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

Abstract

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.