Topological Polarization Singularity Modulation in the Phase Change Material Metasurfaces

Polarization singularities have garnered significant attention due to their special properties and can be modulated by reducing the symmetry of the structure. At present, the regulation of polarization singularity is usually passive, and the polarization characteristics fixed once the device is fabricated. Herein, we investigate the active regulation of polarization singularity based on phase change material (PCM) Sb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>Se<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> tetramer nanodisk metasurface supported by bound states in the continuum (BICs). When the crystallinity of partial Sb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>Se<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanodisks is changed, the BIC transitions into a quasi-BIC (QBIC), corresponding to the splitting of the integer topological charge into two half-topological charges, which produces a pair of C points that can be mapped onto the Poincaré sphere. It is observed that the C point can be dynamically manipulated to transition from the diagonal to y-axis, then back to the diagonal, and finally merged into linearly polarized vortex with a topological charge of +1, maintaining charge conservation throughout the process. Furthermore, when one or two crystalline nanodisks are transitioned to an amorphous state, C point can be regulated on the diagonal and axis. Our finding proposes a novel approach for the dynamic generation and manipulation of various polarization singularities in momentum space, offering a promising strategy for engineering optical singularities.