Generation Of Vector Optical Fields Based On The Control Of Circularly Polarized Vortex States With Plasmonic Metasurface

The manipulation of light field is the arbitrary operations on the basic properties of light waves,such as amplitude,phase,and polarization,which are one of the important research hotspots in the photonics.Compared with the traditional large-volume refractive optical elements,the emerging and rapidly developing metasurface have attracted great interest in the last decade owing to their extraordinary ability to manipulate light wavefronts with a subwavelength resolution and important application prospects.Metasurfaces are composed of artificial inhomogeneous anisotropic metallic or dielectric nanostructures,and by adjusting size,shape,and orientation of the nanostructures,the light waves can be manipulated in the sub-wavelength scale.The study of metasurfaces provides a new way to explore related physical phenomena and laws.Nowadays,metasurfaces have been successfully engineered into versatile micro-nano photonics devices,such as polarizers,multifunctional metalenses,holography,optical vortex plates,and quantum photon sources.These devices have the advantages of ultra-thin,low loss and easy operation,which promote the miniaturization and integration of optical devices,and show the important application prospects in many fields such as military,aerospace,integrated semiconductor devices,precision manufacturing and so on.Vector optical field is a typical field whose polarization and spatial degrees of freedom are inseparable.Its development opens up a new area for manipulation of light field.The concept of high-order Poincarésphere（HOPs）describes the vector optical field as the linear superposition of two orthogonal circularly polarized vortex states with equal but opposite topological charges,which lays a theoretical foundation for the generation and manipulation of vector optical field.At present,macro-scale vector optical fields have developed broad applications in classical physics and quantum science,including optical tweezers,high-resolution microscopy,laser fabrications of microstructures,and optical communication,etc.The spin-orbit conversion of nanostructures in metasurfaces makes it have outstanding advantages in the manipulation of optical vortices and vector optical fields,and the beam size can be extended to the subwavelength scale.Therefore,the manipulation and superposition of circularly polarized vortex states with metasurfaces to realize the nanoscale vector optical fields has become an important frontier topic,and has a good application prospect in the miniaturization and on-chip integration of optical device such as micro particle manipulation,precision measurement and quantum information.This thesis studies the simultaneous manipulation of the polarization and phase degrees of freedom of the light field based on the plasmonic metasurface of orthogonal nano-slit pairs.By adjusting the geometrical arrangement（e.g.Archimedes spiral,Fresnel zone）of nano-slit pairs,the different circularly polarized vortex states and their superposition are manipulated.Correspondingly,the nanoscale vector beams of Bell like states,focused high-order Poincarésphere beams and vector optical lattice fields are realized.Firstly,a spiral metasurface composed of orthogonal nanoslits with the function of quarter wave plates is designed,and based on the matching of three parameters including the handedness of circular polarization,the rotation order of slit pairs and the spiral order,the nanoscale vector beams（VBs）of Bell-like states are generated;then,a Fresnel zone（FZ）metasurface integrating polarization conversion and focusing functions is designed.It combines the propagation phases introduced by the path increment of FZ from the sequential ring to the focal point and the geometric phase introduced by the rotation order of nano-slits,to cancel the incident spin component（ISC）through destructive interference and to focus the converted spin component（CSC）through constructive interference.By controlling the orientation of the nanoslits and adjusting the elliptical polarization of incident light,the focused high-order Poincarésphere beams are realized;later,a FZ metasurface is designed to construct the coaxial superposition of multiple orbital angular momentum（OAM）states,`Chapter 1 presents the research background of the related fields involved in this thesis.Firstly,the basic concepts,properties and applications of surface plasmon polaritons（SPPs）are briefly introduced.Then the research progress and status of metasurface are reviewed and summarized,and typical functional devices based on metasurface are systematically analyzed.Later,several description methods of polarization state and Jones matric representation of related polarization components are presented.Next,the basic concepts,characteristics,generation methods and research progress of vector optical fields are reviewed.Finally,the research status of optical lattice field at micro-nano scale is summarized.Chapter 2 presents the metasurface