Terahertz Computer-generated Holography Based On Metasurface

Computer-generated holography technology has the advantages of low noise,simple operation,and the ability to record and reproduce all the information of an object.Terahertz(THz)waves have the characteristics of low photon energy,strong penetration ability,rich spectrum information and large bandwidth,which is widely used in the fields of security check,biomedical detection and wireless communication.Aiming at the leading direction of THz holographic imaging,this thesis proposes a method which uses all-dielectric metasurface to realize the holographic imaging which is expected to solve the problems of inherent ohmic loss and low PCE of metal metasurfaces,while stimulating the Mie type electric resonance and magnetic resonance to achieve wavefront modulation,providing a new way for the development of efficient,lightweight and low-cost THz functional devices.Different methods of phase control are used to design metasurfaces to achieve holographic imaging with different functions.Metasurfaces based on propagation phase control can realize holographic imaging when the linear polarized THz wave is incident.Metasurfaces based on geometric phase control can realize holographic imaging and verify that the obtained image has specific broadband and position multiplexing holographic imaging.On this basis,the all-dielectric metasurface based on the geometric phase and propagation phase control simultaneously is designed to realize circular polarization multiplexing and multi-polarization holographic imaging.The simulated results verify the feasibility of the methods which are proposed in this thesis.The major research works of this thesis can be outlined as follows:1.The research background and theoretical basis of holographic imaging based on metasurface are introduced in detail.It summarizes the development and application of holography,metasurface,metasurface holography and terahertz computer-generated holography based on metasurface.Then it introduces the basic knowledge about the diffraction theory,such as Huygens-Fresnel principle.The scalar diffraction theory has three forms: Kirchhoff principle,the first Rayleigh-Sommerfeld diffraction and the second Rayleigh-Sommerfeld diffraction according to different Green function.The thesis introduces the first Rayleigh-Sommerfeld diffraction and GS algorithm in detail.Combining Rayleigh-Sommerfeld diffraction and GS algorithm with the help of matrix operation in software tool.Then the basic theory of metasurface resonance phase control,propagation phase control theory,geometric phase control theory detailedly.Finally,the main design ideas and the research process of holographic imaging based on metasurface are summarized in detail.2.The element structure is designed and optimized reasonably and the phase control principle of the element structure is verified.All-gold U-type element structure with high reflectivity(≈100 %)and all-silicon rectangular element structure with high transmittance(>70 %)are designed and the element library is established at the same time with the method of finite difference time domain.Considering the convenience of metasurface processing and the application scenarios of transmittancetype holographic imaging,the all-silicon rectangular columns which have the higher polarization conversion efficiency are selected as the metasurface elements.Meanwhile,the propagation phase and geometric phase control theory of the selected element structure is verified.At the same time,the element structure based on geometric phase control was designed and optimized,The polarization conversion efficiency(PCE)changes with the incident frequency,and the selected element structure has extremely high PCE(up to 76.8 %)at 1 THz.3.Different methods of the phase control are used to design metasurfaces to realize the computer-generated holographic imaging with different functions.First and foremost,the metasurface based on propagation phase control is designed to realize holographic imaging when the linear polarized THz wave is incident.Then the metasurface based on geometric phase control is designed with full-silicon rectangular column which has higher polarization conversion efficiency(> 70%)to realize the0.95 THz-1.2 THz broadband holographic imaging and multi-position holographic imaging which can get different holographic image at 2 mm and 5 mm when left circularly polarized THz wave is incident.On this basis,the metasurface based on the simultaneous control of geometric phase and propagation phase is designed suitably to realize a circular-bias multiplexing function to obtain different holographic images when different polarization states of THz waves are incident and different analysis are applied in the direction of the transmitted waves.The multi-function of metasurface holographic imaging research can store multiple information on a metasurface,which improves the multi-function ability of metasurfaces,provides a new design idea to achieve different functions in the terahertz band for different polarization state,frequency,orbital angular momentum and spatial distribution,promotes the development of miniaturization integrated terahertz band functional devices and has certain application prospects in biomedicine,human security check and art nondestructive testing.