Surface plasmon polariton (SPP) is an electromagnetic oscillation which is produced by the interaction of free electrons on metal surface with incident light field. Using some specific metal metasurface structures, plasmonic vortices can be easily obtained. The SPP can well confine the light field within the interface between metal and medium. The SPP has two excellent characteristics: the ability to break through the diffraction limit and the strong field enhancing effect. Consequently, it has unique advantages in many applications, such as nanoparticle trapping, near-field imaging and sensing.
The key point lies in selecting the incident light and the construction of metasurface structure when we want to produce SPP vortices. Especially the construction of metasurface structure has great influence on the generation of SPP vortices. So far, many structures have been proposed to produce SPP vortices. In 2015, Byoungho Lee research group (Lee S Y, Kim S J, Kwon H, Lee B 2015
IEEE Photonics Technol. Lett.
27705) designed a structure of double-ring distributed nanoslits which can produce high-order plasmonic vortices by circularly polarized light. In addition, the use of Archimedes slit structure to generate optical vortices is currently a more common method. More recently, a novel plasmonic vortex lens was proposed to produce SPP vortices with arbitrary topological charges. The plasmonic vortex lens consists of an array of gold film nanoslits, and the superposing of the SPP field excited by each nanoslit can produce a composite plasmonic vortex. Here, we propose a novel spiral metasurface structure for generating surface plasmonic vortices. Using the combination of theoretical analysis and finite difference time domain (FDTD) simulation, we find that the topological charge quantity of surface plasmon vortices can be changed by adjusting five parameters, namely, the chirality of incident circularly polarized light, the difference in radius between inner and outer nanocavity array, the rotational velocity factor of nanocavity array, the number of segments of spiral, and the pitch of spiral. This metasurface structure has many adjustable parameters and each parameter can influence the final surface plasmon vortices. Therefore, the plasmonic vortices with an arbitrary topological charge quantity can be generated and manipulated simply by using such a metasurface structure. This work can be expected to have a very broad prospect of applications in super resolution microscopy, quantum cryptography, nanoparticle manipulation, optical data storage, and optical communication.
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