with orthogonal nano-slit pairs arranged on spirals to generate VBs of Bell-like states.Using the vector-form Huygens-Fresnel principle,and combining the phase shift of SPP wave field caused by the rotation of the nano-slit pairs during the polarization change process with the spiral order the metasurface,the theoretical expression of wave field distribution in the center region of the spiral is derived.In addition,based on the matching of three parameters including the handedness of circular polarizationσ,the rotation order of nano-slits m and the spiral order n,the equal-weighed linear superposition of two orthogonal circularly polarized vortex eigenmodes is realized,and the influence of the orientation angle of the initial nano-slit pairs on the polarization performance is analyzed.In the simulation calculation,the vector beams of Bell-like states,including radial,azimuthal（|TM>₁,|TE>₁,）andπ-radial,π-azimuthal（|HE^e>₁,HE^o>₁）,are obtained,and the vector beams with slanted polarization states are also created by adjusting the orientation angle of initial nano-slit pair,which is basically consistent with the theoretical analysis.In the experiment,we set up an experimental device to measure the intensity distribution of the wave field,and the experimental results provide a powerful verification for the generation of various polarized vector beams.Chapter 3 presents a novel plasmonic metasurface consisting of nanoslits arranged perpendicularly on the odd and even rings of FZ,denoted as FZ metasurfaces,to realize the generations of focused HOP beams.The metasurface combines the propagation phase introduced by the path increment of FZ from the sequential ring to the focal point and the geometric phase introduced by the rotation order of nano-slits.The combination of the two phases is the key to achieving a focused high-order Poincarébeam.We firstly gave the theoretical analysis of the focused VBs for the FZ metasurface design based on the vector-form Huygens–Fresnel principle and the transmittance property of the nanoslits.When the metasurface was illuminated by circularly polarized light,the propagation phase of FZ causes the ISCs to be eliminated by destructive interference;by contrast,with the combination of the alternate binary geometric and propagation phases,the CSCs are precisely focused by constructive interference.By controlling the orientation of the nanoslits and adjusting the elliptical polarization of incident light,the linear superposition of two CSCs with equal topological charges but opposite signs can be achieved,thus forming the focused HOP beams at the focal plane.Then using finite-difference time domain（FDTD）method,we optimized the metasurfaces and demonstrated the generations of the focused HOP beams;the metasurfaces were designed for wavelength 632.8nm and were also demonstrated to work at wavelengths 532nm and 473nm,respectively.Experimentally,the four samples with different rotation order are were fabricated,and realized the vector beams evolving on the equator and on the prime meridian of the HOP sphere at the wavelength of632.8nm,532nm and 473nm,respectively.The generated HOP beams appeared to be of high quality,and we expect the method proposed in this work will be of significance to related fields in classical and quantum physics.Chapter 4 presents a Fresnel zone（FZ）metasurface with orthogonal nano-slit pairs arranged on concentric rings to generate subwavelength optical lattice fields.The rotation orders m_n of nano-slit pairs arranged on the different rings of FZ are different.Different from the traditional methods that mimicked the multi-beam interference by polygonal slits,segmental structure,or multiple round-apertures,our metasurface manipulates the coaxial/collinear superposition of multiple OAM states by adjusting the rotation orders m_nof nano-slit pairs to essentially generate the optical lattices geometries.Firstly,with the help of the method of M plane wave interference,it is analyzed that the essence of the lattice fields is the superposition of OAM states,and the regulation law of superposition combination of OAM states on the lattice topological states is summarized.By taking theoretical analysis and FDTD simulation,the four forms of circular polarized lattice fields,namely hexagonal,hexagonal vortex,Kagome,and honeycomb lattices are obtained under the illumination of left-circularly polarized light.Further,based on the concept of HOPs proposed by R.R.Alfano et al.,the two orthogonal circularly polarized vortex lattices with the opposite topological charge are manipulated by switching the incident circularly polarized light to linearly polarized light,and the vector lattice fields|TM>₁,|TE>₁,and|HE^e>₁,|HE^o>₁ are generated.Experimentally,the intensity patterns of optical lattice fields are recorded,and the results are analyzed,compared and discussed,which verify the feasibility of metasurface to generate and manipulate subwavelength optical lattice fields.Charpter 5:In this chapter,we summarize the results and the innovations of this thesis and describe the future work.