Research On A Metasurface Micro-optics Device Based On Polarization Conversion

When light is transmitted through one traditional optical element made of natural material,the accumulation effect of dispersion and absorption of light during the propagation can control the polarization,phase,amplitude and spectrum of the light field.Thus,the optical element has some certain specific functions.The dielectric constant and magnetic permeability of natural materials are limited,which needs to utilize the scale change of optical element to control the optical wave front.This requires the traditional optical element with large size.Obviously,it is difficult for traditional optical elements to meet the requirements of modern optical technology for integrated optical elements,and it is also an obstacle to the integration and miniaturization of modern photonic systems.It is of great significance to explore ultra-thin,compact and easily integrated metasurface micro-optics elements to promote the development of integrated optics and photonics.Metasurface are two-dimensional metamaterials with ultra-thin structures that are easy to integrate and flexible to manipulate the light field.Metasurface can precisely control the light field through the interaction between nanostructure and light.The rotation of the anisotropic nanostructure and the change of its shape can regulate the phase,amplitude and polarization of light and optical functional devices such as metalens,beam deflectors and equivalent wave plates are proposed.Functional optical devices based on metasurfaces have potential applications in super-resolution imaging,optical micromanipulation,optical communication and optical integration.Based on the optical metasurface,this thesis designs many micro-optical elements and realizes the effective regulation of the optical wavefront and polarization state by adjusting the parameters of the nano-structure unit.The innovative contents of the thesis include the following aspects.The first one is the design of a kind of bifocal metalenses with controllable intensity ratio based on the wave front regulation of the metasurface.The device can be used in optical communication,particle trapping,optical integration and micromanipulation.The second one isthe design of a metasurface vector light generator of arbitrary order based on the theory of polarization basis transformation.This device can convert linearly and circularly polarized incident light into the vector polarized light and it has potential applications in super-resolution microscopy,particle trapping and micromanipulation and high density optical storage.The third one is the proposition of a single-layer metasurface circular polarizer.This metasurface device consists of rotational symmetric nanostructures,and it has the advantages of ease to manufacture,ultra-thin and compact structure and high circular polarization conversion efficiency.These are advantageous for the applications in optical communication,optical remote sensing and optical display.All the proposed metasurface devices have the characteristics of simplicity,compactness,ultra-thin thickness and flexible control.The content of the thesis is divided into five chapters,which are detailed as follow.The first chapter is the introduction of the paper.The strong regulation ability of metasurface to light field provides a powerful condition for the development of micro-optical devices.The introduction mentions the research background of metasurface,shows the principle of metasurface regulating light field,summarizes the applications of metasurface and mentions the work carried out in this paper.The optical processes such as polarization conversion and phase regulation of metasurface arer described through establishing the relationship of the interaction between the optical field and the metasurface.On this basis,several functional metasurface devices are also introduced.These contents lay a foundation for the following research work of this thesis.In chapter 2,a bifocal metalens with controllable intensity ratio is proposed.This metalens is based on two phase gradient metasurfaces etched on the silver film and two metasurfaces are divided and then recombined.Since the grating which makes up the optical metasurface has a large transmission,the proposed metalens has the high focusing efficiency.The on-axis and off-axis focal spots with high quality are realized by the compound metasurface.Though adjusting the area proportion of two gradient phase metasurface structures in the compound metasurface,the double spots with different intensity ratios are obtained.Numerical simulation and experimental measurement results verify the controllable focusing performance of the proposed metalenses.The proposed bifocal metalens has the advantages of simple and compact structure,high transmission efficiency and controllable intensity distribution,which is very beneficial to promote the flexible applications of multi-focal lens in wave-front shaping,optical imaging,optical integration and optical micromanipulation.The corresponding design method can also be used in the design of focusing vortex metalens with controllable strength.In chapter 3,two kinds of cylindrical vector light generators are proposed using optical metasurfaces.Two vector light generators are composed of nanometer holes etched on the silver film,which can control the polarization state of light at the nanoscale.The first vector light generator works under the illumination of circularly polarized light and the radial and angular vector lights with zero initial phases can be generated.The second kind of vector light generator works under linearly polarized light illumination.It can produce cylindrical vector light of any order and any initial phase.The order of the cylindrical vector light is determined by the rotation of the hole,and the corresponding polarization state changes with the change of incident polarization direction.The numerical simulation results validate the theoretical prediction and they also provide useful parameters for practical metasurface devices.The experimental results verify the optical performance of these two metasurface vector light generators.The advantages of compactness,polarization multiplexing,convenient manufacture and flexible operation can promote the applications of metasurface cylindrical vector light generator in optical integration and optical microcontrol.In chapter 4,a kind of circular polarizer is presented based on metasurface.The metasurface circular polarizer is composed of a single layer of hole arrays etched on the silver film.The metasurface circular polarizer can work under the condition of linearly polarized light and circularly polarized light illumination.Here,the conditions for generating circular polarizer are analyzed on basis of the principle of matrix optics,and the parameters of nanostructures are optimized by using finite-difference time-domain method.The simulation experiment of the optical performance of the designed metasurface circular polarizer fully proves that the proposed circular polarizer has strong circular dichroism.As one simple application,the directional light transmission with controllable polarization is realized.This indicates that the circular dichroicmetasurface can be also used in the design of complex picture.And the wide applications of the proposed design may appear in the fields of optical communication,optical remote sensor and liquid crystal display.The last chapter of this thesis is the conclusions of the above contents.In this chapter,the main viewpoints and the innovations of the thesis are summarized.And the defects and difficulties met during the work are also narrated.The following plane is also introduced in the end.