Giant Goos-Hänchen shifts with high reflection driven by Fabry-Perot quasibound states in the continuum in double-layer gratings

Herein, we achieve giant Goos-H\"anchen (GH) shifts driven by Fabry-Perot quasibound states in the continuum (quasi-BICs) in double-layer gratings. Based on the hybrid coupling model, the linewidths of two resonant modes in a double-layer grating are determined entirely by the far-field coupling strength. When the vertical distance between two single-layer gratings satisfies the Fabry-Perot resonant condition, the linewidth of one of the resonant modes vanishes, giving rise to a Fabry-Perot BIC. The $Q$ factor of the Fabry-Perot quasi-BIC can be tuned over a wide range by the vertical distance between single-layer two gratings. Driven by the angle-sensitive and ultrastrong resonant properties of the Fabry-Perot quasi-BIC, the GH shift can be significantly enhanced to the order of ${10}^{3}\ensuremath{\lambda}$, together with high reflection. Compared with the reported works on giant GH shifts assisted by momentum-mismatch-driven quasi-BICs in dimerized gratings [Phys. Rev. Appl. 12, 014028 (2019); Phys. Rev. A 104, 023518 (2021)], the proposed scheme greatly reduces the difficulty in experiments. Our work not only provides a feasible recipe to observe quasi-BIC-driven giant GH shifts, but also facilitates the development of high-performance sensors and optical switches.