Study About Optical Metalenses And Their Dispersion

Optical lenses have the characteristics of focusing,imaging,and Fourier transform,and they are now widely used in various optical designs and optical information processing.Simple optical lenses are made of optical materials with different thicknesses,and the transmitted light wavefronts of the lenses are changed by using different light transmission phases in the materials.Due to the effect of material dispersion,simple optical lenses exhibit significant dispersion when focusing and imaging.In order to eliminate the dispersion of the lens,multiple optical elements are combined to design optical system,which will inevitably make the overall structure of the system complicated.This greatly increases the manufacturing cost of the lens.Optical lenses usually do not have a beam splitting function,therefore,a lens can only converge the light beam into a focal point.The characteristics of large size,heavy weight,complex structure and no multiplexing function make traditional lenses difficult to meet the requirements of modern optical technology for small,integrated and multifunctional optical components.Optical metasurface refers to a two-dimensional metamaterial composed of nanostructures with ultra-thin thickness.The nanostructures constituting the metasurface can be various structures such as holes,slits or protrusions.Optical metasurface can regulate the amplitude,phase,polarization,and transmission spectrum of light in the sub-wavelength range through the interaction of nanostructures with light.The advantages of ultra-thin thickness,easy fabrication,easy integration,and global light field control make optical metasurface widely used in abnormal transmission and reflection,wavefront control,polarization conversion,and color filtering.With the deepening of research,many new types of optical metasurface structures including ultra-thin plasmonic wave plates,metalenses,and vortex generators have been proposed.The researches on optical metasurface components and their light field micromanipulation are of great significance to promote the miniaturization of flat optical elements to replace traditional optical elements,and even to promote the development of micro-optics and nanophotonics.This thesis focuses on the focusing and dispersion of lenses,and conducts research on optical metalenses and their dispersion problems based on optical metasurface,with the aim of designing metalenses with reusable functions and controllable dispersion.An anisotropic nanostructure etched on a silver film is used to form a metalens,and the phase and polarization of optical field are controlled by the anisotropic nanostructure to effectively control the optical amplitude,optical phase,optical polarization,and optical spectrum.The innovative work of this paper is reflected in the design of two new metalenses,one is to design a multi-focus combination bifocal metalenses through the comprehensive application of transmission and geometric phase shift,and the other is to optimize the design of the metalenses structure to achieve the controllable focusing of three primary wavelengths,including positive dispersion focusing,negative dispersion focusing,and three-wavelength confocal.The specific content of the paper is arranged as follows:Chapter 1 is the introduction of this thesis.Since metasurfaces are the basis of this article,the introduction first introduces the research progress of metasurfaces,how the metasurfaces regulate light wavefronts,and the different metasurfaces such as polarization conversion devices,vortex generators,metasurface holograms,and metalenses.The advancement of surface function devices are benefit to the extensive application of metasurfaces and the former works show the importance of metasurfaces in light field regulation.The introduction also introduces the numerical simulation methods commonly used in the design of metalenses and the fabrication methods for the manufacture of nano-microstructures.Finally,the content arrangement and innovation of the paper are summarized.Chapter 2 mainly introduces the principle for the focusing of traditional lenses,the dispersion mechanism of traditional lenses and the corresponding solution techniques.The design of metalenses and the focusing principle are analyzed.One is to achieve the focusing of metalenses through the contol of the transmission phase shift.One is the use of geometric phase compensation to achieve the focusing of metalenses.The key of both methods is to construct a converging spherical wavefront,which provides basic ideas and methods for the design of metalenses.Numerical simulations about the focusing effect of two metalensesbased on different design principles are performed and the dispersion effects are shown.The comparison of the dispersion effect of metalenses with the traditional lenses shows the different dispersion mechanisms.Finally,the improvement measures taken to eliminate the dispersion of metalenses are summarized and analyzed.Chapter 3 proposes a composite metalenses composed of nanometer rectangular holes and cross holes etched on the silver film.The metalenses uses both the transmission phase mode and geometric phase mode of the nanostructure to adjust the phase of the light field to achieve double foci.The polarization states of the two focal points show different polarization combinations depending on the polarization state of the incident light.In order to obtain as high intensity and low interference as possible,the cross-hole array working in transmission phase mode is located on the periphery of the metasurface,and the rectangular hole array working in geometric phase mode is located on the inside of the metasurface.Numerical simulation results confirm the focusing performance of the polarization reuse of the metalenses.Our proposed metalenses has the characteristics of compact optical design,bifocal focusing and polarization diversities and the advantages of convenient fabrication and simple experimental operation.Chapter 4 designs metalenses with controllable multi-wavelength focusing based on the metasurface.The metalenses we proposed can effectively control the focusing of three primary wavelengths,including focusing with negative longitudinal dispersion effect,positive longitudinal dispersion effect,lateral dispersion effect,and three-wavelength confocal effect.By rationally optimizing the parameters and spatial arrangement of the nanometer rectangular holes,a metalens corresponding to controllable dispersion is obtained.Rotating the rectangular holes introduces a spherical wave phase shift and the light with different wavelength focuses at a set position.Theoretical analysis shows the controllable focusing effect of metalenses,and numerical calculations and experimental measurement confirm the confocal effect and color separation performance of the focusing metalenses.The advantages of multi-wavelength controllable focusing of our designed metalenses are helpful for expanding the applications in color display and integrated imaging.Chapter 5 gives a summary of the thesis.This chapter summarizes the work content of the paper,highlights the innovations in the research content and points out the working advantages of the designed device,explains the shortcomings of the paper and the difficulties encountered in the work,and describes the future work plan.