Research On Phase Metasurfaces For Circularly Polarized Wave-fronts Manipulation

Metasurface,which can be considered as the two-dimensional structure,is composed of periodically arranged electromagnetic(EM)scatters.Through meticulously tailoring the structure dimensions of sub-wavelength meta-atoms,the basic electromagnetic characteristics,including amplitude,phase and polarization,can be artificially manipulated.Meanwhile,phase response is the main factor that affects the wave-front of electromagnetic waves.Based on the phase-modulated metasurface,the output wave-front can be achieved with required functions and polarization states.Metasurface based on geometric phase response is the typical method for circularly polarizated(CP)EM wave-front manipulation.However,the conjugate symmetric phase responses are the main limitation for EM wave modulation with high efficiency and multi-channel multiplexing.Therefore,in order to expand the modulated CP transmitting channels and achieve multi-function integration by single metasurface,the multiplexing methods for orthogonal CP transmitting channels are proposed,which can provide a theoretical foundation for expanding information transmission channel and enhancing transmission rate.Firstly,as for the conjugate symmetry restriction of the geometric phase,the modification effect of propagation phase on the symmetric center of geometric phase is analyzed by using a two-port transmission network model.On this basis,the crosspolarized conjugate responses decoupling method is proposed.A multi-layered metaatom structure with birefringence characteristics is designed,and a cross-polarized channel-multiplexing vortex/convergence bi-functional metasurface is constructed,which improves the cross-polarization reusability and opens up a new application scenario for geometric metasurface.Secondly,as for the problem that co-polarized transmitting channels cannot be phase-modulated by geometry phase,the synchronous manipulation method of coand cross-polarized amplitude and phase responses is proposed.The analytical expression of the amplitude-phase relationship between the co-and cross-polarized components in the transmission electric field is derived,and the consistent phase regulation and continuous amplitude regulation introduced by the propagation phase under the orthogonal CP EM incidences are analyzed.A series of bi-functional metsurfaces with controllable energy ratio between co-and cross-polarized output components are proposed,which work within single input – double output mode,verifying that the full energy in transmitted field can be artificially utilized.On this basis,with the CP orthogonality,a multi-mode vortex integrated metasurface is proposed to produce vortex wave-fronts with five integer and fractional orbital angular momentum(OAM)modes,which can further verify the feasibility of the synchronous manipulation method of co-and cross-polarized amplitude and phase responses.This method provides theoretical design guidance for realizing the functional diversification of single metasurface.Thirdly,in order to realize separate manipulation of two co-polarized components,the chirality-induced phase is introduced,and synthesized effect produced by propagation phase and chirality-assisted phase is analyzed to propose the independent manipulation method of two co-polarized wave-fronts.The chirality characteristics of the meta-atom is realized by rotating every patch layer of meta-atom with different angles,and the theoretical expression of chirality-assisted phase of meta-atom is derived by using the cascade network model.Bi-channel metasurfaces are designed to verify the feasibility of the independent manipulation of the copolarized components.Furthermore,a superunit that can suppress the crosspolarizated energy is designed,and the dual-image holographic meta-lens that can only pass co-polarized energy is proposed,which provides a new idea for increasing the working effecicency of transmitted EM energy.Lastly,in order to realize artificial manipulation of full CP transmitting channels(L-L,L-R,R-L,R-R),the circularly polarized transmission matrix is used to characterize the coherent coupling relationship between four elements,and a comprehensive phase control method was proposed based on the propagation phase,chirality-assisted phase and geometric phase.Through tailoring the multiple structure degree of freedom of meta-atom,a metasurface with four-mode vortex beams integration is proposed to demonstrate the validity of the full CP channel manipulation method,realizing the full usage of CP channels multiplxing.Furthermore,a metasurface with four-angle refraction beams integration is proposed,and the relationship between the output deflection peaks and the input polarizations is studied to analyze the polarization sensitivity of metasurface.This result improves the application potential of full CP channel multiplexing technology in reconfigurable antenna and wireless communication systemIn summary,the research objective is to achieve independent phase manipulation of CP channels with high working efficiency.Based on geometric phase,propagation phase and chirality-assisted phase provided by metasurface,the manipulation methods on CP waves are proposed,and the validity of mechanisms are verified by full-wave simulation and experimental experiments.The results in this dissertation provide theoretical bases and values for polarization multiplexing and multi-funtional integration,which can be applied in modern communication systems.