Research On Holographic Display With Metasurface-based Optical Element

As one of the important solutions of next-generation display technology,holographic display can perfectly "restore" the light field of three-dimensional objects(or scenes).The reconstruction result of hologram is usually determined by multiple parameters such as light energy utilization,amplitude and phase modulation efficiency,polarization response,spectral response,diffraction angle and viewing angle.However,as the core device of holographic video display,traditional space Spatial Light Modulator(SLM)can only achieve modulation of a small number of parameters,which cannot meet the demand.Therefore,we need an optical medium that can effectively encode and express light field information.As a kind of artificial two-dimensional material with sub-wavelength scale,metasurface is expected to achieve a breakthrough in the field of holographic materials.It can be used as an ideal hologram coding material to realize the compound modulation of the light field through the sub-wavelength unit structure of different geometric configurations.However,due to the structural characteristics of the micro-nano unit,the metasurface has the disadvantages of complicated processing,high precision requirements,and susceptibility to external environment.When applied to holographic display,it will cause problems such as degradation of display quality,serious interference,and limited display range.This article focuses on the above-mentioned related issues.The main research contents of the thesis include:1.Propose a method to optimize the effect of metasurface holographic display by using blazed grating.Regardless of the geometric phase type or the resonance phase type,the unit structure or modulation method of the metasurface discretization will cause part of the incident light to be emitted without modulation.Because of its high intensity and concentrated energy,observation in a limited dynamic range will cause serious interference.If the post-polarization selection optical path is increased,the field of view angle will be reduced.The blazed grating is a phase modulation device that can adjust the offset of the light field.It can effectively separate the center of the reconstructed light field from the zero-order light,and obtain an interference-free reconstruction area.Based on the design principle of the composite phase element,this paper first expands the phase hologram into a phase distribution with a macropixel structure,and then superimposes the phase of the blazed grating of the same period and encodes to obtain a new metasurface hologram.The simulation reconstruction experiment results show that this method can effectively separate the zero-order light from the reconstructed light field,and at the same time increase the intensity of the reconstructed light field to obtain a clear holographic reconstruction image.2.It is proposed to apply axilens to super-surface holographic display to form a clear holographic reproduced image in the range of focal depth.Unlike the infinite focal depth of an axicon,an axilens can achieve light field focusing within a certain range of the propagation direction,and at the same time has the advantages of a lens and an axicon.When reconstructing a metasurface hologram,a clear holographic reconstructed image can be obtained on the back focal plane of the Fourier lens,but when the receiving surface deviates from the focal plane,the reconstructed holographic image will become a diffuse spot due to defocus.In this thesis,the composite phase-encoding metasurface obtained by the phase superposition of the hologram and the axilens achieves the effect of continuous and clear imaging in the near-field focal depth range.Simulation experiments were carried out based on the FDTD method,and different focal length and focal depth parameters were set.The experimental results show that when the metasurface axilens is applied to holographic display,the hologram can be clearly reconstructed within the focal depth range.This method effectively reduces accuracy requirements for shortdistance near-field reconstruction.