Generating high-purity orbital angular momentum vortex waves from Cassegrain meta-mirrors
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Generating high-purity orbital angular momentum vortex waves from Cassegrain meta-mirrors Dongxing Gaoa
, Yanling Wang
School of Mechanical, Electrical and Information Engineering, Shandong University, Weihai 264209, China Received: 8 September 2020 / Accepted: 7 November 2020 © Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Cassegrain meta-mirrors are proposed to generate the orbital angular momentum (OAM) vortex waves through dual-reflecting system with two-planar grounded meta-surfaces of properly devised sub-wavelength ring patches. More specifically, electromagnetic fields reflected by the secondary meta-mirror would subsequently be collimated by the primary meta-mirror and transformed into the well-converged OAM vortex waves possessing the merits of high directivities with clear helix wave-fronts. We extend such a design into conformal Cassegrain systems having the primary meta-mirrors of concave and convex profiles, respectively, and also demonstrate excellent syntheses of OAM vortex beams. Our design, using the dual-reflecting meta-mirrors to generate high-purity OAM vortex waves, can obtain the radiations with more stable angular momentum numbers in the long-distance propagation for the OAM wireless communications.
1 Introduction Orbital angular momentum (OAM) vortex waves have attracted numerous attentions nowadays due to the great capacity of enhancing the spectral efficiency in the wireless communication [1–11]. Normally, the twisting wave-fronts of OAM vortex beams are obtained by helicoidally parabolic reflectors with elevating vortex surfaces [1,2] or through using spiral phase plates (SPPs) with specific rotating phases [7–11]. Meta-mirror has also been proved to be a great candidate to generate the well-tuned electromagnetic fields including the helicoidal wave-fronts of the OAM vortex beams or other unique applications [12–22]. Such arrangements of sub-wavelength meta-atom arrays, mimicking the aforementioned parabolic reflectors and spiral phase plates, can demonstrate the same capabilities of generating OAM vortex beams. However, the present syntheses of OAM vortex beams still suffer the drawbacks of less collimated wave-fronts with low purity of the angular momentum number of modes, especially in the long-distance transmission. And this is mainly attributed to the inadequate efficiency of the OAM vortex wave generators, which are often limited by the inherent characteristics of wave divergence and ununiformed illumination from the source. On the other hand, Cassegrain dual-reflecting system [23– 25], possessing the advantages of compact structure and higher aperture efficiency when compared with the conventional reflectors, should be available as an alternative solution to
a e-mail: [email protected] (corresponding author)
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Eur. Phys. J. Plus
(2020) 135:921
generate higher-quality OAM vortex beams. In addition, the meta-surface technology should also be able to offer the opportunities to de
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