4-bit millimeter-wave Janus metasurface enabled polarization-spatial multiplexing holography.

Metasurface holography has become a surging and revolutionized field due to its flexible manipulation of amplitude and/or phase, which enhances the quality and capacity of holographic images. However, the current meta-holograms primarily focus on half-space manipulation, posing a challenge in developing simplified meta-hologram structures for spatial multiplexing. To address this situation, what we believe to be a novel 4-bit "Janus" metasurface combined with the weighted Gerchberg-Saxton (WGS) algorithm is proposed to record and reconstruct two distinct images in millimeter wave band. By meticulously designing the single-layer units, the 4-bit Janus metasurface achieves independent amplitude and phase responses in two orthogonal information channels. Moreover, the imaging ability of the proposed metasurface is investigated under different amplitude and phase dispersion. Comparative analysis also highlights several notable advantages of our work, including a low-profile design, polarization-frequency multiplexing, and enhanced imaging efficiency. The proposed method is verified through theoretical calculations, simulations, and experiments, and promises a versatile platform for applications in data storage, encryption, and auxiliary sensing.