Phase-Controlled Fano Resonances in Hybrid Metasurfaces

Quasi-bound states in the continuum (qBIC) offer high-quality factors resonances with effective far field coupling, making them ideal for applications in lasers, sensors, and nonlinear applications. We report a hybrid dielectric-metallic system that couples a qBIC with a Fabry-Pérot cavity mode, achieving tunable high-Q resonances through Fano interference. Using coupled-mode theory, we analytically establish the phase-dependent interaction between the discrete qBIC resonance and the continuous cavity background, revealing controllable Q factor, Fano asymmetry inversion, and phase singularity behavior. Coupled-mode theory further predicts that Q factor and Fano parameter are tunable by cavity thickness, scaling ratio of the qBIC metasurface, asymmetry, and effective refractive index. Full-wave simulations confirm the coupled-mode theory prediction, showing a maximum Q factor of 2.9 × 10⁴ near the phase singularity point and revealing a rich resonance behavior through such tuning. Experimental realization in the near-infrared confirms the existence of phase singularity points and reveals a refractive index sensitivity of 732 nm·RIU^-1 in mixed chlorobenzene-methylaniline solutions. This work establishes a phase-tunable hybrid photonic platform for strong light-matter interaction and high-performance optical sensing.