Directional Flow of Confined Polaritons in CrSBr

Nanoscale control of energy transport is a central challenge in modern photonics. Utilization of exciton-polaritons hybrid light-matter quasiparticles is one viable approach, but it typically demands complex device engineering to enable directional transport. Here, we demonstrate that the van der Waals magnet CrSBr offers an inherent avenue for steering polariton transport leveraging a unique combination of intrinsic optical anisotropy, high refractive index, and excitons dressed by photons. This combination enables low-loss guided modes that propagate tens of microns along the crystal $a$-axis, while simultaneously inducing strong one-dimensional confinement along the orthogonal $b$-axis. By embedding CrSBr flakes in a microcavity, we further enhance the confinement, as evidenced by energy modes that are discretized along the $b$-axis but continuous along the $a$-axis. Moreover, the magneto-exciton coupling characteristic of CrSBr allows unprecedented control over both unidirectional propagation and confinement. Our results establish CrSBr as a versatile polaritonic platform for integrated optoelectronic device applications, including energy-efficient optical modulators and switches.

💡 Research Summary

This paper investigates the use of the van der Waals antiferromagnet CrSBr as an intrinsic platform for directional exciton‑polariton transport and one‑dimensional confinement. CrSBr possesses a highly anisotropic orthorhombic lattice, resulting in markedly different refractive indices and loss coefficients along the in‑plane a‑ and b‑axes. Two strong excitonic resonances (≈1.37 eV and 1.77 eV) exhibit large oscillator strengths; the lower‑energy exciton shows mixed Frenkel‑Wannier character and is sensitive to the magnetic order, shifting by ~15 meV across the Néel transition (T_N ≈ 132 K). This magnetic tunability provides a direct handle on both the exciton energy and the material’s dielectric tensor.

Bare CrSBr flakes (thickness > 90 nm) deposited on SiO₂/Si act as high‑index waveguides. Photoluminescence (PL) excited with a 532 nm continuous‑wave laser at 4 K reveals strong emission at the excitation spot (polarized along the b‑axis) and, importantly, bright edge emission extending tens of micrometers along the a‑axis. The edge emission is much stronger for edges parallel to the b‑axis, reflecting the intrinsic dipole orientation. By scanning the excitation position along the a‑axis and collecting at a fixed edge, the authors extract distance‑dependent PL decay curves for several polariton branches. The decay length (propagation length) depends on the detuning ΔE = E_X − E between the bare exciton energy (E_X) and the polariton energy (E). Exciton‑like branches (small ΔE) decay rapidly (L_prop ≈ 2 µm), whereas photon‑like branches (large ΔE) propagate up to ~9 µm. Numerical simulations (COMSOL) of the guided‑mode dispersion in finite‑width slabs reproduce TE₀ₙ and TE₁ₙ modes, where n denotes the number of field maxima across the b‑direction. The simulated propagation lengths follow the same trend as the experiment, confirming that the measured L_prop is a weighted average over many supported modes.

To enhance confinement, the authors embed a CrSBr flake between two distributed Bragg reflectors (DBRs) forming a planar microcavity. The cavity increases vertical photon confinement and, together with the natural lateral limitation of the flake width, yields strong quantization of the in‑plane wavevector along the b‑axis while leaving the a‑axis dispersion continuous. Fourier‑plane PL shows a parabolic dispersion along a (continuous k_a) and discrete sub‑branches along b (quantized k_b), directly visualizing one‑dimensional confinement.

A key advantage of CrSBr is its magnetic tunability. Applying an external magnetic field shifts the exciton resonance, which in turn red‑shifts the entire polariton dispersion. This provides a reversible, non‑mechanical method to control both propagation length and confinement, a capability rarely available in other polaritonic platforms.

The paper combines analytical slab‑waveguide theory, full‑wave numerical modeling, and extensive PL imaging to build a coherent picture: (i) intrinsic anisotropy yields low‑loss guided polariton modes preferentially along a; (ii) the high refractive index enables self‑hybridization without external cavities; (iii) embedding in a DBR cavity quantizes the orthogonal direction, creating a 1D polariton waveguide; and (iv) magnetic fields allow dynamic tuning of the polariton bandstructure.

Overall, CrSBr emerges as a compact, broadband, and easily integrable polaritonic material that can support long‑range, low‑loss, direction‑selective transport while offering magnetic control. This makes it a promising candidate for on‑chip optical modulators, switches, and more complex polaritonic circuits without the need for elaborate nanofabrication or metallic waveguides.