Transmissive High-Q Resonance via Quasi-BIC Distributed Bragg Reflector Mode Coupling

Combining a narrow high-Q passband with broadband background suppression in a transmissive planar platform remains challenging. Here, we report and experimentally demonstrate a transmissive metasurface filter combining a grating-supported quasi-bound state in the continuum (q-BIC) resonance with a distributed Bragg reflector (DBR). In-plane symmetry breaking controls the radiative coupling of the grating-supported resonance, while the DBR provides broadband reflection and wavelength-dependent phase feedback. The coherent interaction between the grating-supported q-BIC resonance and DBR-mediated reflection and phase feedback opens a sharp transmission passband within a broad, flat suppression band. Mechanistically, this design effectively suppresses the multiple parasitic interference peaks common in conventional q-BIC devices, thereby realizing spectral filtering with a high signal-to-noise ratio. Numerical simulations predict a Q-factor of 1,937 and a peak transmission of 96%. We designed and fabricated a device operating in the 1550 nm telecommunication band. Experimental results indicate that the filter exhibits excellent angular tolerance, achieving a maximum Q-factor of 721 and a peak transmission of ∼70%. As proof-of-concept applications, the device is used for optical feature extraction and for classical polarization-state preparation and reconstruction, yielding a fidelity of F = 0.964 ± 0.015 between the reconstructed output polarization state and the target state.