Reflective Metasurface for Controlling the Beam Divergence Angles of Dual-mode OAM Waves

For dealing with the problem of receiving multimode orbital-angular momentum (OAM) waves resulting from different beam divergence angles of OAM waves with different topological charges, it is essential to control the beam divergence angles of OAM waves. In this paper, a method is proposed to control the beam divergence angles of multi-mode OAM waves without modifying the inherent topological charges by a reflective metasurface. Based on this method, a reflective metasurface is properly designed to generate dual-mode OAM waves with equal beam divergence angle. As a dual-mode OAM waves generator, the reflective metasurface is composed of 31x31 metal-insulator-metal elements which is capable to manipulate dual-polarized electromagnetic waves (EMWs) independently. Furthermore, as a beam divergence controller, the reflective metasurface is properly divided into two regions that are responsible for +1 mode and +3 mode OAM waves generations. In this way, dual-mode OAM waves with equal beam divergence angle can be generated by the proposed reflective metasurface. The proposed design provides an efficient technology to control the beam divergence angles of multi-mode OAM waves flexibly, which is conductive to simplify the receiving device in OAM based communication systems.