Half-Integer Topological Charge Polarization of Quasi-Dirac Bound States in the Continuum

The non-trivial polarization topology of bound states in the continuum (BICs) provides new strategies in nanophotonics. The polarization topology depends on the geometric parameters and energy-momentum dispersion of the system and can be engineered to add specific functionalities for light molding. Herein, such a possibility is investigated by studying the topology of the polarization states associated with the optical field radiated by BICs when Dirac-cone-degeneracy is lifted. The opening of a pseudogap in the Dirac cone dispersion of square-lattice dielectric photonic crystal slabs is achieved by tuning the slab thickness. First, the emergence of half-integer topological charges without the requirement of BIC annihilation is theoretically shown, which instead occurs when in-plane inversion symmetry is broken. Then, using spin-to-orbital angular momentum conversion, the theory of half-integer topological charges mediated by BICs is demonstrated and experimentally proved. The same device is able to give rise to vortices with different orbital angular momentum depending on the way it is illuminated, thus improving the potential of optical multiplexing. In addition, the additive character of the topology-induced phase-vortex generation is finally demonstrated for both integer and half-integer charges using also vortex states as input beams, which is of relevance for information delivery.