Ultrahigh-Q spin-selective dual quasi-BIC resonances in ultraviolet dielectric metasurfaces

Ultraviolet (UV) dielectric metasurfaces featuring ultrahigh-quality-factor (high-Q) resonances are essential for advanced applications such as biosensing, nonlinear optics, and spectral filtering. However, achieving high-Q resonances in the UV range remains a significant challenge due to inherent material limitations and fabrication complexities. In this study, we introduce a single-layer UV metasurface comprising wide-bandgap silicon nitride (SiN_x) cross-shaped nano-resonators that support spin-selective, dual quasi-bound states in the continuum (q-BICs). By breaking in-plane symmetry, a symmetry-protected mode is split into two distinct resonances—each selectively excited by left- and right-handed circularly polarized light—resulting in near-unity circular dichroism (CD) and Q-factors exceeding 10³. These resonances can be tuned through geometric design, angular rotation, and modulation of the surrounding analyte's refractive index, all while preserving their high-Q characteristics. Notably, angular perturbation enhances the Q-factor from approximately 693 to 1875 by effectively controlling radiation leakage. Furthermore, analyte layers with dielectric constant ranging from 1.00 to 1.20 induce resonance shifts of approximately 10–12 nm, enabling independent tuning of both CD and Q. This fabrication-compatible platform presents a promising pathway toward next-generation UV chiral photonic technologies, including ultrasensitive biosensors, low-threshold lasers, and nonlinear optical devices.