| Metasurface is a new kind of two-dimensional(2D)or quasi-two-dimension(Q2D)artificial electromagnetic surface consisting of subwavelength microstructure units,which can flexibly control the propagation of electromagnetic waves.Reflective metasurface is a new concept and new method,which develops quickly in recent years.Based on the research of reflective metasurfaces,this dissertation is aiming at the applications of electromagnetic vortex wave with orbital angular momentum(OAM),near field electromagnetic focusing and digital metasurface design,etc.The electromagnetic vortex waves with OAM have great potential applications for new communication modes,which deserves further investigation.The generating technique based on metasurfaces is a promising way to solve the spectrum congestion.Electromagnetic focusing transfer technology can be applied to many civilian fields,such as wireless power transmission(WPT),radio frequency identification(RFID),and electromagnetic therapy,etc.In addition,the focusing energy technique can be also applied to high energy microwave weapons.So,it is of strategic importance to study electromagnetic focusing transfer technology.Because electromagnetic waves can be flexibly regulated by metasurfaces,it has many advantages in the design of focusing antennas by using metasurfaces.Digitizing technology is a new design technology in the field of the metasurfaces recently,which will open up new possibilities for metasurface and its applications.The detail contents of this dissertation include the following aspects:1.Generating method for electromagnetic vortex wave with OAM by using reflective metasurfaces is studied.In this dissertation,the space structure and phase distribution of the OAM vortex waves are investigated.Based on the concept of optical vortex,the intrinsic OAM modes and waveform characteristics of the electromagnetic vortex waves are discussed.Through in-depth study of the basic theory of OAM and metasurface,a new way for generating OAM electromagnetic vortex waves by using reflective metasurface is proposed.With tri-diople unit cells,a reflective metasurface prototype is designed,fabricated,and experimentally demonstrated to generate a vortex wave with OAM mode 2 at 5.8 GHz.Besides,the key techniques to produce electromagnetic vortex waves with different OAM modes,and propagation characteristics of OAM electromagnetic vortex waves in near field and far field are also studied and analyzed.2.Multiple electromagnetic vortex waves with different OAM modes are effectively generated by using single reflective metasurface.Based on the previous technique,this dissertation further proposes a metasurface to produce multiple OAM electromagnetic vortices,which can provide theoretical basis for OAM multiplex and OAM channels modulation.Besides,the dual-polarized OAM vortex beams based on the cross-dipole metasurface is proposed.Results show that the designed metasurface can generate the OAM mode is 1 in the horizontal polarization,while the OAM mode is 2 in vertical polarization.Because of the polarization orthogonality,the dual polarized OAM electromagnetic vortex waves do not interfere with each other.Thus,two different signals can be transmitted or received in the same frequency band,which can provide a theoretical basis for polarization multiplex in the future OAM wireless communication systems.3.The technique of detection and reception for OAM electromagnetic vortex based on reflective metasurfaces is studied.It is a hot research topic how to effectively detect and receive the OAM vortex electromagnetic waves.This dissertation first discusses the existing OAM mode detection methods.Based on the OAM waves generation techniques,this dissertation analyzes the propagation characteristics of electromagnetic vortex waves.And then the detecting and receiving methods for different OAM modes are discussed by using reflective metasurfaces.For the reception of single OAM-mode,a 3T3R(3 Transfers and 3 Recievers)OAM transmission system based on reflective metasurface is analyzed in nearfield condition,and the isolation characteristics between different OAM channels are verified.Besides,the detecting and receiving method for mixed-mode OAM based on metasurface is also experimentally studied in this dissertation.4.Study of near-field electromagnetic focusing transfer technique based on reflective metasurfaces.An electromagnetic focusing antenna needs to form a specific phase distribution on aperture.Since metasurfaces have many advantages in the modulating phase distribution of electromagnetic waves,it has a great potential to design electromagnetic focusing transfer by introducing metasurfaces.Based on the basic theory of reflective metasurfaces,this dissertation gives the detail study of near-field focused reflective metasurface.Furthermore,four type focusing transfer issues are raised,namely single focus and single feed case,mutiple foci and single feed case,single focus and mutiple feeds case,and mutiple focus and mutiple feeds case.Phase shift compensation formulas and theoretical models are presented in detail,and a design procedure for near-field focused metasurface with multi-focus characteristics is presented.In order to further verify the effectiveness of the design,this dissertation presents the implementations of the two near-field focused metasurfaces with 50cm×50cm at 5.8GHz for single-focus and two-focus,respectively.Furthermore,the planar near-field measurement technique is used to measure the focusing plane and validates the proposed approach.The results show that the proposed metasurfaces with tri-dipole elements can achieve a good near-field focused performance.Besides,the effective epsilon-near-zero(ENZ)characteristics and tunneling effect in a near-field magnetic resonant wireless power transfer(WPT)system are also disscussed.5.The digital design for reflective metasurface is stuied in this dissertation.The traditional reflective metasurface is usually composed of a series of units,which usually requires a continuously reflection phase characteristics.Between the digital metasurface and the traditional metasurface,the biggest difference is the introduction of the digital concept of unit,for example,1-Bit corresponds to only two types of "0" and "1",and 2-bit corresponds to four types of "00","01","10","11".By arranging these digitized units on the metasurface,we can get the reflected beam characteristics of different functions.In this dissertation,we discuss the design method of wideband digital reflective metasurface,and propose a nonresonant broadband 1-Bit digital reflective unit.Then,we give a detail analysis and design of "0" and "1" digitization units.Since the proposed "0" element is a non-resonant structure,it has a smooth and highly linear in-phase reflection characteristic,and therefore has a better broadband response.In order to verify the correctness of the design,this dissertation actually fabricated a metasurface of 30 cm × 30 cm with 1-Bit digital reflective units.The measured results show that-1d B bandwidth of about 25%,covering 8.4 GHz to 10.8 GHz,with a good broadband response.The digitization concept can greatly simplify the design of reflective metasurface,and lay a foundation for digital metasurface integrated PIN switches and programmable logic arrays in the future. |
PDF Full Text Request