ABSTRACT Bound states in the continuum (BICs) enable ultra‐narrow resonances with strong field confinement, critical for high‐performance sensing. In plasmonic systems, quasi‐BICs are limited by nonradiative losses, making the balance between radiative and absorptive decay crucial. Here, we theoretically investigate substrate‐induced hybridization in silver‐based plasmonic quasi‐BIC metasurfaces, revealing a thickness‐dependent phase diagram that governs refractometric sensitivity. The metasurface supports a quasi‐BIC induced by diagonal symmetry , serving as a reference to extract the intrinsic nonradiative loss at critical coupling. By varying film thickness relative to the silver skin depth ( nm), we identify three regimes: purely plasmonic (), substrate‐modified (), and strongly hybridized (), producing characteristic transitions in refractometric response from linear to exponential and back. Crucially, a dual‐symmetry control — in‐plane mirror symmetry , tuning nonradiative loss and diagonal inversion symmetry tuning radiative decay — enables systematic exploration of the coupling landscape, maximizing field confinement and sensing performance. The optimized near‐infrared metasurface hosting a nonradiative‐matched quasi‐BIC at normal incidence achieves bulk sensitivity nm/RIU (increasing to 360 nm/RIU in the visible range), surface sensitivity nm/nm (increasing to 1.58 nm/nm in the visible range), quality factor and an unprecedented figure of merit FOM = 378 in a purely plasmonic metasurface. This dual‐symmetry framework establishes a robust strategy for designing plasmonic quasi‐BICs with tunable radiative and absorptive losses for high‐sensitivity refractometric sensors.
Singh et al. (Wed,) studied this question.