Geometry-Driven Lattice of Photonic Spin-Meron Tubes in Free Space

We theoretically demonstrate the first photonic spin-meron tube lattice in free space using spin-angular momentum vectors. Square-block diffraction creates $C_{4}$-symmetric beams with $π/2$ phase steps. Non-paraxial spin-orbit coupling then forms finite-length meron tubes ($N_{\mathrm{sk}} \approx \pm 1/2$, $> 25,λ$). Extending the formalism, we show that $C_{3}$ geometry of a triangular block yields a spin-skyrmion tube. Stratton-Chu theory and full-vectorial finite-difference time-domain calculations both support this material-agnostic, geometry-driven approach as a platform to explore symmetry-driven free-space topology.