At its core, the Tellegen (or magnetoelectric) effect describes a strange situation where electricity and magnetism are permanently linked inside a material — apply an electric field and you also get a magnetic response, and vice versa, even if nothing is changing. In ordinary materials, electricity and magnetism mainly interact when things move or vary in time, like in generators. A Tellegen material, however, would have this connection built in at a fundamental level.
Strong effect with metasurfaces
Now, researchers from the University of Gothenburg, Aalto University in Finland, and Stanford University report what they describe as the first experimental detection of the Tellegen effect for visible light. Instead of relying on naturally occurring materials, the team engineered a metasurface — a carefully designed layer of nanoscopic building blocks known as meta-atoms — achieving an effect about 100 times stronger than in any known natural material
Rather than sending light through a thick sample, the researchers focused on what happens right at the surface, where light first meets the material. There, the Tellegen effect reveals itself most clearly in reflection, producing a distinctive cross-polarized signal that changes depending on the direction of the incoming light. In simple terms, this is not an ordinary mirror — it’s more like a mirror that behaves differently depending on how light approaches it.
“This is about giving light a kind of one-way ‘memory’ at a surface,” says Alexander Friemann Dmitriev, Professor and Head of the Department of Physics. “In ordinary materials, if you reverse the direction of light, the interaction looks essentially the same. In our metasurface, the reflection can change in a way that signals true nonreciprocal behavior.”
Where light meets the material
The team emphasizes that the boundary itself is crucial. “The easiest way to see the Tellegen effect is not by sending light through a thick block,” explains Dr. Ihar Faniayeu. “It shows up most strongly at the boundary—right where light meets the material. That’s why reflection is the key signature.”
In their experiments, the researchers observed a reflected signal known as cross-polarized reflection, indicating that the surface is doing something beyond standard optics. Importantly, the response is nonreciprocal — it depends on the direction of propagation and cannot be fully reversed by simply retracing the same optical path. “If you think of many optical surfaces as mirrors that follow the same rules in both directions,” says Faniayeu, “this is more like a mirror that can behave differently depending on how the light approaches it.”
By moving from bulk materials to a purpose-built, resonant metasurface — and by designing a system that does not require an external magnetic bias field — the researchers have opened a practical route toward bias-free nonreciprocal optical components. Beyond its technological promise, the achievement brings a long-standing theoretical idea into clear experimental view, showing that carefully engineered surfaces can make light behave in ways nature rarely allows.
