Negative refraction, in which light bends the wrong way at an interface, has generally required metamaterials: artificial structures patterned at subwavelength scale, difficult to fabricate and lossy in the optical range. Work led from HKU, carried out with collaborators at Wuhan University and South China Normal University and with guidance from the State Key Laboratory of Optical Quantum Materials, achieves the effect in a naturally occurring magnetic crystal instead.
The material is chromium sulfur bromide. In a very thin layer, its magnetic ordering changes how the crystal interacts with light, causing light to bend backwards when that order is active. The researchers guided light into the material on a chip and confirmed the backward bending visually, which is a more direct demonstration than the indirect spectroscopic signatures often used to infer negative index behaviour.
Two features make the result practically interesting. The team built a miniature hyperlens that focuses light into extremely small spots, exploiting the same mechanism, and the device can be switched on and off either with magnets or by changing temperature. Reconfigurability of this kind is hard to achieve in a fabricated metamaterial, where the optical response is fixed once the structure is patterned. The study appeared in Nature Nanotechnology.
Suggested applications include ultra-compact lenses, high-resolution microscopy, reconfigurable optical devices, medical imaging and quantum computing, though no numerical performance metrics are reported. The deeper interest for condensed matter physicists is the coupling itself: magnetic order controlling optical refraction in a single material links two domains that device engineering has usually had to bridge with separate components.