| As important properties of the light fields,amplitude,phase and polarization play a crucial role in manipulation of light fields.In recent years,various methods and experimental devices have been designed to flexibly control the properties of the light fields,metasurfaces stand out among many devices due to the advantages of high resolution,miniaturization and feasibility for fabrications,and have rapidly become a hot research area,and have found important applications in a wide range of areas such as metalenses,optical holography,medical detection,and structural color.Meanwhile,metasurfaces also play an important role in generating light fields with singularities such as vortex beams,vector beams,and complex superposed fields,which not only enriches the variety of structured light,but also greatly expands the practical applications of metasurfaces in various fields.Vector beams(VBs)are light fields with spatially inhomogeneous distribution of polarizations.Over the past decade,VBs have attracted significant research interest due to their unique properties,and have found applications in a wide range of fields such as high-resolution microscopy,particle manipulations,quantum walk,and quantum encryption.In recent years,the generation of nanoscale VBs with metasurfaces has achieved rapid advancements.However,the generated VBs have a familiar characteristic that the doughnut size in the intensity profiles is enlarged with the increase of the topological charges,according to research,the doughnuts of different sizes caused difficulties to couple multiple orbital angular momentum beams of different topological charges into a fiber with an annular refractive index profile.Moreover,this characteristic is associated with changes in the radial direction,so the adjustment of the doughnut size is actually the control of the radial mode.Subsequently,the perfect VBs have also been generated by the metasurfaces;though the perfect VBs have found important applications in ultrasecure optical encryption,high-capacity optical communications,and particle trapping,they have a diameter of the doughnut much larger than the tightly focused VBs,which weakened ability for strong trapping and manipulations of particles.Compared to the perfect VBs,tightly-focused beams have supported a series of important phenomena such as spin-orbit interactions,topological structures in light field and super-resolution.However,in generating the tightly-focused VBs with metasurfaces,the doughnuts of intensity profiles for the generated beams are usually dependent on the topological charges,and how to tune the doughnut sizes so as to make them independent of the topological charges still needs to be tackled.Furthermore,though as the superpositions of the circularly polarized vortices,generations of the ordinary VBs are widely studied,the superposition of tightly-focused VBs with intensity profiles-tunable has been rarely investigated.In this paper,the polarization and phase properties of the light fields are modulated based on metasurfaces composed of rectangular nanoslit units,and the superimposed fields of profiletunable tightly-focused VBs and superimposed fields of profile-equal tightly-focused higher-order Poincar(?) VBs are also generated.The metasurface is composed of the nano-slit pairs arranged on two sets of rings,each slit-pair acts as a half wave plate and realizes the chiral conversion of incident circularly polarized light;meanwhile,the circularly polarized vortex beams with helical profiles based on the rotation of each sets of nano-slit pairs are produced,and two Bessel VBs are generated by the diffraction of wavefronts from the two sets of slit-pairs on corresponding rings under illumination of linearly polarized light.In addtion,the focal length is set as f = 5 μm,being of the same order as the radius of the slit-pair ring,and such setting corresponds to a larger numerical aperture of focusing,therefore the two Bessel VBs to be tightly focused.By precisely changing the diameters of two sets of rings,the doughnuts of the two Bessel VBs of different orders are tuned to be of the same size,thus realizing the superposition of two profile-tunable tightly focused VBs.Based on the above work,by adjusting the ellipticity of the illumination light,the metasurfaces can be extended to realize the superposition of profile-tunable higher-order Poincar(?) VBs,with the resultant structured light fields being achieved correspondingly.The main work of this paper is summarized as follows:1.Chapter 1 firstly introduces the research progress on metasurfaces,which mainly includes the basic concept,propagation phase metasurfaces,geometric phase metasurfaces,metasurfaces combined with propagation and geometric phase based on the principle of phase manipulation,and the applications,such as metalens,vortex manipulation,and optical holography.Secondly,we give a brief introduction to the polarization properties of the optical fields.Finally,we give a relevant introduction to the vector beams,such as the basic concepts,properties,the polarization state of vector beam based on higher order Poincar(?) spheres and hybrid order Poincar(?) spheres,and the related progress in generating vector beams with metasurface.2.In Chapter 2,by spatially multiplexing the orthogonal nano-slit pairs arranged on circular rings,we design a metasurface to realize the superposition of profile-tunable tightly-focused VBs and to generate the involved structured light.Each nano-slit pair acts as a half wave plate and realizes the chiral conversion of incident circularly polarized light;meanwhile,two Bessel VBs are formed by the diffraction of wavefronts from the two sets of nano-slit pairs under illumination of linearly polarized light.By finely varying the diameter of the slit-pair rings,two doughnuts of Bessel vector beams are adjusted to the same size,thus the superposition of two Bessel vector beams with equal-profile is realized.To perform the metasurface design and demonstrate the superposition of the VBs,we gave the theoretical analysis of the superimposed fields produced by the metasurface;then we performed the simulations using finite difference in time domain(FDTD)to optimize the designed metasurfaces;finally,we fabricated metasurface samples and the carried out experimental measurements.3.Based on the work of Chapter 2,in Chapter 3,by adjusting the ellipticity of the illumination light in our experiment,the four metasurface samples are extended to realize the superposition of profile-equal higher-order Poincar(?) VBs,with the resultant structured light fields being achieved correspondingly.With the help of the samples in chapter 2,by adjusting the position of incident light on the Poincar(?) sphere,the superposition of left-handed and right-handed circularly polarized light with unequal amplitude is firstly realized and the elliptically polarized incident light is also produced;because the ring diameter of metasurface samples has been modulated,therefore the superposition of profile-equal higher-order Poincar(?) VBs is realized under the illumination of elliptical polarization.In order to prove the feasibility of this method,we give the theoretical analysis,FDTD simulation and experimental measurement,successively. |
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