| Vortex electromagnetic waves carrying Orbital Angular Momentum(OAM)are a unique beam with a helical phase structure and a toroidal amplitude field compared to conventional planar electromagnetic waves.Due to the infinity of OAM modes and the orthogonality between different modes,it provides a new degree of freedom independent of the time,frequency,and polarization domains.OAM waves have great potential in terms of channel expansion and improving spectral efficiency.The problem of how to generate broadband high-efficiency multimode OAM beams using compact and low-cost solutions has become a pressing issue with the continuous development of OAM technology.The passive transmission metasurface(TMS)can beam modulate the conventional electromagnetic waves to generate transmission waves carrying OAM,which happens to have the advantages of miniaturization,low cost,and flexible deployment.However,for now,the OAM waves generated by the transmission metasurface have the pain points of low mode purity and low efficiency.In this paper,we address this problem by conducting an in-depth study around OAM wave generation,and the main work and innovations are as follows:1)A Pancharatnam-Berry transmission metasurface with parallel stripe double-arrow array unit cell is designed to achieve broadband and high mode purity OAM wave generation in Ku-band.A scattering field model of a parallel stripe double-arrow unit cell is constructed by introducing the Jones matrix.A multilayer parallel stripe double-arrow periodic structure with a transmission loss of less than 2.92 d B and a phase shift range of2π is designed.The OAM transmission metasurface with l=±1,±2,±3 is designed with a phase compensation algorithm and the Pancharatnam-Berry phase principle on the basis of the parallel strip double-arrow array unit cell,which can stably generate OAM waves in six modes.The l=2 transmission metasurface was machined and assembled to produce broadband OAM waves with high mode purity(>77.2%).The measured and simulated results are in good agreement,indicating that the OAM beam generated by this TMS has high mode purity and broadband.It is important for OAM wave generation in Ku-band and can be used for OAM communication in UAV and satellite in the future.2)A Fabry-Perot Cavity-Transmission Metasurface(FPC-TMS)is proposed to solve the problems of poor directionality and low efficiency of OAM wave generation by TMS,and achieve the dual functions of perfect circular polarization conversion and multi-mode OAM wave generation in X-band.A TMS-based improved Fabry-Perot resonant cavity principle is proposed,and a periodic unit cell composed of an amplitude-phase modulation unit and a Fabry-Perot polarization conversion unit is designed,and the simulation results of the unit demonstrate that the metasurface composed of it can generate OAM waves with high efficiency.Further,the FPC-TMS with modes ±1 and ±3 is designed,and the high directivity and stability at the transmission distance of 30.0λ0(900 mm)is demonstrated by the transmission amplitude and phase field distribution.The polarization conversion rate and transmission efficiency of the FPC-TMS were measured,with a peak polarization conversion rate of 94.1% and a maximum transmission efficiency of 76.2%.The OAM wave generated by the FPC-TMS is more useful for OAM communication or target detection in X-band.In conclusion,the transmission super-surface proposed in this paper can generate broadband OAM beams with high mode purity and high efficiency,providing a stable wave source with multiple modes for OAM communication and high-precision target detection,extending the technical solution of OAM super-surface,which is of significance for fire control radar,Unmanned Aerial Vehicle,and satellite communication. |
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