The Efficient Generation Of Plasmonic Vortex Field And Its Manipulation Based On Artificial Microstructure

Light beams carrying orbital angular momentum(OAM)in the form of optical vortices have attracted great interest due to their capability of providing a new dimension and approach to manipulate light-matter interactions.However,due to the vigorous development of optical communication,optical micro-manipulation and other research fields,the requirement for miniaturization of devices is becoming more and more important.Usually,the traditional methods for generating vortex beams require complex optical paths or large-scale optical devices,which are not suitable for the integration of optical devices.-Due to the subwavelength light field manipulation and near-field enhancement effect,surface plasmons is considered as one of best ways to manipulate light field at nanoscale.Thus,how to manipulate the topological properties of localized optical field by using the sub-wavelength characteristics of surface plasmons has attracted much attention.However,considering the dispersion of materials,it is still a big challenge to realize plasmonic vortices at low-frequency regions(far-infrared terahertz or microwave).Recently,it has been found that the precise design of metal artificial microstructures can effectively control the propagating or localized modes of low-frequency electromagnetic wave,which provides a potential way to realize plasmonic near-field optical vortices.This paper focuses on how to effectively excite and manipulate plasmonic vortices working at low frequency.It mainly includes the following two aspects:1.We realize the efficient generation of deep-subwavelength plasmonic near-field optical vortices,using the asymmetrical spatial field provided by spoof surface plasmon waveguide to excite the metallic meta-particle in its vicinity.Firstly,we design and propose a meta-particle,which can support two degenerate localized vortex modes rotating in the opposite directions.The two degenerate vortex modes could be excited simultaneously,and thus show the typical field pattern as a dipole resonance,in which the OAM information carried by vortices cancels each other.In order to destroy the degenerate state,we put the meta-particle close to a comb-shaped waveguide.Due to the asymmetric field provided by the comb-shaped stripe waveguide,we can excite the single vortex mode supported by the meta-particle.Next,we propose an equivalent physical model is to explain the generation mechanism of plasmonic vortices,which considers the coupling parameters between the waveguide and the meta-particle,and the radiation loss of the meta-particle.The experimental results show in good match with electromagnetic simulation and theoretical model.It's worthwhile to note that the equivalent radius of plasmonic vortex equals to 1/16 of the corresponding free space wavelength.Our work breaks the high frequency limitation of plasmonic vortices,which could be very useful for integrated compact elements and devices operating at low-frequency regions.2.We realize the efficient generation of localized near-field optical vortices carrying multiple topological charges,designing and preparing a new kind of meta-particle supporting multiple localized resonance modes and using the asymmetrical excitation field.Considering the equivalent dispersion,the coupling strength and the radiation loss in spoof plasmon system,we propose a meta-particle supporting multiple localized resonance modes.Using the asymmetrical excitation field provided by plasmonic waveguide,we obtain localized near-field optical vortices carrying multiple topological charges.The microwave experiments match the simulation results and theoretical analyses well.Our work provides an effective way to obtain multiple OAM of plasmonic vortices working at low frequencies,which may find potential applications in integrated devices for information encoding or signal processing in the future.