Objective Metasurfaces, composed of subwavelength meta-atoms with carefully designed geometric parameters, provide versatile and precise control over the phase, amplitude, and polarization of electromagnetic waves. This unique capability enables compact, lightweight, and multifunctional integrated devices that can replace bulky conventional optics in a wide range of applications, from imaging systems to wireless communications. Among the various types of metasurfaces, Janus metasurfaces—named after the two-faced Roman god—have attracted significant interest because they exhibit distinct asymmetric responses when excited from opposite propagation directions, offering a promising route to enhance information channel capacity and design flexibility. However, despite these advantages, the vast majority of existing Janus metasurface implementations are strictly limited to transmission-mode operation, leaving the reflection channel almost completely unused under both forward and backward illuminations. This underutilization of the available spatial degrees of freedom fundamentally restricts the potential for system integration and multi-channel multiplexing. To address this key limitation, we propose a bidirectional four-channel Janus metasurface that independently and simultaneously manipulates both reflection and transmission channels under forward and backward illumination. Unlike conventional schemes that rely on intricate polarization conversions or interleaved arrangements, our cascaded design simplifies fabrication while ensuring high efficiency and low crosstalk. By fully leveraging all four scattering quadrants—forward-reflection, forward-transmission, backward-reflection, and backward-transmission—our design achieves complete decoupling of four independent information channels. As proof-of-concept, we numerically demonstrate versatile wavefront shaping functionalities, including multi-angle beam deflection, dual-focus metalens, and four-channel holographic image reconstruction. Our approach thus provides a new paradigm for full-space electromagnetic control, paving the way for highly integrated, high-capacity communication and sensing systems.

Methods We propose a cascaded architecture that integrates polarization-selective reflective meta-atoms and asymmetric transmissive meta-atoms to enable independent control of four scattering channels. The reflective unit (M₁) comprises a top rectangular copper patch, a dielectric spacer (F4B, ε_r = 2.65, loss tangent 0.001, thickness 1.524 mm), and a bottom metallic grating with a period of 0.5 mm and linewidth 0.25 mm. This grating serves as a polarization filter that reflects x-polarized waves while transmitting y-polarized waves. By systematically varying the patch length, we obtain four discrete reflection phases (step π/2) covering 0–2π at 16 GHz, with a simulated reflection amplitude efficiency of 99.6%. The transmissive unit (M₂) adopts a three-metal-layer design interleaved with two F4B dielectric layers. The top and bottom layers are orthogonal metallic gratings that form a Fabry–Pérot cavity, enabling high-efficiency cross-polarization conversion. The intermediate layer is a C-shaped ring resonator; tuning its opening angles and radii yields four discrete transmission phases with an efficiency of 98.5% for x-to-y and y-to-x conversion. Both units exhibit asymmetric responses under forward and backward illuminations due to the different interaction sequences of the incident wave with the asymmetric multilayer structures. By cascading M₁ and M₂, and then mirroring the combined structure along the z-direction to obtain complementary supercells (M'), we construct a spatial-multiplexed supercell where M and M′ are arranged in alternating rows. In this layout, for x-polarized incidence, both the reflection channel (phase α) and the transmission channel (phase β) are excited in both forward and backward directions, while y-polarized incidence only excites the transmission channel (phase β), thus fully utilizing the available degrees of freedom. Based on this design, we numerically demonstrate two proof-of-concept devices. The first is a 180×180 mm² wavefront manipulator: its front and back reflective layers are encoded with linear phase gradients (according to the generalized Snell’s law) to deflect reflected beams to +15° (forward) and −30° (backward), while the intermediate transmissive layer is designed as a metalens with focal lengths of −90 mm (forward) and +120 mm (backward) using spherical phase compensation. The second is a 400×400 mm² holographic metasurface that encodes four independent target letters M, E, T, A via the point-source superposition method, where each image point acts as a source and the total phase on the metasurface is the coherent sum. Full-wave simulations are performed using CST Microwave Studio, and the metalens focusing is further validated by vector diffraction theory.

Results and Discussions Full-wave simulations confirm independent control of all four channels. The beam deflector produces clear anomalous reflections at +15° (forward) and −30° (backward), with deflection efficiencies of 37.79% and 43.89%, respectively. These are below the 50% theoretical limit imposed by spatial multiplexing, with losses mainly due to 4-level phase quantization errors and near-field coupling between adjacent meta-atoms, which also generate specular residues of 18.86% (forward) and 15.55% (backward). The metalens yields well-defined focal spots at z = −90 mm (forward) and +120 mm (backward), with slight shifts caused by coupling and discretization effects. For holography, reconstructed images clearly show the letters M, E, T, and A in their corresponding channels. NCC values against target images are 34% (forward-reflection), 79.4% (forward-transmission), 80.6% (backward-transmission), and 37.6% (backward-reflection); the lower reflection values arise from incident-wave interference, yet the patterns remain visually distinguishable. Cross-talk NCCs between unintended channels are all below 17.1%, confirming negligible inter-channel interference. Overall, these results demonstrate that the proposed design successfully decouples the four scattering channels, achieving independent wavefront shaping with high efficiency, low cross-talk, and satisfactory image fidelity across both reflection and transmission half-spaces.

Conclusions In summary, we propose and numerically verify a bidirectional four-channel Janus metasurface that independently shapes two reflection and two transmission channels under opposite illuminations, with reflection and transmission efficiencies of 99.6% and 98.5%, respectively. By synergistically combining spatial multiplexing, polarization selectivity, and asymmetric resonant coupling, we demonstrate four-channel beam deflection and holography. This work provides a full-space manipulation paradigm that exploits all scattering quadrants and offers a scalable platform for multifunctional integrated devices. Future extensions may incorporate Jones-matrix decoupling or orbital angular momentum multiplexing to further increase channel density.