Anisotropy-induced evolution of topological half vortices in a dielectric photonic crystal slab

Circular polarization points (C points), which evolve from bound states in the continuum and feature topological half vortices, are important in manipulating chiroptical effects and controlling valley exciton emission in photonic systems. The methods for generating and manipulating C points usually rely on symmetry breaking of structures, but new strategies are required for more refined control. In this work, we investigate the generation and evolution of C points in photonic crystal slabs (PCSs) composed of anisotropic media while keeping the geometrical structure unchanged. By adjusting the optical axis of the anisotropic media, we introduce two degrees of freedom including azimuth and rotation angles, leading to the V point to split into two C points with opposite handedness. Our approach offers flexible control of C points by tuning material properties, presenting new, to the best of our knowledge, opportunities for direction- and spin-dependent optical devices.