Avoided-crossing engineering of quasi-BIC resonances for flat-band metadevices with suppressed angular dispersion

High-Q resonant nanostructures generally exhibit severe angular dispersion, where tiny deviations in incident angle induce resonance-wavelength shifts comparable to the mode linewidth, degrading spectral stability. Here, we propose a metal-dielectric hybrid asymmetric nanoribbon (MHAN) metadevice supporting symmetry-protected bound states in the continuum (SP-BIC) and avoided-crossing flat-band engineering to suppress angular dispersion. Controlled in-plane symmetry breaking converts dark SP-BIC into a high-Q quasi-BIC with finite radiative leakage. Near the avoided-crossing region formed by inter-mode coupling, modal characteristics redistribute markedly; tuning structural and dielectric parameters reshapes hybridization to offset intrinsic guided-mode dispersion, yielding a flat-band resonance with reduced wavelength drift for incidence angles from -12° to 12°. Equipped with a gold back mirror to fully suppress transmission, the flat-band quasi-BIC achieves low-angular-dispersion narrowband infrared absorption. Ethanol-based thermal modulation of the refractive index (RI) further enables dynamic spectral tuning with a thermal spectral sensitivity of 0.131 nm/°C. This work establishes a design scheme for low-angular-dispersion infrared photonic devices, thermally tunable absorbers, and potential high-precision optical sensors.