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 LiNbO₃ 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 LiNbO₃ metasurface is accounted for, and the C_4v symmetry is maintained. According to the theoretical derivation, four quasi-BICs are obtained, with the highest quality factor (Q-factor) reaching 2.1×10⁴ and the figure of merit being 5707RIU^-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 LiNbO₃ in advanced integrated nonlinear optics for integrated optics, biosensing, filtering, and lasers.