Quasi-bound states in the continuum in asymmetric hetero-bilayer metasurfaces

Bound states in the continuum (BICs) are completely confined states of open optical structures existing above the light line. However, BICs must be turned into quasi-BICs having a finite Q-factor to be accessible. To achieve quasi-BICs with a high Q-factor at normal incidence, diverse optical structures have been explored by inducing asymmetry by etching a tiny part of the structure. However, it is difficult to introduce a small asymmetry by the etching process for a high Q-factor. Here, we propose novel asymmetric hetero-bilayer metasurface using a low-refractive-index top grating and high-refractive-index bottom grating. The thin upper grating is intended to introduce perturbation accurately, and the bottom grating functions as a broadband reflector. We identified the effect of the geometric perturbation and the top grating material on the Q-factor, resonance wavelength, and background reflectance. As the geometric asymmetry increases, both the Q-factor and the background reflectance decreased, and the resonance wavelength changed accordingly. The Q-factor followed the inverse quadratic of the geometric perturbation and the inverse cube of the relative permittivity. The effect of geometric perturbation decreased up to 10 times with a lower refractive index. Taking advantage of a high-Q hetero-bilayer metasurface, a gas sensor was numerically investigated, and it distinguished a change in the refractive index of 5 × 10⁻⁷. Furthermore, the asymmetric hetero-bilayer metasurface showed a quasi-BIC with a high Q-factor on a rigid substrate.