Dynamically tunable robust ultrahigh-Q merging bound states in the continuum in phase-change materials metasurface

Bound states in the continuum (BICs) are localized states within the radiative continuum that exhibit high quality-factor (Q-factor) resonance, which significantly boosts light-matter interactions. However, out-of-plane radiation losses can arise from inherent material absorption and inevitable technological imperfections during fabrication process. Merging BICs have been introduced as a solution to address the issue of out-of-plane radiation losses. By merging BICs, it is possible to expand the area of high Q-factor resonance in momentum space, thereby enhancing the system's robustness against external perturbations. However, achieving this enhancement is contingent upon altering the geometrical parameters of the structure, which inherently restricts its dynamic tunability. Here, we propose an emerging approach that integrates phase change materials (PCMs) into photonic crystal slabs (PCs) metasurface, enabling dynamically tuning of merged BICs. By utilizing low-loss Sb₂S₃ as a tunable PCMs, we demonstrate that altering its phase state can merge BICs, leading to a substantial increase in the high Q-factor across an extended range of wave vectors space. Furthermore, this study validates the universality and robustness of merging BICs against common unit-cell topology fabrication defects. Additionally, by twisting the square holes to break in-plane symmetry, asymmetric merging and inversion of topological charge at the -point are achieved. This approach leverages phase-transition states of PCMs to enable reconfigurable polarization distribution of radiation field without scale and parameter changes, which is tunable and offers promising potential applications in optical vortices and nano-lasers.