Study Of Vector Beam And Near-field Orbital Angular Momentum Superposition Based On Metasurface

Light field manipulation is a hot research topic in optics.Due to the large size and single function of traditional optical components,they cannot meet the development requirements of today's integration.The nanostructures that make up the metasurface have an optical resonance response that excites surface plasmonics field.By changing the size,shape and arrangement of the structural units,the light field manipulation can be realized in the subwavelength.Therefore,metasurfaces are used in metalens,holography,polarization converter,vortex and vector beams generator,etc.Vector beams is a new type of light field with non-uniform distribution of polarization states,which is used in quantum information,tight focusing and laser acceleration.In order to describe the complex polarization states of vector beams,the high-order Poincare(HOP)sphere is proposed.The use of metasurface to generate vector beams is of great significance to promote the development of vector beams to the subwavelength.Beams carrying orbital angular momentum(OAM),called vortex beams,are used in fields such as micromanipulation,microscopy,and optical communications.The composite vortex beam,that is,the superposition of OAM states,has a more complex light field distribution.Manipulating the superposition of OAM states at subwavelengths using plasmonic nanostructures is important for the development of plasmonic devices.On the basis of the existing theory,this dissertation conducts in-depth research on the generation of HOP sphere beams based on metasurface and the arbitrary superposition of plasmonic OAM states with nanostructures.The major work and highlights are as follows:1.A metasurface composed of orthogonal nanoslit pairs arranged on concentric rings is proposed.The metasurface is irradiated by a circularly polarized vortex beam,and HOP sphere beams can be obtained at the focal point.We first simulated the evolution of the vector beams on the equator of HOP sphere by adjusting the initial angle of nanoslits,and verified the result by experiments.And the evolution of the 2-order and 3-order vector beams on a meridional coil of a HOP sphere is obtained by adjusting the distance of nanoslit pairs from simulation and experiment.The metasurface proposed by us is simple in structure,convenient in adjustment,and ingenious in design,which is of great significance for the development of HOP sphere beams to small scales.2.We design a nanostructure that combines nanoslit pairs and segmented spirals,and achieve arbitrary superposition of OAM states by simultaneously manipulating the geometric and dynamic phases.We completed the equal-weight and unequal-weight superposition of two OAM states in simulation and experiment respectively,and realized the evolution of the superposition of OAM states on the entire Bloch sphere.In addition,the superposition of two OAM states with different absolute values of topological charges is also completed.This work opens up a new way for the manipulation of OAM states in near-field,and can promote the application of superposition of OAM states in classical physics and quantum fields,as well as the design of chip integrated systems.