Near-infrared high-<i>Q</i> all-dielectric metasurface biosensor based on quasi-bound state in continuum

In recent years, bound states in the continuum (BICs) have become a hot research topic because of their strong ability to facilitate light-matter interactions, and they are also an ideal platform for realizing optical resonances with ultra-high quality factors (<i>Q</i>). Nowadays, BICs have been found to exist in various photonic microstructures and nanostructures such as waveguides, gratings, and metasurfaces, among which metasurfaces have attracted much attention due to their ease of adjustment and considerable robustness. Traditional precious metal-based metasurfaces inevitably have low <i>Q</i>-factors due to the inherent defect of high ohmic losses. In contrast, due to lower ohmic losses, all-dielectric metasurfaces can be an excellent alternative to metallic metasurface structures. In this work, an all-dielectric metasurface is designed, with a silicon disc as the unit cell, and symmetric protected BIC (SP-BIC) is observed on the metasurface. When introducing eccentric holes to break the symmetry in the structural plane (QBIC), the SP-BIC can be transformed into a quasi-BIC, with radiation dominated by magnetic dipoles and has a high-quality <i>Q</i>-factor. For QBICs formed on the metasurface, the resonance wavelength is usually greatly dependent on the refractive index of the surroundings due to the strong localization of the electric field within the cell. As the refractive index of the background changes, the positions of the resonance peaks change accordingly, and identification sensing of some biological components is achieved by this principle. This metasurface-based bio-refractive index sensor is less invasive in free space and is expected to overcome the drawbacks of traditional electrochemical-based biosensing technologies, which have cumbersome detection steps and high time and material costs. In terms of sensing parameters, due to the quadratic inverse relationship between the quality factor and asymmetric parameters, by adjusting the asymmetric parameters, the quality factor will also change, thereby enhancing and adjusting the sensing performance. After adjusting, the refractive index sensing sensitivity and figure of merit of this metasurface reach 162.55 nm/RIU and 1711.05 RIU<sup>–1</sup>, respectively, which are higher than those achieved in many other existing studies. This high <i>Q</i>-factor all-dielectric metasurface design provides a new avenue for achieving high-sensitivity and high-precision bio-detection.