Anisotropy-induced band transition and merging of bound states in the continuum in gratings

We propose the grating systems composed of anisotropic material and study the tunability of such anisotropy to the evolutions of bound states in the continuum (BICs) and leaky mode, as well as the band flip between odd- and even-mode. The dependence of band transitions on the permittivity anisotropy is given in the single-layer grating lattice. By further introducing the freedom of interlayer shift, different numbers of band closures (zero, two and four) can be achieved during one shift period in a bilayer grating system. Besides, we also investigate the merging and transition of accidental BICs in a thicker grating structure, and analyze the asymptotic behavior of Q factor in such process. These studies elucidate the important effect of anisotropy on leaky-mode resonance and may supplement the research of band dynamics in isotropic grating systems.