Dual polarization-independent quasi-BICs enabled by out-of-plane symmetry breaking in nanodisk tetramer metasurface.

Polarization-independent quasi-bound states in the continuum (quasi-BICs) based on asymmetric all-dielectric metasurfaces have attracted significant attention due to their insensitivity to the polarization state of incident light. However, the conventional paradigm of in-plane symmetry breaking overlooks vertical design space, fundamentally limiting integration density and practical deployment in photonic systems. In this Letter, we present dual polarization-independent quasi-BIC resonances in a rotationally symmetric all-dielectric nanodisk tetramer metasurface. By introducing height perturbations in nanodisks along one diagonal, the out-of-plane symmetry breaking excites dual quasi-BIC resonances while enhancing structural compactness. We demonstrate that these resonant modes in the near-infrared exhibit flexible tunability through modulation of the asymmetry parameter. It is also found that the dual quasi-BIC resonances are polarization-independent through transmission spectra analysis. Multipole decomposition and near-field analysis reveal the formation mechanism of the dual quasi-BIC resonances. Additionally, the proposed metasurface exhibits remarkable sensing sensitivity of 311 nm/RIU and a figure of merit (FOM) of 1777.14 RIU^-1. Our work holds significant promise for high-performance label-free sensing applications.