Ultra-high-<i>Q</i> and ultra-high-figure of merit sensor based on accidental quasi-bound states in the continuum at normal incidence in the visible range

Accidental quasi-bound states in the continuum (A-QBICs) designed for normal incidence in the visible spectrum have been utilized for refractive index (RI) sensing, for both transverse electric (TE) and transverse magnetic (TM) polarizations of light. The A-QBICs are supported on a simple-to-fabricate, optimized one-dimensional (1-D) symmetric silicon oxynitride (SiON) metagrating on a silica (SiO 2 ) substrate. Simulations show extraordinary sensing performance with ultra-narrow full width at half-maximum ( FWHM ) of 3 pm (TE) and 15.7 pm (TM), ultra-high figures of merit ( FoM s) of 1.59×10 4 RIU −1 (TE) and 1.14×10 3 RIU −1 (TM), and Q -factors ( Q ) of 1.7×10 5 (TE) and 3.67×10 4 (TM), bulk sensitivities ( S b ) of 47.69 nm/RIU (TE) and 17.91 nm/RIU (TM), and surface sensitivities ( S s ) of 0.428 nm/nm (TE) and 0.114 nm/nm (TM). The exceptional performance is attributed to the enhanced near-field localization at A-QBIC resonances, driven by toroidal dipole and magnetic quadrupole moments. Moreover, this sensor maintains ultra-high Q across varied SiON stoichiometries and substrates, highlighting its experimental feasibility. The presence of multiple A-QBICs expands its application potential for multiplexed-multi-analyte sensing for advancements in environmental monitoring, biomedical diagnostics, etc.