Design and Validation of an All-Dielectric Metamaterial Medium for Collimating Orbital-Angular-Momentum Vortex Waves at Microwave Frequencies

In wireless communications, electromagnetic (EM) waves carrying orbital angular momentum (OAM) can obviously improve data transmission efficiency due to their particular characteristic of multiple orthogonal modes. However, the divergent peculiarity of vortex waves limits seriously the propagation distance of communication. In this paper, a medium performing as a collimating lens for vortex waves is designed with a transformation optics concept and manufactured by additive printing technology. The design concept is presented and the lens is both numerically simulated and experimentally measured at microwave frequencies. The measured near-field amplitude distribution indicates that the divergence angle and the diameter of the doughnut-shaped vortex wave passing through the collimating lens are greatly reduced over a broad frequency range. The far-field antenna gain patterns show that the energy density of the vortex wave is significantly improved, while the phase profile and mode purity show that the topology charge of the vortex wave remains unchanged after passing through the collimating lens. The proposed all-dielectric collimating lens allows us to increase the realized gain of vortex waves and may extend the propagation distance, laying the foundation for future vortex wave communications.