Giant-tunable bidirectional Goos–Hänchen shifts via phase change material-based metasurfaces with quasi-bound states in continuum

We propose a novel approach for investigating the tunable Goos–Hänchen (GH) shift via an all-dielectric metasurface that incorporates phase change materials (PCMs). By introducing material asymmetry through the reconfigurable characteristic of PCMs while maintaining fixed geometric parameters, we can achieve tunable dual quasi-bound states in the continuum with ultrahigh quality factors (Q factors). Enabled by such tunable dual modes with significant phase changes, the PCM-based metasurface exhibits giant-tunable bidirectional GH shifts compared to conventional metasurfaces. Notably, the GH shift exhibits multidimensional tunability, including PCM-driven switching (amorphous to crystalline), incident-angle dependence (θ), and wavelength selectivity (λ). The maximum observed shift reaches approximately 10⁴ wavelengths, accompanied by a corresponding Q factor of 10⁷. Our work demonstrates its potential for applications in ultrahigh-precision multifunctional devices, from biosensing to reconfigurable nanophotonic switches.