Polarization-independent and rotationally symmetric LiNbO₃ tetramer metasurface supported by bound states in the continuum

Metasurface transmission, where light of different wavelengths or modes is redistributed and regulated by the metasurface structure, plays a crucial role in modern photonics and nano-optics research. Herein, an in-plane symmetry-breaking approach is designed to excite quadruple transmission dips using the <i>L</i><i>i</i><i>N</i><i>b</i><i>O</i>₃ metasurface based on the leaked plasmonic bound states in the continuum (BIC). The influence of the difference between the major and minor diameters of the ellipse of the <i>L</i><i>i</i><i>N</i><i>b</i><i>O</i>₃ metasurface is accounted for, and the <i>C</i>_4<i>v</i> symmetry is maintained. According to the theoretical derivation, four quasi-BICs are obtained, with the highest quality factor (<i>Q</i>-factor) reaching 2.1×10⁴ and the figure of merit being 5707<i>R</i><i>I</i><i>U</i>^-1. The multi-level decomposition and near-field analysis of the four specific BIC modes indicate that the modes are excited by toroidal dipoles and magnetic quadrupoles rather than any related guided-mode resonances. The results reveal that the four resonances are polarization-independent, and their properties are maintained even for circularly polarized light. The results provide insights into the utilization of <i>L</i><i>i</i><i>N</i><i>b</i><i>O</i>₃ in advanced integrated nonlinear optics for integrated optics, biosensing, filtering, and lasers.