Modifying the Quality Factors of the Bound States in the Continuum in a Dielectric Metasurface by Mode Coupling

As optical resonances with high-quality (Q) factors, bound states in the continuum (BICs) supported in a metamaterial with low optical loss have attracted great interest due to their potential applications in photonic devices with different functionalities. However, most studies focus on the influence of the symmetry breaking in the constituent elements on the Q factors of the quasi-BICs, and less attention is paid to the coupling between the constituent elements. Here, we investigate numerically and experimentally the quasi-BICs revealed in metasurfaces composed of regularly arranged silicon (Si) nanoparticles with and without a geometric symmetry. It is found that the quasi-BICs appear in the transmission spectra of the metasurfaces as Fano lineshapes with different asymmetry parameters, depending strongly on the relative locations of the two optical modes involved in the interference. Apart from symmetry-protected BICs dominated by optical modes that radiate only in the direction parallel to the metasurface, accidental BICs at-Γ formed by the destructive interference of two optical modes are also revealed. The leakage of the quasi-BICs in the direction of light incidence originates from the interference of such modes with other optical modes existing in the same spectral range, which limits the maximum Q factors of the quasi-BICs. Consequently, quasi-BICs with high-Q factors (1010 to 1013) are also observed in metasurfaces composed of Si nanoparticles with geometric symmetry (or without symmetry breaking). More interestingly, the Q factors of the quasi-BICs in a metasurface composed of Si nanoparticles with symmetry breaking exhibit a strong dependence on the period of the metasurface. An exponential decrease of the Q factor is observed when the period of the metasurface deviates from the value at which the maximum Q factor is achieved, implying the crucial role of mode coupling in determining the Q factor. As a result, the Q factor of a quasi-BIC may not decrease exponentially when the symmetry breaking of the Si nanoparticle is increased. In comparison, the quasi-BICs in a metasurface composed of Si nanoparticles with geometric symmetry are not sensitive to the period of the metasurface. By deliberately choosing the period, the maximum Q factors of the quasi-BICs achieved in the metasurface with symmetry breaking are close to those realized in the metasurface without symmetry breaking. Our findings are important for understanding the physical origin of the so-called symmetry-protected BICs supported by metasurfaces and helpful for designing photonic devices based on quasi-BICs.