Ultra-narrow dual-band notch filter based on merging bound states in the continuum

Bound states in the continuum (BIC) transforms into quasi-BIC in symmetry-broken metasuface structure, which will result in ultra-high quality (Q) factors and has promising applications in sensors and filters. Here we propose an the all-dielectric single-layer metasurface (SLM) and double-layer metasurface (DLM) consisting of amorphous silicon (a-Si) nanorods. The results show that the SLM structure exhibits symmetry-protected BIC (SP-BIC), while the DLM structure reveals not only SP-BIC, but also merging BIC from combination of multiple Fabry-Pérot BICs (FP-BICs). Two types of BIC of the DLM structure could be used to design ultra-narrow dual-band notch filter with the full width at half maximum (FWHM) of 0.02 nm and 0.12 nm. The generation of the merging BIC can be controlled by adjusting the distance between both layers. Breaking through limitation that the ultra-high Q resonances need an extremely small perturbation, the ultra-high Q with the magnitude of 106, which can't be achieved in the SLM structure, can could be obtained from the transformation of the merging BIC even at large perturbations. Additionally, the sensing properties of the proposed dual-band notch filter are investigated is used as a refractive index sensor with good linear performance. The sensor exhibits high spectral sensitivity (Sλ) of 871.8 nm/RIU for the mode Ⅰ and 822.9 nm/RIU for the mode Ⅱ. The maximal figure-of-merit (FOM) of mode1 and mode Ⅱ is 119179.3 and 4145.6, respectively.