Nonvolatile Spectral Tuning of UGR-like Resonances via Near-Merging C-Point Pairs in a GSST Phase-Change Metasurface

Unidirectional guided resonances (UGRs) provide a compact route to highly directional radiation without metallic reflectors, but their realization usually relies on precise structural tuning and is often restricted to isolated points in momentum or parameter space. Here, we propose a Ge2Sb2Se4Te1 (GSST)-based phase-change metasurface that enables nonvolatile spectral tuning of UGR-like resonances by controlling the momentum-space evolution of paired circular-polarization singularities. Instead of relying on an exact C-point merging condition at a single phase state, the proposed structure controls the approach and departure of a symmetry-related C-point pair relative to the ky=0 line, with the closest sampled approach occurring near an intermediate crystallization state. This near-merging evolution selectively suppresses air-side radiation, giving rise to continuously tunable UGR-like resonances with persistent high directionality. At the high-directionality operating point selected for each of the crystallization states considered, the eigenwavelength shifts by 202.2nm, exceeding three times the largest eigenmode linewidth. Meanwhile, the downward-to-upward radiation asymmetry ratio η consistently exceeds 102 over the investigated phase states and reaches approximately 2.4×104 near the optimal crystallization state. A self-consistent fixed-angle analysis further shows that a fixed substrate-side direction of 16.52∘, equivalent to 22.93∘ in air, retains 119.6≤η≤191.3 throughout all 11 states considered while preserving a 186.6nm tuning range. Despite increased crystalline-state absorption, the intrinsic substrate-side modal branching fraction remains at least 75.7%. These results establish a nonvolatile strategy for combining spectrally resolvable tuning, persistent directional radiation, and substantial dominant-side modal out-coupling in a single phase-change metasurface.