Tetramer extreme Huygens’ metasurfaces based on quasi-bound states in the continuum and their high-resolution near-infrared refractive index sensing

Traditional Huygens’ metasurfaces achieving background-free full transmission are often limited by radiation leakage, challenging the simultaneous realization of high-quality-factor (Q-factor) and strong light–matter interactions. To overcome this, we propose an all-dielectric metasurface based on a rectangular silicon pillar tetramer, constructing an extreme Huygens’ optical platform by combining the Huygens’ condition with quasi-bound states in the continuum. By tuning geometric parameters, this system achieves strict frequency degeneracy and high-quality-factor matching of orthogonal electric and magnetic quasi-bound modes, alongside their cross-order-of-magnitude modulation. This mechanism generates dispersive phase modulation and extreme slow-light effects while maintaining background-free full transmission. Applying this extreme Huygens’ system to refractive index sensing demonstrates exceptional capabilities relying on the sensitive response of the group index (ng) to environmental changes. This work expands the implementation of the extreme Huygens’ mechanism and provides a solid physical foundation for designing next-generation high-resolution sensors, slow-light dispersion engineering, and nonlinear photonic devices.