Fig. 6. Evolution of far-field polarization states in the upward with varying rotation angle αtop of the top hole in the twisted PhCS.
a Polarization ellipses evolve from linear state with αtop = 45°, elliptical state with αtop = 45°~18°, to circular state with αtop = 18°. b The corresponding values of χ show uniform ellipticity in vicinity of each BIC. cThe orientation angle ψ changes from −90° to 90° twice demonstrating BIC topology. d Poincaré sphere illustrates the evolution of polarization states around BIC. At every αtop, the polarization distribution matches a parallel on the sphere, in accord with different χ. Full coverage of Poincaré sphere can be achieved by changing αtop. e The topological charge maintains 1 for vortex with different χ