Modulation Of Electromagnetic Waves Carrying Orbital Angular Momentum Based On Metasurface

Orbital angular momentum(OAM)multiplexing technology holds great potential in the achievement of high-capacity wireless communications.Nevertheless,the wide use of OAM communication systems still faces some challenges and problems,including the inevitable diffractive divergence of OAM waves,the stringent alignment requirement between an OAM transmitter and receiver,and the generation of multiple OAM waves.Therefore,the key problem for an OAM communication system is to achieve flexible modulation of OAM waves,alleviating the dispersion of OAM waves and achieving OAM multiplexing.As for the modulation of OAM waves,metasurfaces are potential candidates as they are characterized by their excellent ability to modulate electromagnetic waves(EMWs),simple structure,and easy processing.Consequently,this paper investigates metasurface-based techniques for achieving flexible modulation of OAM waves,which is of great importance for the practical applications of OAM communication systems and the realization of high-capacity communications in the future.Aiming to deal with the problems inhibiting the wide use of OAM communication systems,methods for achieving flexible modulation of OAM waves based on metasurfaces are studied,and three designs are proposed in this paper.Firstly,metasurfaces for the modulation of quasi-non-diffractive OAM waves are proposed to alleviate the inevitable diffraction divergence of OAM waves and to meet the stringent alignment requirement between an OAM transmitter and receiver.Based on the concept of the optical conical lens,an effective approach for controlling quasi-non-diffractive OAM waves based on an alldielectric metasurface is proposed.Specifically,the proposed alldielectric metasurface can be regarded as a synthesis of an OAM source and a quasi-zero-order Bessel beam launcher with an equivalent surface perpendicular to the propagation path of the generated EMWs.For proof-of-concept,two all-dielectric metasurfaces are designed and 3D printed to create single and dual deflected quasi-non-diffractive OAM waves,respectively.Measured by a THz imaging camera,the desired OAM waves were observed in predesigned directions with a non-diffractive depth predefined theoretically,indicating that the proposed all-dielectric metasurface can modulate quasi-non-diffractive OAM waves flexibly,controlling the beam number,the topological charges,the deflection angles,and the non-diffractive distances of the generated OAM waves independently.Secondly,metasurfaces for multi-dimensional multiplexing are proposed to enhance the performance of OAM multiplexing technologies.As the diffractive divergence of OAM waves with large topological charges is so severe that it restricts their application in wireless communications,the number of independent channels featuring orthogonal OAM waves is limited for an OAM wireless communication system.In order to achieve channel capacity enhancement in wireless communications,three metasurfaces are proposed in this paper,which can produce eight dual linearly polarized,dual circularly polarized,and dual-band OAM waves,respectively.Taking advantage of the proposed metasurfaces,multimode OAM waves can be manipulated independently,and hence multi-dimensional multiplexing of OAM,polarization,and frequency domains can be achieved.Finally,metasurfaces for controlling the beam divergence angles of multi-mode OAM waves are proposed to deal with the problem of receiving coaxial multi-mode OAM waves.For the generation of multi-mode OAM waves with equivalent beam divergence angles,the proposed metasurface is required to be a synthesis of multiple metasurface-based OAM launchers with different element numbers,which are responsible for the generation of OAM waves with different topological charges.Hence,anisotropic elements are employed in the design of the proposed metasurfaces.In this way,multi-mode OAM waves with equivalent beam divergence angles can be generated coaxially.Here,two metasurfaces are designed,which successfully produce dual-mode and four-mode OAM waves with equivalent beam divergence angles,respectively.In this paper,metasurfaces for the modulation of the vortex modes,deflecting angles and beam divergence angles of OAM waves are investigated in order to alleviate the diffraction divergence of OAM waves and extend the link-achievable distance of OAM wireless communication systems.Moreover,metasurfaces for multidimensional multiplexing of frequency,polarization,and OAM domains are proposed to boost the channel capacity of wireless communication systems.These metasurface-based techniques for the modulation of OAM waves provide effective approaches for the achievement of high-capacity communications.