A broadband orbital angular momentum generator utilizing polarization conversion metasurface at microwave frequencies

Abstract

In this paper, we achieved first- and second-order orbital angular momentum (OAM) vortex beams by a single-layer polarization conversion metasurface (PCM). Firstly, we present the basic PCM unit, composed of a short line and two pairs of oval split rings along the diagonal direction attached to a substrate layer. The agreement of numerical simulations and experimental measurements proves that the employed structure’s polarization conversion ratio (PCR) is more than 90% from 8.12 to 22.81 GHz under normal incidence (corresponding to the fractional bandwidth of 94.9%). We reveal the physical mechanism of the proposed PCM by an equivalent circuit method in broadband. Furthermore, to construct the reflective PCM array, the reflection phase of 2π can be obtained by adjusting the geometric parameters of the units. The OAM vortex beams with different topological charges (l= −1, l= −2) can be achieved by specific compensation phases. Simultaneously, we calculate the purity of different OAM modes from 10 to 15 GHz, demonstrating that the main-mode dominates broadband. Finally, the proposed reflective array realizes a stable response to the variations in the incidence angle from 0° to 45°. The proposed method provides an opportunity to realize different topological charges of OAM waves by PCM units in broadband.