Abstract
In this paper, we review the generation of vortex beams carrying orbital angular momentum in the microwave domain. We firstly present the theory of Laguerre–Gaussian beams where it is demonstrated that they carry such type of momentum. We further provide an overview of the classical methods used to generate orbital angular momentum vortex beams, which rely on two main methods; plane wave to vortex wave conversion and direct generation using radiating antennas. Then, we present recent progress in the physics of metasurfaces devoted to the generation of vortex beams with a discussion about reflective and transmissive metasurfaces for plane wave to vortex wave conversion as well as methods to reduce the intrinsic divergence characteristics of vortex beams. Finally, we conclude on this rapidly developing research field.
Highlights
From Maxwell’s theory, it is well known that electromagnetic (EM) waves carry both linear momentum and angular momentum
The last approach concerns the direct generation of orbital angular momentum (OAM) beams through the application of uniform circular arrays (UCA) of radiating elements with a particular feeding system that allows specifying the required phase shift for each radiator corresponding to the generation of the selected
We have presented the theory of Laguerre–Gaussian beams, where it is shown that they possess a well-defined orbital angular momentum
Summary
From Maxwell’s theory, it is well known that electromagnetic (EM) waves carry both linear momentum and angular momentum. OAM has only attracted valuable attention since 1992 when Allen et al [1] showed that Laguerre–Gaussian (LG) beams possess an orbital angular momentum of lhper photon, where l is an integer and refers to the topological charge (order) of the OAM mode, i.e., the number of phase twists around the vortex. It provides a simpler way to generate OAM, that is, vortex beams can be excited with a plane wave. For applications in microwave domain, the OAM wave generation method is very important to ensure optimal performances of the whole system.
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