Manipulation Of Optical Lattices And Vector Beam Arrays Based On Spin-orbit Interactions Of Metasurface

The exploration and regulation of the novel spatial structure light field is one of the “hot spots” in optics,and it is the key to control the phase and polarization of light in an ideal way.In recent years,optical metasurface has attracted wide attention due to its peculiar electromagnetic characteristics.Metasurface is composed of spatially inhomogeneous anisotropic nanostructures,showing strong spin-orbit interaction effects.By changing the orientation,geometry,and size of the nanostructures in metasurface,the modulation of the excitation,transmission and the spatial distribution of the light field can be realized flexibly.At present,various applications of metasurface have been achieved,such as broadband achromatic metalens,holography,highly efficient polarizers,vortex and vector beam generators,etc.,which are superior to the conventional optical elements.As a special potential field with periodic structured distribution,optical lattice was originally used to trap the ultracold atoms and to manipulate single atoms.With the development of science and technology,optical lattice field has been applied to many research fields,such as photonic crystal lithography,microfluidic sorting,hyperresolution imaging,etc.Combining vortex characteristics and traditional lattice light fields to form a two-dimensional lattice light field with periodic phase singularity has attracted wide attention of researchers.Topologically tunable subwavelength lattice light fields generated by metasurface can greatly simplify the volume of optical systems,and may be applied in classical physics and topological photonics.Vector beam is a new type of structured light field with orbital angular momentum(OAM)while having an anisotropic distribution of polarization.It can be described by Poincaré sphere,higher order Poincaré sphere,hybrid Poincaré sphere,and generalized Poincaré sphere.The array of the vector light field is a number of vector beams arranged in order,which plays an important role in optical trap,optical processing and quantum science due to its periodic spatial distribution and coherence in propagation.At present,the generation of vector beam array mainly depends on conventional optical elements,the periodic vector beams of highorder Poincare sphere generated by metasurface is of great significance to promote the development of vector beam arrays.Based on the control over phase and polarization state by metasurface,the nanoscale lattice field and the array of vector light field on the higher order Poincaré sphere are realized.The lattice field with independent and controllable topologies in two spin channel are obtained,proving that the manipulation of light field based on SOI effect in metasurface enables higher polarization degree of freedom for light manipulation at nanoscale.The content of the thesis is as follows:1.Chapter 1 presents the basic concepts,characteristics and applications of SPPs,the concept of the metasurface,the generalized Snell law based on the metasurface,the classification of phase regulation of the metasurface,and the typical functional devices of the metasurface.It presents the origin and important applications of spin-orbit interaction phenomenon in optics.Then it introduces the description of polarization state and polarization devices;and the basic characteristics of vortex beam,vector beam and the description of vector beam by the higher order Poincaré sphere.Finally,the characteristics and generation methods of the macroscopic lattice light field are introduced,and the study of the subwavelength lattice light field based on the metasurface are summarized.2.Chapter 2 presents metasurface with nano-slit pairs arranged on a segmented spiral to generate circular polarized optical lattice field at subwavelength scale.Firstly,the excitation of a slit pair under the incidence of circularly polarized light is derived.By analogy with multi-beam interference,each pair of nano-slits can be regarded as a halfwave plate rotating arranged on the segmented spiral.The geometric phase and dynamic phase can be adjusted respectively by the rotation of the slit pair and the pitch of the spitral,leading to a phase difference of the excitation from the adjacent two segments of the spiral.Based on the Huygens-Fresnel principle of SPPs,the expression of the optical field near the center region of the metasurface is deduced.With the combination of geometric phase and dynamic phase,we take a theoretical analysis of the regulation of the optical lattice morphology,and then we can obtain hexagonal,hexagonal vortex,kagome and honeycomb optical lattice fields.As for honeycomb lattice,the evolution of optical lattice field is discussed by changing the length of each segment of the segmented spiral.According to the formula,the theoretical results are calculated and compared with the FDTD simulation results.The Mach-Zehnder interference system was built to record the intensity patterns of the four optical lattice fields and the interference intensities with the reference light.Finally,the experimental results are discussed and analyzed to verify the feasibility of generating and regulating subwavelength optical lattice fields using segmented spiral metasurface.3.Chapter 3 proposes that the spin-orbit interaction of light in the metasurface can realize the regulation of the two-channel light fields.Based on the principle of multi-beam interference,two kinds of metasurfaces with circular aperture arranged on circle and spiral line are designed,and the subwavelength optical lattice field with independent morphology of two-polarization-channel is realized for verification.Firstly,the excitation field of a single nano-slit under incidence of circularly polarized light is deduced.Based on the Huygens-Fresnel principle of SPPs,the expression of the superposition of the lattice light field in the space of the two orthogonal spin channels in the center region of the metasurface is deduced.The process of polarization filtering is deduced theoretically,and the expression of lattice light field of two opposite spin channels is finally filtered out in two linear polarization directions perpendicular to each other.The optical lattice morphology of each channel are summarized.FDTD simulation results of metasurface are analyzed and discussed.The Mach-Zehnder interference system and the polarization filter system were built to record the intensity pattern of the two-spin-channel lattice field and the interference patterns with the reference light.The phase distribution of the lattice field in each spin channels was extracted by calculation.Finally,the experimental results are discussed and analyzed to verify the simulation results.4.Chapter 4 presents metasurfaces with nano-slit arranged on concentric arc segments arrays in discrete sector region to realize the focused subwavelength vector optical lattice fields.The nano-slits are arranged within the discrete sector region to imitate multi-beam interference with uniform phase difference between adjacent beams.Two sets of submetasurface are designed for left and right spin circular polarization incident light,and each sub-metasurfae has radial lens phase and azimuthal vortex phase distribution to realize the superposition of two sets of vortex lattice light fields with opposite topological charge and orthogonal circular polarization.Then the initial orientation angle of nanoslits in one set of sub-metasurfae is changed to adjust the phase difference between the two sets of lattice light fields to obtain the array vector light fields of the four Bell states on the equator of the Poincaré sphere,namely the |TM>,|TE>,|HEe> and |HEo> modes.This chapter theoretically introduces the expression of phase distribution of metalens and vortex and the combination of the two phases.Based on the Huygens-Fresnel principle,the expression of two sets of vortex array light fields with orthogonal circular polarization and opposite topological charge is derived,and the expression of the superimposition of the two sets of vortex array light fields is also derived.The FDTD simulation results of the metasurface are analyzed and discussed.The polarization state distribution of the light field is calculated by the simulation data to verify the feasibility of the theory.