Non-spreading meronic spin defects around optical vortices

Optical vortices are singularity lines where the light field intensity vanishes and its phase is undefined. These threads of darkness are adorned by Gauss's law as lines of pure longitudinal polarization where the polarization plane tilts and winds around. The resulting spin field is as a unique structure with both features of topological texture and defect, as it includes a point defect of undefined spin enclosed by a meronic texture which spans half the spin unit sphere. This topological structure does not spread in propagation: the normalized spin field maintains a deep-subwavelength confinement around the vortex line while the underlying vortex beam continues to diffract. From their intrinsic nature and subwavelength confinement, these textures exhibit enhanced robustness upon isotropic perturbations such as atmospheric turbulence compared with other tailored polarization textures. Here we describe these topologies of transverse spin which decorate the phase singularity of paraxial vortex beams, highlighting the diversity of topological structures that arise in two different spaces-the spin unit sphere and the transverse-axial Poincaré sphere-and discuss the underlying aspects behind their subwavelength localization.