Tunable optically anisotropic materials can dynamically and actively modulate the propagation, polarization, reflection, and absorption of light, making them highly promising for miniaturized polarization devices and critically important for next-generation intelligent photonics and optoelectronics. The exploration of 2D van der Waals materials with in-plane anisotropy has greatly expanded the family of optically anisotropic systems and accelerated the development of compact polarization devices. However, the linear dichroism exhibited by these materials has so far shown only limited responsiveness to external stimuli, particularly magnetic fields. As a multifunctional and non-contact means of control, magnetic field offers an attractive route for device regulation and is essential for achieving efficient manipulation in practical photonic applications.
Recently, the research group of Associate Professor Qing Zhang at the School of Materials Science and Engineering, Peking University, developed a quasi-1D microcavity exciton-polariton platform based on the antiferromagnetic van der Waals layered CrSBr. This platform not only realizes giant magneto-optical linear dichroism over a broad temperature range and a wide spectral window, but also enables precise modulation of the far-field distribution of left- and right-circularly polarized light through anisotropy-induced optical spin-orbit coupling within the microcavity. The work, entitled “Cavity-Enhanced Giant Magneto-Optical Linear Dichroism in van der Waals CrSBr,” has been published in the Journal of the American Chemical Society.
As shown in Figure 1a, the anisotropic exciton-photon interaction in CrSBr gives rise to broadband linear dichroism. By measuring the reflection linear dichroism of CrSBr crystals, the researchers observed a broadband response spanning 500–1000 nm. At room temperature, the linear dichroism amplitude reaches 0.8 at 920 nm and exhibits a pronounced cavity-enhancement effect (Figure 1b). In the near-infrared region from 800 to 1000 nm, the room-temperature linear dichroism of CrSBr ranks among the strongest reported for anisotropic van der Waals materials (Figure 1f).
Figure 1. In-plane cavity-enhanced optical linear dichroism in CrSBr. (a) Schematic illustration of the crystal structure of CrSBr, its in-plane magnetic ordering, and the anisotropy of the coupling mechanism. (b) Linear dichroism spectra of an isotropic reference material, CrSBr without cavity modes, and CrSBr with cavity modes. (c) Comparison of the linear dichroism amplitude of CrSBr with those of other representative anisotropic van der Waals materials.
Magnetic-field-dependent linear dichroism spectra further reveal that an external magnetic field can effectively reconstruct the spectral profile and peak evolution of the dichroic response. As the magnetic field approaches the critical point of the antiferromagnetic-to-ferromagnetic transition (0.4 T), the linear dichroism peak exhibits a pronounced redshift (Figures 2a–c). Reflection spectra measured under different magnetic fields with the polarization aligned along the b-axis further show that the exciton-polariton energy corresponding to the dichroism peak also redshifts. This result indicates that the peak shift mainly originates from the strong magnetic-field response of the exciton-polariton distributed along the b-axis (Figures 2d–f). In addition, the linear dichroism amplitude can be tuned over a wide range and even undergoes sign reversal at specific wavelengths, enabling reversible switching from positive to negative values, or vice versa (Figures 2g–i).
Figure 2. Magnetic response of in-plane optical linear dichroism in CrSBr. (a–c) Two-dimensional magnetic-field-dependent maps of the linear dichroism spectra for three CrSBr samples with different thicknesses, demonstrating the magnetic tunability of the dichroism peak position. (d–f) Reflection spectra polarized along the crystallographic a and b axes for the three CrSBr samples under different magnetic fields, showing that the peak shift originates from the magnetic response of the exciton-polariton along the b-axis. (g–i) Magnetic-field-dependent linear dichroism curves, demonstrating wide-range tunability and reversible sign inversion.
Temperature- and thickness-dependent studies further highlight the robustness of CrSBr as a platform for miniaturized polarization devices and its high degree of design flexibility. Temperature-dependent measurements show that CrSBr maintains pronounced linear dichroism over a broad temperature range from 6 to 298 K, with the dichroic response becoming even stronger at low temperatures. Meanwhile, the peak position shows multiple temperature-dependent evolution behaviors, which can be attributed to the combined effects of exciton-magnon coupling, variations in exciton-photon coupling strength, and exciton-phonon interactions at elevated temperatures. From a structural perspective, thickness serves as a key geometric parameter that directly tunes the resonance condition of the microcavity, thereby enabling highly flexible control over both the peak position and the amplitude of the linear dichroism. At room temperature, the cavity-mode resonance energy shifts with thickness and produces resonant enhancement within specific spectral ranges. Under low-temperature strong-coupling conditions, the polariton branches inherit the thickness-dependent resonance-energy shift through their cavity-photon component, thereby driving the synchronous shift of the linear dichroism peak and enabling amplitude modulation.
Finally, taking advantage of the pronounced optical anisotropy of CrSBr, the researchers fabricated a double distributed Bragg reflector microcavity based on CrSBr (Figures 3a, b) to investigate its role in modulating optical spin-orbit coupling. The anisotropy introduced by CrSBr gives rise to transverse-electric (TE) and transverse-magnetic (TM) mode splitting, which acts as an effective in-plane magnetic field in momentum space. This effective field drives the precession of the photonic pseudospin and provides a route for controlling the distribution of left- and right-circularly polarized light. Angle-resolved spectrum of the Stokes parameter S3 shows a clear alternating distribution of left- and right-circular polarization (Figure 3c). By rotating the CrSBr crystal, the far-field angular distribution of left- and right-circularly polarized light rotates correspondingly, a behavior absent in isotropic media (Figures 3d–f). Furthermore, under resonant excitation, the researchers observed a four-lobed separation pattern of left- and right-circularly polarized light in both real space and momentum space (Figures 3g, h), opening up an important pathway for future studies of the optical spin Hall effect and its magnetic-field response.
Figure 3. Modulation of the far-field angular distribution of left- and right-circularly polarized light induced by optical spin-orbit coupling in strongly anisotropic CrSBr. (a) Schematic of the CrSBr double-DBR structure. (b) Scanning electron microscopy image of the double-DBR structure. (c) Angle-resolved spectrum of the Stokes parameter S3 for the CrSBr microcavity. (d–f) Control of the far-field angular distribution of left- and right-circularly polarized light by rotating CrSBr to different orientations. (g, h) Real-space (g) and momentum-space (h) distributions of left- and right-circularly polarized light under resonant excitation.
Author Information and Acknowledgements
Kwok Kwan Tang, a Ph.D. student enrolled in 2025 at the School of Materials Science and Engineering, Peking University, is the first author of the paper. Associate Professor Qing Zhang of the School of Materials Science and Engineering, Peking University, is the sole corresponding author. The study also benefited from the support of Professor Xuekai Ma at Paderborn University, Researcher Xinfeng Liu at the National Center for Nanoscience and Technology, Chinese Academy of Sciences, Researcher Chao Shen at the Institute of Semiconductors, Chinese Academy of Sciences, and Professor Handong Sun at the University of Macau. This work was supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, the Open Research Fund of the State Key Laboratory of Quantum Functional Materials, and the Science and Technology Development Fund from Macao SAR.
DOI: 10.1021/jacs.5c19443
