Directional Flow of Confined Polaritons in CrSBr

TL;DR

This work addresses how to achieve directional energy transport and confinement of exciton–polaritons in a compact, on-chip platform. It leverages CrSBr’s intrinsic optical anisotropy, high index, and magneto-exciton coupling to realize long-range propagation along the a-axis and one-dimensional confinement along the b-axis, both in bare flakes and inside a DBR microcavity. Key contributions include demonstration of edge-assisted out-coupling revealing directional polariton transport, observation of discrete confined modes in the cavity geometry, and magnetic-field tunability of both propagation and confinement spectra. These findings establish CrSBr as a versatile, reconfigurable polaritonic platform with potential for integrated optoelectronic devices such as modulators, switches, and non-reciprocal components.

Abstract

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.

Figures (4)

• Figure 1: Polariton propagation in CrSBr. (a) Crystal structure of CrSBr projected onto the a--b plane (top) and b--c plane (bottom). Arrows indicate the spin orientation of each layer, illustrating the AFM ordering between layers. (b) Refractive index ($n$) and extinction coefficient ($k$) of CrSBr, illustrating the optical anisotropy. (c) Microscopic image of a 90 nm thick CrSBr flake. Due to the material’s high anisotropy, the flake is oriented with its long side aligned along the a-axis. (d) Schematic illustration of polariton propagation in the CrSBr and the measurement setup used to observe it. Upper panel shows the CrSBr flake exfoliated onto a SiO2/Si substrate and excited with a 532 nm continuous-wave (CW) laser. Photoluminescence (PL) emission is detected both at the laser spot and at the edge of the flake due to the polariton propagation. Lower panel shows the polariton propagation with varying photonic components. More photon-like polariton branches propagate over longer distances than more exciton-like branches. (e) Real-space map of PL emission collected from the black dashed region in (c). The white dashed line marks the edge of the flake. The brightest spot at the center corresponds to emission from the laser point, while additional bright spots near the edge indicate polariton emission after propagation. (f) PL spectra collected at the laser spot (top) and the edge (bottom) for a 100 nm thick CrSBr flake in the AFM and FM state. $P_1, P_2$, and $P_3$ in the top spectrum are the peaks for the three polariton branches. $P'$ may be attributed to a defect-related state or a surface exciton.
• Figure 2: Polariton propagation length. (a) 3D plot of the PL spectra of CrSBr collected from a fixed edge (aligned along the $b$-axis) while varying the excitation spot along the $a$-axis of the flake. The horizontal axis denotes the distance between the excitation spot and the detection edge. Each spectrum is normalized to the peak intensity at its corresponding excitation position. Inset: Logarithmic decay of the PL intensity $I$ normalized to $I_0$, plotted as a function of energy offset $\Delta E = E_X - E$, where $E_X=1.37$ eV is the exciton energy and $E$ is the polariton energy. The intensity $I$ is taken from different spectra at various distances along a propagation path (indicated by same-colored dashed lines in the 3D plot), and $I_0$ is the intensity at the same $E$ measured at distance $= 0$. (b) Propagation length as a function of $\Delta E$ derived from experimental data. Each data point corresponds to a decay curve of the same color shown in the inset of panel (a). (c) Simulated propagation length as a function of $\Delta E$, showing guided modes obtained from the calculation. The color represents the effective refractive index of each mode, normalized to the refractive index along the b-axis. The blue and green curves correspond to the dispersions of the first two $TE$ modes of a CrSBr slab waveguide. Inset: Simulated electric field distributions of the $TE_0$ and $TE_1$ modes in the $b$--$c$ plane of the waveguide, along with a higher-order guided mode derived from the $TE_0$ family.
• Figure 3: Exciton-polariton confinement in CrSBr cavities. (a) Schematic illustration of the DBR/CrSBr/DBR microcavity, consisting of $\sim$130 nm-thick, 5.0 µ m-width CrSBr flakes encapsulated between two DBR mirrors. (b,c) Experimentally measured momentum-resolved PL spectra of the 5.0 µ m-width CrSBr flake with momentum $k$ aligned along the $k_a$ (b) and $k_b$ (c) directions in Fourier space. Here, $k_0 = {2\pi}/{\lambda_0}$, ${\lambda_0}$ is the wavelength of the uncoupled cavity mode at $k_{a,b} = 0$. (d,e) Experimentally measured spatially resolved PL spectrum along the a-axis (d) and the b-axis (e) of the same 5.0 µ m-wide flake. (f) Spatial distribution of the measured PL intensity along the b-axis of the CrSBr flake at energies of 1.301 eV, 1.303 eV, 1.306 eV, 1.309 eV, 1.314 eV, and 1.319 eV, corresponding to mode number ($m$) from 1 to 6. Each curve corresponds to the dashed lines of the same color in panel (e). (g) Simulated momentum-resolved PL spectrum of the DBR/CrSBr/DBR microcavity with a 5.0 µ m-wide CrSBr flake. Momentum $k$ is aligned along the $k_b$ direction in Fourier space. PL intensities are normalized to the maximum intensity within each respective panel. Insets in panels (a-d) indicate the orientation of the flake with respect to the spectrometer slit.
• Figure 4: Tuning polariton confinement using magnetic field. (a-e) Momentum-resolved PL spectra along the $b$-axis at magnetic fields of 0 T (a), 0.5 T (b), 1.0 T (c), 1.5 T (d), and 2.0 T (e), applied along the c-axis. The exciton-polariton energy exhibits redshifts due to spin-canting transitions induced by the magnetic field. PL intensities are normalized to the maximum intensity within each respective panel.