A Toroidal-Fano-Resonant Metasurface with Optimal Cross-Polarization Efficiency and Switchable Nonlinearity in the Near-Infrared

The recent progress in plasmonic metasurfaces gives rise to an intense evolution of controlling light properties such as phase, amplitude, polarization, and frequency. In this work, a new paradigm is established to control the light properties centered on low-loss toroidal multipoles with high field enhancement in contrast to most of the previous plasmonic metasurfaces that are optimized through electric and magnetic multipolar resonances. Through a proof-of-concept demonstration, a linear cross-polarization conversion efficiency reaching 22.9%, remarked as the optimal value that can exist in a single-layer plasmonic metasurface in the near-infrared spectrum, is experimentally realized. A polarization-insensitive toroidal response, that previously was accessible only in isotropic high-index metasurfaces, is also observed. Furthermore, a giant anisotropic (polarization-sensitive) generation of the second-harmonic frequency is demonstrated with the proposed polarization-independent toroidal metasurface that provides different levels of electric energy storage within the metasurface. These findings open a new path for keeping low-efficiency plasmonic components on track when one engineers a metasurface based on the toroidal multipole family.