This paper presents a high-performance energy-selective structure (ESS) design methodology based on a stacked slotline. By leveraging the unique characteristics of three-dimensional stacked structures, the method efficiently converts spatial waves into guided waves in slotline transmission lines, which can be tailored via lumped-circuit design to achieve precise energy-selective functionality. The proposed design approach systematically extends previous work by providing a clear theoretical foundation for decoupling and independently optimizing multiple ESS performance indicators. This allows the design of structures with flexibly selectable frequency bands and high shielding efficiency. Two prototypes were fabricated to validate the method. Prototype I targets bandwidth expansion, achieving an operational range from 2.2 to 8.2 GHz (115.4% relative bandwidth), with less than 1 dB insertion loss and over 10 dB shielding effectiveness across the band. Prototype II emphasizes shielding performance, reaching a shielding efficiency greater than 33 dB between 3.9 and 6.3 GHz, with a maximum of 40.3 dB. Both prototypes were fabricated and validated through experimental measurements, showing agreement with the simulation results. The performance of the two designed structures far exceeds other existing ESSs in terms of broadband or high shielding efficiency, indicating that the comprehensive design method has great potential to significantly improve the design of targeted technical specifications.
The design of large-scale coding metasurfaces poses significant computational challenges, often limited by the prohibitive time required for full-wave simulations necessary for optimization. This paper proposes an efficient design strategy based on a Hybrid Genetic Algorithm, validated through the design, fabrication, and characterization of an X-band metasurface for Radar Cross Section reduction. The proposed design strategy relies on a two-stage optimization process: a fast pre-optimization phase, based on the analytical Huygens-Fresnel principle, generates a preliminary solution which is subsequently refined by a second optimization stage utilizing fullwave simulations. Specifically, the optimization targets a 1-bit coding scheme, where meta-atoms switch between two distinct states with a phase difference of 180 +/- 37 degrees. This hybrid approach demonstrates optimal convergence, reducing computational time by 25% compared to traditional full-wave-only techniques. Furthermore, a novel "spiralling cross" unit cell topology is introduced. Owing to its delay-line geometry, this structure provides additional degrees of freedom for spectral tuning and supports intermediate phase shifts, thus enabling encoding schemes beyond traditional 1-bit configurations. Experimental results confirm the validity of the proposed approach, demonstrating how the combination of versatile geometry and hybrid optimization effectively overcomes the trade-offs between numerical accuracy and computational efficiency.
