An ultrathin spiral phase plate for generation of OAM radio waves

Recently, orbital angular momentum (OAM) has attracted much attention in radio frequencies due to the unique properties that radio waves carrying different OAMs are inherently orthogonal. Consequently, OAM-based multiplexing has potentials to increase the capacity of communication system. Here, a novel ultrathin slot-based planar-spiral phase plate (planar-SPP) is designed for generating the OAM radio waves. The proposed planar-SPP is designed using the concept of transmit array antenna with the ability to individually control the transmission phase of each element to generate the radio beams carrying OAMs with a specific topological charge. The unit cell used here consists of two identical components placed on top of each other. Each component is composed of two closely-placed thin dielectric layers, in which the outer layers are etched with annular ring slots and the inner layer is placed with a uniplanar compact photonic bandgap (UC-PBG) element. The UC-PBG element functions as an additional resonator and coupler to reduce the requirement of an extra layer. The proposed four-layer unit cell is compact with a total thickness of λ
0
/15 at the frequency of 10 GHz, which achieves a full transmission-phase range of 360° and transmission magnitude equal to or better than −1 dB. Besides, the phase error is less than 5° when the oblique incidence angle is less than 40°. Such a unit cell of superior performance ensures the proposed planar-SPP to have high efficiency. The simulated far-field radiation patterns and near-field phase distributions clearly show the generation of OAM-carrying radio waves, which verifies the feasibility of the proposed design. The planar-SPP has advantages of high efficiency and low profile, having potential applications in the radio communication systems.