Engineering circular polarization states via bound states in the continuum on a chiral metasurface

We present a chiral metasurface design that enables momentum-space engineering of circularly polarized (CP) light via bound states in the continuum (BICs). The metasurface consists of rotationally symmetric tri-arm structures arranged in a triangular lattice atop a perovskite thin film, providing a strong chiroptical response with simplified fabrication. By introducing symmetry-breaking in the meta-atom design, we induce transitions from vortex polarization singularities (V points) to separated CP states (C points) in the far field. Additionally, we demonstrate tunable, all-directional separation of C points by rotating the top layer of a bilayer meta-atom. This work offers a compact, flexible platform for manipulating optical chirality and highlights the potential of chiral metasurfaces for integrated chiral photonics and active optoelectronic applications.