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

Abstract

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 $\pi /2$ phase steps. Non-paraxial spin-orbit coupling then forms finite-length meron tubes ($N_{\mathrm{sk}}\approx \pm 1/2$, $>25,\lambda$). 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.