Experimental Realization of the Optical Tellegen Effect in Nonreciprocal Metasurfaces

Nonreciprocal magnetoelectric coupling, known as the Tellegen effect, promises pioneering potential for applications such as magnet-free isolators and axion-like electrodynamics, yet its realization in optical regimes remains elusive. Here, we present the first experimental demonstration of an optical Tellegen metasurface. Composed of cobalt-silicon nanocones with pronounced shape anisotropy, this metasurface leverages spontaneous magnetization to induce a robust Tellegen response, alongside gyroelectric and gyromagnetic effects, enabling nonreciprocal light reflection. Furthermore, we introduce a novel method to independently quantify the amplitudes of these effects using a standard magneto-optical measurement. Our results show a resonant Tellegen response that is 100 times stronger than that of any known natural material.