Controlling the Bidirectional Circular Polarization States Using Ultrathin Back-to-Back Quarter-Wave Plates Cavity

Efficiently manipulating the polarization states of electromagnetic waves is of great importance for communication, imaging, and sensing. In this paper, we aim to control the circular polarization states, e.g., left-hand, or right-hand, for the bidirectional radiated waves utilizing a pair of back-to-back quarter-wave plates, which are integrated within an ultrathin cavity. As an example, a bidirectional circularly polarized wave with the same helicity in forward and backward is generated based on numerical analyses, and proved by experiments in microwave region. The proposed ultrathin back-to-back quarter-wave plates cavity can be adopted to higher frequencies, e.g., terahertz and mid-infrared ranges, with lower metallic conductivity. The proposed method exhibits the advantages of compact dimension and low-cost implementation in engineering the bidirectional polarization states of electromagnetic waves.