Nonreciprocal interactions between light and matter lie at the heart of many exotic physical phenomena, from magnet-free optical isolation to axion-inspired electrodynamics. One particularly intriguing example is the Tellegen effect, a nonreciprocal magnetoelectric coupling predicted more than 75 years ago but long considered weak and negligible at optical frequencies. “In natural materials, the optical Tellegen effect is extraordinarily weak, making it challenging to observe experimentally,” the scientists explained.
In a new paper published in eLight , an international team of scientists reports the first experimental realization of a strong optical Tellegen effect using an engineered metasurface. The research team designed a nanostructured surface composed of cobalt–silicon nanocones that exhibit spontaneous magnetization without requiring any external magnetic field. Remarkably, the achieved Tellegen response is about 100 times stronger than that found in any known natural material.
The key to this breakthrough lies in the careful design of the nanoscale building blocks. Each nanocone combines magnetic and dielectric components that resonate with visible and near-infrared light, enabling electric fields to directly induce magnetic responses in a nonreciprocal manner. Because the Tellegen effect primarily manifests at interfaces, the metasurface geometry provides an ideal platform for observing this elusive phenomenon through cross-polarized light reflection.
“Beyond demonstrating the effect itself, we developed a new method that allows us to disentangle the Tellegen response from other magneto-optical effects that occur simultaneously. By measuring light reflected from carefully designed metasurfaces, we were able to independently quantify gyroelectric, gyromagnetic, and Tellegen contributions with high accuracy,’’ the scientists noted.
Looking ahead, the demonstrated metasurface platform is scalable and compatible with large-area nanofabrication techniques. This opens the door to practical, bias-free nonreciprocal optical devices, as well as experimental platforms for studying axion-like physics and other exotic electromagnetic phenomena in the laboratory. The work marks a significant step toward controlling light in fundamentally new ways using engineered materials.
eLight
Realization of the tellegen effect in resonant optical metasurfaces