Traditional plasmonic vortex lenses (PVLs) typically generate plasmonic vortices in a single vortex mode, whereas vortex arrays are sometimes required in practical applications. To overcome this limitation, this study proposes a composite PVL structure design and systematically investigates the characteristics of the surface plasmonic vortex fields generated under circularly polarized light excitation. The research findings hold potential application value in fields such as nanophotonic devices and near-field micro/nanoparticle manipulation.

This study employed the finite-difference time-domain (FDTD) method to conduct systematic numerical simulations and analysis of the proposed composite PVL structure, aiming to investigate the characteristics of the excited optical fields under different incident conditions and structural parameters. The designed composite PVL consisted of left-handed and right-handed Archimedean spiral slits mirror-symmetrically superimposed on the same metal plane. The two sets of slits were identical in geometric parameters and arranged in strict symmetrical distribution (Fig.2a).

Under illumination by left-handed and right-handed circularly polarized light, the optical field excited by the composite PVL exhibited a six-fold symmetric petal-like vortex array at the center, with a vortex of lower energy present at the very center (Fig.2b, Fig.2e). Further analysis revealed that the topological charge sign of the central derived vortex changes with the polarization state of the incident light, while the topological charges of the peripheral ring-shaped vortex array remained unchanged (Fig.2c, Fig.2f). The study also demonstrated that the optical field distribution could be flexibly controlled by adjusting the geometric structure of the composite plasmonic vortex and the incident conditions. When the number of Archimedean spiral segments was 5, 6, and 7, the central region of the optical field exhibited vortex distributions with five-fold, six-fold, and seven-fold symmetry, respectively (Fig.3). When the topological charge of the incident light was −1, the central vortex was completely canceled out. When the topological charge was +1, the topological charge of the central vortex became +2 (Fig.4).Compared to traditional structures, the composite PVL exhibited two major advantages in optical field manipulation. Firstly, it could generate multi-vortex arrays with complex topological charges. Secondly, it reduced the dependence of the orbital angular momentum on the spin of the incident light. Based on theoretical simulations, this study also provided an important reference for future experimental fabrication.

To overcome the limitations of traditional PVLs, which can only excite a single vortex, this study proposes and validates a composite PVL structure. The study shows that this structure can excite multi-symmetry vortex arrays under different conditions, with a derived vortex at the center whose topological charge varies with the polarization state of the incident light. The research findings show broad potential in nanophotonic devices and near-field micro/nanoparticle manipulation.