Achieving high-Q Friedrich–Wintgen BICs in terahertz metasurfaces: design, sensing, and fast-light enhancement

Abstract High-quality (Q) resonances are critical for enhancing light-matter interactions in micro-nano optical systems, and achieving these resonances is often facilitated by bound states in the continuum (BICs), where energy is trapped within the resonant cavity without leakage. Moreover, quasi-BICs, arising from perturbations to the system, offer a viable alternative with ultra-high but finite Q factors that can be detected and utilized in real-world applications. This study explores the integration of BIC concepts into terahertz (THz) metasurfaces, specifically focusing on the Friedrich–Wintgen BIC (FW-BIC) mechanism, which allows flexible control over Q factors through resonance detuning and coupling manipulation. We design a metallic metasurface composed of a cut wire and coupled split ring resonators, traditionally used to achieve electromagnetically induced transparency, but show that tuning the coupling parameters leads to the realization of FW-BIC modes. The metasurface demonstrates a Q factor approaching 538, with notable performance in environmental refractive index, intensity, and phase sensing. The quasi-BIC peak also exhibits significant dispersion, with a maximum group delay of −302 ps, highlighting the potential for high-sensitivity THz sensors and fast-light devices. Our results open new avenues for the design of metasurfaces with enhanced light-matter interactions in the THz regime.