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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. [ABSTRACT FROM AUTHOR]

Metasurfaces have provided a novel and promising platform for realizing compact and high‐performance optical devices. The conventional metasurface design approach assumes periodic boundary conditions for each element, which is inaccurate in most cases since near‐field coupling effects between elements will change when the element is surrounded by nonidentical structures. In this paper, a deep learning approach is proposed to predict the actual electromagnetic (EM) responses of each target meta‐atom placed in a large array with near‐field coupling effects taken into account. The predicting neural network takes the physical specifications of the target meta‐atom and its neighbors as input, and calculates its actual phase and amplitude in milliseconds. This approach can be used to optimize metasurfaces' efficiencies when combined with optimization algorithms. To demonstrate the efficacy of this methodology, large improvements in efficiency for a beam deflector and a metalens over the conventional design approach are obtained. Moreover, it is shown that the correlations between a metasurface's performance and its design errors caused by mutual coupling are not bound to certain specifications (materials, shapes, etc.). As such, it is envisioned that this approach can be readily applied to explore the mutual coupling effects and improve the performance of various metasurface designs. [ABSTRACT FROM AUTHOR]