Self-referenced spectral-purity sensing enabled by iso-spectral dual Q-factor quasi-BIC DUV absorber

A quasi-bound state in the continuum (BIC) near-perfect absorption meta-surface for the deep ultraviolet (DUV) band, which is composed of a solid substrate fused silica/Al/SiO₂/HfO₂ multilayer structure, is proposed and numerically validated. The device exhibits an iso-spectral dual-Q response where the resonant wavelength remains nearly constant across all polarization angles, while the Q-factor gradually decreases with increasing polarization angle, ranging from ~140 to ~410 in the DUV band. It enables near-perfect absorption with tunable resonant peaks in the 240-280 nm solar-blind region. The underlying leakage channels and quasi-BIC characteristics are revealed by fitting with the temporal coupled-mode theory, combined with multipole decomposition analysis and eigenmode/band structure calculations. By using the iso-spectral dual-Q feature, we further propose a ratio-based health factor that can be used for spectrometer-free spectral purity detection. Fabrication tolerance analysis and empirical scaling formulas for rough design in the solar-blind band are also provided.