Negative refraction is one of the core physical foundations for cutting-edge applications such as super-resolution imaging and electromagnetic wave invisibility. Historically, the realization of this phenomenon has primarily relied on plasmon resonance in metallic nanostructures or phonon polaritons in polar crystals. Excitons, as composite quasiparticles of bound electron-hole pairs in semiconductors, dominate light absorption and emission, but their potential for manipulating light propagation—particularly in achieving negative refraction—had long remained experimentally unverified. Theoretical predictions suggested that strong excitonic resonances in two-dimensional materials could form unique "hyperbolic" dispersion, offering a pathway to excitonic negative refraction, yet its experimental realization and control have posed persistent challenges.

    A collaborative team led by Academician Zhang Xiang of The University of Hong Kong, along with Professor Liu Xiaoze of Wuhan University, Associate Researcher Chen Zuxin of South China Normal University, and other co-authors, has achieved a breakthrough in the natural two-dimensional magnetic semiconductor CrSBr. They have experimentally observed, for the first time, magnetic-order-mediated excitonic negative refraction and, based on this principle, successfully developed a miniature, chip-integrable "exciton superlens." This work establishes a new paradigm for manipulating nanoscale light propagation via magnetic order, laying a critical foundation for developing next-generation magneto-optical devices, on-chip super-resolution imaging systems, and light-magnet quantum interfaces. The findings were published in Nature Nanotechnology. The co-first authors of the paper are Research Assistant Professor Ma Jingwen and Postdoctoral Fellow Wang Xiong of The University of Hong Kong, with Academician Zhang Xiang, Professor Liu Xiaoze, and Associate Researcher Chen Zuxin serving as co-corresponding authors. Professors Cui Xiaodong, Yin Xiaobo, and Zhang Shuang from the State Key Laboratory of Optical Quantum Matter at The University of Hong Kong provided important guidance for this work.

    Using the van der Waals layered magnet CrSBr as their platform—a material exhibiting unique intralayer ferromagnetic and interlayer antiferromagnetic order at low temperatures, with strongly anisotropic excitonic resonances tightly coupled to its magnetic order—the research team made a key discovery. They found that in the magnetically ordered phase, the magnetic order significantly enhances the excitonic resonance along specific crystal axes, causing the real part of the dielectric constant along that direction to become negative. This results in a "hyperbolic" optical iso-frequency surface that supports negative refraction. To directly observe this, they integrated CrSBr thin flakes with a precisely designed on-chip nanophotonic circuit. By guiding light to the material boundary via waveguides, they directly captured far-field images of negative refraction, where the outgoing light and incident light lie on opposite sides of the normal. Building on this effect, the team further constructed an "exciton superlens" device. By leveraging the material's wavelength-dependent negative refraction behavior to control the wavefront of incident light, they successfully focused a diverging beam into a diffraction-limited focal spot, achieving micro/nanoscale on-chip optical field manipulation. Notably, this negative refraction and focusing functionality exhibited a distinct "magnetic-control" switching characteristic: when the temperature was raised to transition the material into a paramagnetic phase, the optical function was immediately deactivated. This magnetic-order-dependent control dimension surpasses traditional plasmonic or phonon polariton systems, offering a novel approach for developing dynamically reconfigurable nanophotonic devices.

    This work marks the first realization of magnetic-order-mediated excitonic negative refraction and a superlens in a two-dimensional magnetic semiconductor, representing a deep integration of excitonic physics, magnetism, and nanophotonics. It not only confirms the immense potential of excitons as a new dimension for photon manipulation but also opens a new pathway for dynamically controlling light propagation using magnetic order as an external degree of freedom. This discovery is expected to directly advance the development of compact, tunable magneto-optical modulators, on-chip super-resolution imaging systems, and light-magnet quantum interfaces, providing crucial materials and physical foundations for integrated photonics and quantum information technologies. The work was supported by grants from the Hong Kong Research Grants Council (N_HKU750/22, 17208725), the National Natural Science Foundation of China (62261160386, 62104073), and several other projects. The research team acknowledges valuable discussions with Professor Liu Zhaowei of the University of California, San Diego, Professor Dong Jianwen of Sun Yat-sen University, and Professor Guo Xiangdong of Shanghai Jiao Tong University, as well as technical support in nanofabrication from relevant teams at The University of Hong Kong.

Article Link:
Jingwen Ma, Xiong Wang, Yuanhao Gong, et al. "Excitonic negative refraction mediated by magnetic orders." Nature Nanotechnology (2026).
https://url.scnu.edu.cn/record/view/index.html?key=36ee4ebf917b6e8b6965aec9972c9a49