Angle-enhanced phase-change tuning of a guided-mode resonance in a GSST-integrated compound grating metasurface

High-Q resonant metasurfaces are attractive for narrowband spectral control, and integrating phase-change materials enables their resonant features to be tuned after fabrication. Beyond this material-state modulation, the external excitation condition can provide an additional degree of freedom for controlling the observable resonant response. Here, we propose a GSST-integrated one-dimensional compound grating metasurface for programmable guided-mode-resonance (GMR) tuning in the mid-infrared region. The structure consists of two Si/Ge2Sb2Se4Te1 (GSST)/Si multilayer ridges within one supercell of period A. A lateral displacement between the ridges breaks the half-period translational periodicity of the equally spaced reference grating, increasing the primitive period from A/2 to A. This period-doubling perturbation opens radiative coupling to an otherwise dark folded guided branch and forms an observable displacement-activated folded GMR with a deep transmission notch. Full-wave simulations show that the observable amorphous-to-crystalline tuning span increases from 7.13 nm at normal incidence to 26.20 nm at 10° incidence, while strong transmission suppression is maintained. Eigenmode analysis shows that accessing finite-kx operating points shifts the operating window toward shorter wavelengths along a strongly dispersive folded GMR branch and enlarges the separation between GSST states, rather than increasing the modal field concentration inside the GSST layer. Loss-channel analysis further explains the gradual lifting of the transmission minimum at larger incident angles. These results provide a practical strategy for dual-degree-of-freedom programmable spectral control in phase-change GMR metasurfaces.