Negative light refraction demonstrated with natural magnetic material
Researchers at the University of Hong Kong demonstrate negative light refraction with the natural magnetic material CrSBr – an alternative to metamaterials.
(Image: University of Hong Kong / Bearbeitung heise medien)
Scientists at the University of Hong Kong (HKU) have demonstrated negative light refraction with a natural magnetic material. The work published in Nature Nanotechnology, titled “Excitonic negative refraction mediated by magnetic orders,” utilizes an alternative approach to the commonly used artificial metamaterials.
Negative Light Refraction in a Thin CrSBr Layer
Normally, when a light beam enters water or glass, it is deflected in a specific direction – an effect observable when looking at a straw in a glass of water. With negative light refraction, the opposite occurs: the light beam bends in the opposite direction, as if defying usual physical rules. The study describes how this phenomenon was observed in an excitonic van der Waals magnet, namely chromium sulfide bromide (CrSBr).
The research team, led by Professor Xiang Zhang, used a very thin layer of the air-stable, two-dimensional magnetic semiconductor. The material undergoes an antiferromagnetic transition at approximately 133 Kelvin (-140 degrees Celsius) and exhibits both air stability and a comparatively high magnetic ordering temperature.
The magnetic structure of the thin CrSBr layer causes magnetic atoms within and between the layers to align differently. This magnetic ordering influences light propagation within the material, enabling negative refraction. The advantage of the thin layer is that it can be integrated onto a chip, allowing for compact optical components.
(Image:Â University of Hong Kong)
Experimental Setup
The Hong Kong researchers developed an excitonic hyperlens on an integrated nanophotonic chip, whose functionality is mediated by the magnetic orders of CrSBr. They claim to have created a “hyperlens” capable of focusing light onto tiny areas – relevant for high-resolution imaging and optical data processing.
A special feature of the system, according to the report, is its controllability: the effect can be switched on and off using external magnetic fields or temperature changes. This distinguishes the approach from conventional metamaterials, whose optical properties are typically fixed. Potential application areas mentioned by the research team include high-resolution medical imaging, optical data processing, compact optical components, and quantum technology.
Theoretical and Experimental Foundations
The theoretical foundations were developed in 1967 by Victor Veselago. The experimental proof was achieved in 2001 using metamaterials, which are artificially manufactured structures. However, these are complex to produce and often exhibit significant energy losses.
In February 2025, researchers from Lancaster University and NTT Basic Research Laboratories demonstrated negative light refraction in atomic media through a numerical simulation using precisely arranged atomic layers in optical lattices. An experimental proof is still pending.
(vza)
