Manipulation of Surface Plasmon Polariton Vortices via Positive Elliptically Polarized Light and Geometric Phase

The Surface Plasmon Polariton （SPP） vortices confined to a metal-dielectric interface， are characterized by the near-field enhancement and orbital angular momentum. They find broad applications in particle manipulation， super-resolution imaging， biochemical sensing， and so on. Recent advances in Plasmonic Vortex Lenses （PVLs） for SPP vortex generation have progressively simplified the excitation source requirements—evolving from complex vector beams inherently carrying orbital angular momentum to scalar fields employing circularly or linearly polarized light. Although positive elliptically polarized light， as a fundamental polarization state of scalar fields， has been utilized for PVL excitation， the existing schemes necessitate the incorporation of additional transmission phases—achieved through spiral contour designs—to generate arbitrary-order SPP vortices under positive elliptically polarized light. The SPPs generated by the meta-atoms arranged at the end of the spiral have more attenuation because of the more optical paths they have to travel than those generated by the slit pairs at the front， which ultimately leads to an uneven field distribution of the SPP vortex. Furthermore， an issue arises as the size of the PVL increases with the radius of the spiral contour， presenting a challenge for miniaturizing the PVLs. Furthermore， these devices exhibit inherent polarization sensitivity， which substantially restricts the application scenarios of both SPP vortices and PVLs.To address the aforementioned challenges， this work proposes a geometric phase-based PVL. The lens comprises composite meta-atoms arranged along a circular contour， with each meta-atom integrating four rectangular slits. By systematically controlling the rotation angles of slits within meta-atoms， the proposed PVL not only generates SPP vortices sensitive or insensitive to the amplitude ratio and handedness of incident positive elliptically polarized light but also enables flexible manipulation of the Topological Charge （TC） of the SPP vortices. Specifically， the rotation angles of slits are adjusted through four parameters， namely a， b， c， and d， to construct different configurations of PVL1 and PVL2. Under illumination by the positive elliptically polarized light of arbitrary handedness and amplitude ratio， PVL1 generates SPP vortices with a TC of l=1-2a, and the intensity of the vortex field varies with changes in the incident light's handedness and amplitude ratio. When illuminated by circularly polarized light of opposite handedness， PVL2 produces SPP vortices with TCs of lLCP=2c-1 and  lRCP=1-2a. Notably， under the condition a+c-1=0 and b+d=π/4， PVL2 generates SPP vortices with l=1-2a for any positive elliptically polarized incident light， and the vortex field intensity remains invariant to changes in the incident polarization's handedness or amplitude ratio. By modulating the slit rotation parameter a， the TC of SPP vortices generated by both PVL1 and PVL2 can be tuned. To demonstrate the properties of PVL1 and PVL2， eight structures with a of 0.5，1，1.5 and 2 respectively were designed. Finite-difference Time-domain （FDTD） simulations reveal that PVL1-1 and PVL2-1 both generate SPP focal spots with l=0. When a is 1， 1.5 and 2 respectively， both PVL1 and PVL2 produce SPP vortices with TC of -1， -2 and -3 respectively. Crucially， the intensity of the SPP field generated by PVL1 varies with the amplitude ratio and handedness of the incident positive elliptically polarized light， confirming its polarization-sensitive nature. In contrast， the SPP field intensity from PVL2 remains unchanged under identical variations， validating its polarization-insensitive functionality.Leveraging these functionalities， the proposed PVLs can be deployed in optical tweezers， optical field manipulation， and optical switching systems， thereby expanding the application domains and practical scenarios for plasmonic vortex lenses. Beyond this extended applicability， the device architecture offers inherent advantages in simplified fabrication workflows and miniaturization. This stems from its exclusive use of single-shaped， uniformly sized slit structures and the absence of introduced transmission phases. These features collectively facilitate the streamlined design and on-chip integration of advanced photonic devices.