Generation Of Plasmonic Vortex Fields And Their Manipulations

Optical vortex is a special light that carries orbital angular momentum(OAM)and has a spiral phase distribution.Optical vortex is widely used in optical communication,optical detection,and quantum information research because it greatly increases information capacity by carrying OAM.More interestingly,the optical vortex acts on the particles and can achieve non-contact and high-precision manipulation of the microscopic particles through the transmission of OAM.The experimental generation of optical vortices often requires complex optical path systems,which have poor anti-interference ability and are not conducive to the integration of optical devices.Surface plasmon polaritons(SPPs)break the diffraction limit and manipulate light at nanoscales owing to the specialty of surface confined and near-field enhancement.The plasmonic vortex carries the OAM like the optical vortex and is more advantageous in capturing metal nano-particles.Its sub-wavelength localization is more conducive to the miniaturization of photonic devices.In this paper,we propose a novel plasmonic vortex len(PVL)based on nanoslit-pair structure to generate and manipulate plasmonic vortex by circular or linear polarized light.1.The plasmonic vortices excited by two opposite spins have independent and adjustable topological charge(TC).The phase of SPP excited by nanoslit consists of two components,one is the dynamical phase resulted from the propagation path of SPP,the other is the geometrical phase relying on the orientation angle of nanoslit.In contrast to dynamical phase,the plus or minus of geometrical phase depends on the incident spin.Therefore both phases are also defined as symmetric phase and antisymmetric phase,respectively.In the previous scheme of exciting plasmonic vortex by metal nanostructures,the TC of the plasmonic vortex is limited by the incident spin.In order to achieve the plasmonic vortex with independent and adjustable TC excited by two opposite spins.We propose a novel PVL with an array of orthogonal nanoslit pairs perforated in gold film,and the gold film is evaporated on a SiO2substrate.With the idea of reverse design,the phase of the plasmonic vortex excited by two opposite spin is first preset,and then the propagation phase and geometric phase of the SPP field excited by the nanoslit are obtained.Finally,The spatial position and spatial orientation of the nanoslit pairs are determined,and the structure of the plasmonic vortex lens is determined.All these properties of our PVL are derived theoretically and demonstrated numerically.The simulations results based on the finite element method(FEM)agree well with our theoretical derivations.It is proved that the plasmonic vortice with independent and adjustable TC can be excited by two opposite spins,respectively.2.The composite plasmonic vortex is excited by two opposite spin simultaneously.The plasmonic vortex fields excited by two opposite spin are independent of each other.So,a composite plasmonic vortex because of the coherent superposition of two palsmonic vortex will be excited by two opposite spin simultaneously.A linear polarized light including two opposite spin components can excite two independent plasmonic vortex simultaneously.The composite plasmonic vortex shows a symmetry-breaking field intensity distribution with radial opening appearing in the primary bright ring.Interestingly,the designed PVL can work like a miniaturized analyzer at the special case.3.The plasmonic vortices can be excited at different positions by two opposite spins and have independent and adjustable topological charge(TC).Previous PVL can only excite plasmonic vortex fields at their centers,which limits its further application.The structure of the PVL we proposed is obtained by the idea of reverse design.By adding the position-shift phase to the preset phase,the spatial position and spatial orientation of the nanoslit pairs are changed based on the original structure.It is possible to excite the plasmonic vortex with adjustable TC at any position of PVL and the two spins can respectively excite the plasmonic vortex at different positions,further embodying mutually independent properties.