Research On Angular-Momentum-Wave Generator Based On Metasurfaces

The demand for higher data transmission rates has been increasing steadily.At the same time,there is a growing need for low-energy,integrated systems that can support various emerging technologies such as Internet of Things,6G communication,and autonomous vehicles.Orbital angular momentum(OAM)become a promising candidate to address these challenges by providing the spatial freedom to encode information in electromagnetic waves.On the other hand,metasurface-based devices offer a low-cost and flexible solution for generating complex beam patterns,making them an ideal platform for OAM-based applications.Here,this dissertation focuses on the bandwidth of generated vortex wave,the mode purity of the beam,the multimode design of the beam,and the corresponding mode modulation.The contents are divided into four parts as follows.1.A design method that fabricates a broadband,reflective vortex metasurface is proposed.This method combining the transmission line model and geometric-phase theory is proposed to address the challenge of the time-consuming design process of metaatoms in broadband geometric-phase metasurfaces.Based on the proposed method,a single dielectric structure etched with a rectangular metal ring is proposed to achieve105.6% relative bandwidth from 7 GHz to 21 GHz.The geometric-phase bandwidth is increased by more than 20% compared with the works during the same period.When excited by an Archimedean circularly polarized spiral antenna,the proposed metasurface can produce fractional,integer,and high-order vortex waves within the operation bandwidth.The vortex wave generator is characterized by its small size and lightweight,capable of generating specific mode vortex waves within a wide bandwidth.2.The relationship between the mode purity and designed metasurfaces is investigated from four aspects,including the arrangement of meta-atoms,the period of meta-atoms,the aperture,and the topological structure of the metasurface.Hexagonal meta-atoms are proposed,which effectively weaken the coupling fluctuation caused by the rotation arrangement of meta-atoms,thereby improving the purity of generated vortex beams.The Nyquist sampling theorem and the conservation of angular momentum theorem are applied to restrict the period and the aperture(radius)of metasurface.A big enough aperture of the metasurface and sufficiently dense meta-atoms are crucial to achieving high-purity vortex beams.This dissertation further investigates the quasicontinuous metasurface,which allows us to achieve a high-purity vortex wave with mode l = 2.3.The metasurfaces-based generation and control of multi-mode vortex waves are investigated.The relationship between the OAM mode and the shape of metasurface is established.An equivalent amplitude control is achieved by tailoring the shape of the metasurface.Compared with traditional metasurfaces,the shape-related design not only introduces new design degrees of freedom to achieve amplitude control but also suppresses the crosstalk modes by removing multiple-mode phase singularities.Finally,some specific shape-tailored metasurfaces are designed to generate the OAM modulation spectrum and the OAM comb spectrum,which verify the effectiveness of the proposed method.4.The broadband and high-efficiency spin-decoupled(polarization multiplexing)metasurface is designed.The dipole-loaded quasi-I-shaped meta-atoms are proposed,in which the additional dipole introduces independent and controllable resonance to improve the polarization conversion efficiency(efficiency>90%).The constructed metasurface can achieve efficient spin-decoupling phase design in the frequency range of 8.6 GHz to17.8 GHz.In addition,the phase compensation method and the multi-feed technique are involved for the proposed metasurface to achieve switchable modes and polarizations of generated vortex beams.Finally,a prototype of a dual-polarization and multi-mode communication system is established,which enabled four-mode OAM communication under dual-circular polarization.It will promote the development and application of multi-mode and multi-polarization vortex wave communication systems.