Plasmonic Spin Meron Lattices with Height-Sensitive Topology Evolution

TL;DR

This work addresses height-dependent topology of plasmonic spin textures above a finite metallic square under circular polarization. It decomposes the scattered field into evanescent SPP and propagating diffracted components, enabling a height-resolved transition from a Néel-type meron lattice near the interface to a Bloch-type lattice in the far field, with a rapid intermediate crossover. Defect nucleation in the in-plane spin phase drives fractional, height-dependent site charges, and a combined SPP-superposition plus Stratton-Chu model plus FDTD validation establishes the mechanism. The results offer a geometry-driven route to engineer and control topological spin textures in plasmonic systems, with potential implications for near-to-far-field spin photonics.

Abstract

We demonstrate height-controlled topological switching of plasmonic spin meron lattices above a metallic square coupling structure under circularly polarized illumination. Near the interface, an evanescent surface plasmon polariton (SPP) channel yields a Néel-type meron lattice with $\pm\frac{1}{2}$ like effective site charges. At larger heights, diffracted fields from the square edges dominate and convert the lattice into a Bloch-type configuration. Over a range of intermediate heights, crossover between the evanescent SPP and edge diffraction gives rise to rich rapid topology evolutions. The switching is accompanied by nucleation of off-boundary vortex-anti vortex pairs in the in-plane spin phase, producing height-dependent fractional site charges. Our findings are analytically formulated by linear superposition of SPPs in the plasmonic regime and Stratton-Chu model in diffraction regime and confirmed via full-wave finite-difference time-domain simulations.

Figures (4)

• Figure 1: Schematic of height-dependent topological switching of plasmonic spin meron lattices. Circularly polarized illumination of a square slit in an Ag film excites SPPs and generates a diffracted field. Spin textures are evaluated on monitor planes at increasing heights above the metal-dielectric interface. In the near field, the SPP-dominant regime corresponds to a Néel-type meron lattice associated with SPP standing-wave interference and spin-momentum locking. At intermediate heights, coexistence of residual SPPs and diffracted fields yields a twisted meron lattice. In the far field, where diffraction dominates, the spin texture evolves into a Bloch-type meron lattice, indicating height-controlled topological switching.
• Figure 2: Height-dependent evolution of spin structure in the $XY$ plane. (a-c) Normalized out-of-plane spin component $S_z$ (color scale) with overlaid in-plane spin vectors $(S_x,S_y)$, evaluated at three monitor heights above the Ag surface: (a) $h=100\,\mathrm{nm}$ (SPP-dominant), (b) $h=2\lambda$ (intermediate), and (c) $h=10\lambda$ (diffraction-dominant). (d-f) Line profiles along $y=0$ quantify the redistribution of spin components with height. The upper panels compare $S_z$ and the in-plane spin magnitude $|S_{\perp}|=\sqrt{S_x^2+S_y^2}$, while the lower panels show $S_x$ and $S_y$, capturing the progression from Néel-like to Bloch-like in-plane rotation as the observation height increases.
• Figure 3: Defect nucleation in the in-plane spin phase across the crossover height. (a-c) Spin-dominance indicator $\chi$ in the $XY$ plane, with $\chi\approx 0$ contours delineating the evolving lattice boundaries at (a) $h=100\,\mathrm{nm}$, (b) $h=3\lambda$, and (c) $h=10\lambda$. (d-f) In-plane spin phase $\psi=\arg(S_x+iS_y)$ at the same heights with overlaid in-plane Poynting vector, showing an ordered phase-singularity lattice in the near field (d), the emergence of off-boundary vortex-antivortex pairs in the intermediate regime (f), and the reconfigured singularity distribution in the diffraction-dominant regime (e). Insets label clockwise (red) and counterclockwise (blue) winding around representative singularities. (g) $\psi(x,z)$ map at $y=0$ showing the onset of an additional singularity within the crossover height range and its persistence for larger $z$.
• Figure 4: Height-controlled topological switching of the meron lattice. (a,b) Skyrmion-density maps in the $XY$ plane at representative heights in the SPP-dominant and diffraction-dominant regimes. (c) Total skyrmion number $N_{\mathrm{Sk}}$ versus height $h$ evaluated for a square unit cell containing $2\times 2$ sites and for a diamond unit cell containing two sites. (d) Site-resolved $N_{\mathrm{Sk}}$ versus height for a central site and its axial-adjacent site, which carry equal-magnitude and opposite-sign values in the SPP-dominant regime and undergo switching across the crossover height range.