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

Abstract

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.

Polarization-Addressable Optical Movement of Plasmonic Nanoparticles and Hotspot Spin Vortices

“…Thus, the nanoparticles are not only affected by p O but also by p s [14]. The optical forces applied on the nanoparticles are [38] ⟨F⟩ = δV ⟨T⟩ • n(r)dS (7) where the force and the Maxwell tensor (T see Equation ( 2)) are time-averaged and n(r) is the normal along the surface of the nanoparticle and δV is the boundary where the force is evaluated.…”

“…In nanophotonic systems, a variety of extraordinary properties and phenomena, leading to key technological innovations, descend from the coupling of photon spin angular momentum (SAM) with its spatial degrees of freedom [1,2]. Spin-orbit coupling can be observed in basic optical phenomena, such as the light propagation in anisotropic media [3][4][5][6][7], nonparaxial optics [8][9][10], and reflection/transmission at dielectric interfaces, manifesting the spin Hall effect of light [11,12]. It has been theoretically and experimentally demonstrated that electromagnetic waves also reveal a quantum spin Hall effect in analogy with the electronic effect in topological insulators [13].…”