Research On Computer-generated Holography Utilizing Huygens' Metasurface

Accurate manipulation of electromagnetic(EM)waves is one of significant fields in modern communication and information technology and application.Particularly,computer-generated holography can be regarded as the representative for the manipulation of wavefront amplitude and phase distribution.Different from traditional holographic imaging technology,computer-generated holography can achieve the wavefront reconstruction by calculating the diffraction process utilizing numerical simulation to obtain the complex array as the computer-generated hologram instead of sampling the interference between the actual incidence and object wave in the optical system.The working schematic is to utilize specific optical device to physically implement the designated hologram under incidence.Commonly used holographic devices are based on the modulation of surface topology or the refractive index of materials.However,due to the limitation of the manipulation dimension and energy assumption,these methods face the challenges of low-resolution imaging,limited field of view,high-order diffraction,dual images and complex fabrication processing.Huygens metasurface,as the emerging multifunctional device,can manipulate the phase,magnitude,polarization,resonance properties of microwaves within sub-wavelength scale.Hence,the utilization of Huygens metasurface enables the high resolution,low noise,high accuracy of the reconstructed image.Compared with traditional implementation of computergenerated holography,Huygens metasurface successfully avoids the unnecessary diffractions with sub-wavelength meta-atoms.Moreover,compared with traditional optical devices,the planar Huygens metasurface is suitable to manufacture into big size with large information capacity,presenting outstanding industrial prospect.Firstly,the imaging schematic and design process of computer-generated hologram utilizing Huygens metasurface are elaborately analyzed.The implementation of technology is mainly divided into two parts,which contain the algorithm for holographic phase map and the design of corresponding devices for superior imaging performances.The weighted Gerchberg-Saxton(GSW)algorithm is adopted here and modified to extend the working frequencies from optical to microwave regime.Particularly,the algorithm achieves both the energy converging and the uniformity of focal strength.Then,the Huygens metaatoms with designated phase shift are designed based on the calculated phase profiles by GSW algorithm.According to boundary condition and Huygens principle,the electric and magnetic surface impedance is related with the reflection and transmission coefficient by derivation.Hence,the required impedance values are obtained for the predesigned electromagnetic response,as the reference parameters for the design of specific electric and magnetic dipoles.Then,the transmission-and reflection-type Huygens metasurface are designed by modulating the geometrical parameters of dipoles.These two structures enable the efficient and full-phase manipulation within sub-wavelength scale,which promise the high image quality.Then,to add a new degree of freedom to the manipulation of electromagnetic fields,the weight factor is introduced into the holographic algorithm.The imaging field can achieve in-phase superposition at focal points and designated focal energy distribution step by step in iterative procedure.The multi-focus metamirror and transmission-type hologram utilizing Huygens metasurface are proposed to verify the feasibility and accuracy of the mechanism,extending the design mechanisms of complex holographic imaging with multi-level gray scale.Next,subsequent research focuses on the design of Huygens metaatom to improve the performance of holographic imaging.First,dual-polarized Huygens metasurface,as the polarization selector,are applied in holographic im aging with high-efficiency and full-phase control of microwaves under orthogonally polarized incidence.The multi-polarized method can significantly increase the information capacity of single surface,providing a simple and efficient scheme of multi-channel communication without integrating any tunable devices.Moreover,the working frequencies have been extended for addressing the narrow-band problems which normally exist in metasurface.Utilizing the proposed Huygens metasurface,the holography can implement the broadband imaging while keeping superior performances of high resolution and imaging efficiency.Besides,the imaging parameters varying with the frequencies are analyzed as the guideline of broadband imaging.Above mechanisms can significantly improve the functionalities of Huygens metasurface,extending the route to multi-mode and high-capacity imaging system.Finally,image transformation is implemented based on the properties of Fourier transform.In the view of the Fourier relationship between the interfacial electric field on Huygens metasurface and scattered field distribution,by modulating the phase distribution on Huygens metasurface in frequency domain,the holographic image in time domain is self-rotated,projected in the specific or scaled angle respectively,avoiding the reconstruction of the holographic system and providing new methods for image processing techniques.Besides,the research achieves the analog Fourier computation within sub-wavelength scale utilizing Huygens metasurface,instead of series of devices in computer-generated computation system,empowdering the design of high-throughput computational materials.