Microwave Encrypted Hologram Based On Coding Metasurface

As an efficient and practical method,metasurfaces are widely used to manipulate electromagnetic waves,and many functional devices have been developed.Meta-materialbased hyper-lens,invisibility cloak,beam deflection,beam splitting,electromagnetic wave gathering,radar cross-section reduction,etc.have all been fully studied.Programmable metasurface links the physical properties of metamaterials with digital domain control,and realizes real-time regulation of electromagnetic waves by changing the functions of metasurfaces by controlling the coding sequence.Microwave encryption hologram based on metamaterials is a typical image encryption method.The attributes involved in the encryption process,such as wavelength,distance,and polarization,can be used as the multi-dimensional key of the encryption system.This article systematically introduces the realization methods of metamaterial microwave hologram and typical methods of information confidentiality,including metasurface holographic algorithms,realization of linear polarization multiplexing encoding metasurface hologram,circular polarization hologram,and programmable metasurface hologram.The design basically covers the main aspects of microwave holograms.At the same time,the significant influence of the feed signal,holographic distance,polarization state,etc.as the key on information extraction is also discussed.The main research contents and contributions of this paper are summarized as follows:1.A coding metasurface holographic algorithm is proposed in the microwave frequency.By introducing the commonly used phase recovery G-S algorithm and constructing the operation matrix through the Huygens-Fresnel principle,it is applied to the microwave frequency band through matrix transformation.In order to overcome the limitations of the G-S algorithm,such as being easy to fall into the local minimum,a design method of genetic algorithm NSGA-II and G-S algorithm cascade is proposed and used to further optimize the results;a holographic algorithm that optimizes the quantized phase is proposed based on the digitally coding metasurface.The phase quantization is applied to the optimization loop to reduce the loss of information caused by the phase quantization.2.A linear polarization multiplexing hologram based on anisotropic metasurface is proposed and designed.The unit is designed based on the transmission phase realization principle,and the full-phase spatial transmission electromagnetic wave control is realized through the stacking of the same structure layers.Faced with the defect that the phase of the unit part changes drastically with the size,the phase quantization is used as much as possible to use only the area where the phase changes smoothly.Through the design of the unit,the independent control of the orthogonal linear polarization is realized,and the polarized and encrypted hologram of the metasurface in the orthogonal directions are realized.In order to embody the multi-key system of microwave hologram,the influence of the feed source and distance as the information extraction key on the holographic pattern effect is mainly discussed.3.A circularly polarized metasurface letter hologram is designed and realized in the microwave band.The circular polarization metasurface is designed using the principle of geometric phase(P-B phase),that is,the metasurface is composed of different phase units by rotating the unit.The non-uniform wavefront of the circularly polarized horn antenna is equivalent by a suitable method,and the feed information is compensated in the algorithm in advance,and finally the circularly polarized hologram is successfully realized.It shows that the imaging method can be extended to any polarization,and the proposed imaging method has universal applicability.4.A reprogrammable metasurface hologram is designed and verified.In order to overcome the shortcomings that the microwave hologram function is fixed and cannot be changed,the metamaterial hologram method and the re-programmable metasurface encoding are combined,and the dynamic change of the imaging pattern is realized by changing the control sequence of the programmable metasurface.