44 . In CrSBr, in-plane HEPs exhibit wavefronts that develop with oscillatory out-of-plane electric fields propagating through the bulk-like waveguide modes.…”