The impact of light polarization and the nature of modes in the formation of quasi-bound states in the continuum at near-normal incidence

Bound states in the continuum (BIC) is a peculiar resonant mode with an infinite radiative lifetime and quality factor ( Q -factor) embedded within the radiation continuum, which find applications in sensing, lasing, and quantum photonics. While an ideal BIC with an infinite Q -factor can only occur in theory, observing quasi-BIC in realistic samples with finite sizes is possible. However, the robustness of quasi-BIC depends primarily on the trapped electromagnetic modes. Here, we discuss the polarization dependence and the nature of quasi-BIC mode in a two-dimensional array of gallium arsenide (GaAs) scatterers through finite difference time domain simulations and analytical calculations. The calculated angle- and polarization-dependent transmission spectra show quasi-BIC evolution with high Q -factor at near-normal incidence only for transverse magnetic polarization. The calculated total scattering cross-section implies the dominant contribution from electric dipole moments in generating the quasi-BIC. The evolution of quasi-BIC mode is discussed in terms of Mie or Fabry–Perot modes using geometry-dependent transmission and field intensity calculations. The proposed GaAs metasurfaces with quasi-BIC at 638 nm, corresponding to the zero phonon line of nitrogen-vacancy centers in diamond are useful for applications in photonic quantum technologies.