Tunable Bound States in the Continuum in Toroidal Metasurfaces

The properties of toroidal metasurfaces such as ultrahigh-Q bound states in the continuum (BIC) resonances and nonradiating anapole modes, have attracted significant attention. High-Q resonances are of particular interest for creating sensors, detectors and tunable structures. In this work, we focus on tunable toroidal metasurfaces that utilise vanadium dioxide, a widely used phase change material for active photonics and room-temperature bolometric detectors. The phase change transition of vanadium dioxide occurs in a narrow temperature range providing a large variation in material resistivity. Through heating thin film patches of vanadium dioxide integrated into a metasurface comprising gold split-ring resonators on a sapphire substrate, we achieve remarkable control over the amplitude and frequency of toroidal dipole BIC resonances due to their high sensitivity to losses present in the system. Breaking the symmetry of meta-atoms reveals enhanced tunability. The predicted maximum change in the amplitude of toroidal dipole BIC resonances reaches 14 dB with a temperature variation of approximately 10°C. The proposed tunable metasurface holds promise for various applications, including active photonic systems and room temperature bolometers.