High-Efficiency Full-Space Complex-Amplitude Metasurfaces Enabled by a Bi-Spectral Single-Substrate-Layer Meta-Atom

Full-space metasurfaces have attracted significant interest due to their unprecedented abilities to tailor the electromagnetic wavefronts in both transmission and reflection half-spaces. However, it remains difficult and challenging to achieve high-efficiency complex-amplitude modulation within a single-substrate-layer structure for circularly polarized (CP) waves, which can enhance the power distribution regulation. Herein, a universal design strategy for the frequency-multiplexed full-space meta-device is proposed by employing a single-substrate-layer meta-atom with complex-amplitude modulations. The full-space metasurface is composed of a subwavelength-thickness substrate and two discrepant metallic layers, where a modified complementary split-ring resonator and an electric field coupled resonator are selected to refract and reflect the CP incidences to the cross- and co-polarized components at two distinct frequencies. Based on this emerging meta-atom, a multifunctional bi-spectral metasurface is designed and verified by both full-wave simulations and experiments, which generates the quad-vortex beam and the hologram in transmission and reflection modes, respectively. The proposed method employs a single-substrate layer to maintain high efficiency, while incorporating the complex-amplitude modulation, which has potential applications in imaging and communication systems.