Research On Phase-type Metasurface For Orthogonal Circularly-polarized Wave Wavefronts Modulation

Metasurface,a two-dimensional structure,consists of a series of artificially designed subwavelength electromagnetic scatters arranged in a periodic or quasi-periodic manner.Through finely design the subwavelength meta-atom,the electromagnetic response can be changed,and thus enabling flexible manipulation of the electromagnetic wave fundamental characteristics,such as polarization,phase and amplitude.Recently,metasurface based on geometric phase have attracted much attention and been extensively studied,due to its superior features of nondispersive,broadband and easy implementation,that is the main method for achieving wavefront manipulation of circularly-polarized（CP）electromagnetic wave.However,this modulation manner exhibits a conjugate symmetric phase response to orthogonal CP wave,which limits the degree of freedom for polarization modulation of both spin waves and results in a single working performance and low utilization of metasurface.Therefore,according to phase theory,this thesis analyzes the methods of decoupling independent modulation of orthogonal circularly polarized waves,and carries out the study of wavefront modulation of orthogonal circularly polarized electromagnetic waves by using phase-type metasurface.The research provides a certain foundation for enriching the degree of freedom of metasurface modulating CP wave and extending the polarization multiplexing method.The main research contents are summarized as follows:Firstly,based on the geometric phase theory,a single layer reflective metasurface composed of an H-shaped metal resonant structure is proposed,which can achieve a high co-polarization conversion efficiency over 92%for incident CP waves in the ultra-wide operating band from 7.3 GHz to 21 GHz,the relative working bandwidth is up to96.8%.A metasurface array is designed to generate vortex waves carrying 2 mode orbital momentum angular（OAM）.The research results obtained by numerical simulation and experimental measurement show that the metasurface modulated by geometric phase can realize efficient generation of vortex waves in ultra-broad working band.Additionally,the conjugate symmetry of the conventional geometric phase modulation for left-and right-handed CP waves is verified as linearly polarized incident wave illuminating the metasurface.Secondly,based on the geometric phase and propagation phase,a single-layer and ultra-thin reflective metasurface consisting of an anisotropic metal resonator is proposed.According to the Jones matrix theory,the conjugate symmetry of conventional geometric phase can be broken by applying the independent phase response of the anisotropic structure for two orthogonal linearly polarized waves.The decoupled wavefront modulation is realized at a single operating band by endowing independent wavefront phase distribution functions for orthogonal CP waves.The thickness of metasurface is only 0.07λ₀at the central operating frequency of 14 GHz.The desired propagation phase can be achieved by varying the physical sizes along x and y directions,and the desired geometric phase can be achieved by rotating the structure.Additionally,three two-bite encoded phase-type metasurface are constructed and applied to realize convergent beams with different focal lengths,convergent vortex beams with different OAM modes,and deflected beams with different outgoing directions under orthogonal CP incident waves illuminating,respectively.The feasibility of this decoupled modulation design method is demonstrated by simulation analysis and experiments,and the polarization multiplexing rate of metasurface manipulating CP waves is effectively improved.Thirdly,based on the circular dichroism of the chiral structure and geometric phase theory,a frequency-multiplexing chiral metasurface is proposed to realize spin-selective absorption and reflection functions for dual-orthogonal CP incident waves.The working mechanism for the spin-selective absorption of CP waves is analyzed,and then the necessary conditions of structural symmetry for the metasurface design are obtained.Combined with the circular dichroism of the chiral structure,a chiral metasurface is designed,which is composed of two asymmetrical split-ring resonators with different radii and widths and same asymmetric opening angles,and one half waveplate inclined at a certain angle.Furthermore,a gradient chiral metasurface and a vortex chiral metasurface are constructed.Numerical simulations show that the incident right-handed CP wave is highly absorbed at the resonant frequencies f₁ and f₃,while the incident left-handed CP wave is reflected and converted into deflected beams or vortex beams with the OAM mode of-2.On the contrary,the incident left-handed CP wave is highly absorbed at the resonant frequency f₂,and the incident right-handed CP wave is directionally reflected or vortex wave carrying 2 mode OAM is generated.Through fabricating and measuring two metasurface prototypes,the working performance of spin-selective absorption or reflection for CP electromagnetic waves are verified.The research results provide a promising route for the independent wavefront modulation of orthogonal CP waves with phase-type metasurface.Lastly,based on the composite geometric and propagation phase theories,a broadband single-layer reflective metasurface composed of an axisymmetric metal resonator is proposed.The decoupled wavefront modulation is realized in the frequency range of 10-16 GHz.In line with the transmission matrix theory,when the phase difference under two orthogonal linearly polarized waves illuminating equals to 180°,the decoupled independent manipulation for orthogonal CP waves in different transmission channels can be easily realized by endowing a consistent propagation phase response and conjugate symmetric geometrical phase response to the orthogonally circularly polarized incident wave.The meta-atom consisting of two rings with symmetry opening angles on the left and right sides and one rectangular half waveplate is designed.The desired propagation phase can be obtained by adjusting the opening angle on both sides,and the rotate of units can obtain the geometric phase.Moreover,two deflected multiplexing metasurfaces are constructed to induce gradient phase distributions of 45°\90°and 30°\60°for left-and right-handed CP waves,respectively.And 3-bit encoded spin-decoupled holographic multiplexed metasurface and vortex multiplexed metasurface are separately constructed.The effectiveness of the proposed broadband spin-decoupled metasurface for decoupled independent wavefront modulation of orthogonal CP waves is verified by full-wave simulations and experimental measurements.The research results provide a new scheme for its practical application.In this thesis,the independent wavefront manipulation of orthogonal CP waves as the research objective,through combing the theories of geometric phase,propagation phase and circular dichroism of chiral structure,the spin-decoupled wavefront control methods are investigated.The research results of this thesis have promising application prospects in the fields of phase-type metasurfaces in polarization channel multiplexing,CP electromagnetic waves detection and modern wireless communication systems.