Nature-inspired design of soft, hard and hybrid metasurfaces

Recently, there has been a growing interest in metasurfaces such as high impedance (HIZ) surfaces [1]. These structures have the advantage of exhibiting tailored reflection phase characteristics and a frequency band gap through which the surface wave cannot propagate. They have been employed to realize compact low profile antennas, TEM waveguides, microwave filters, and antenna substrates to suppress surface waves [2]. As a subcategory of metasurfaces, soft and hard surfaces can stop the propagation of waves or support a transverse electromagnetic wave along the surface, which are particularly useful in designing waveguides as well as soft and hard horn antennas [3-4]. Another example would be a hybrid surface that satisfies the balanced hybrid condition. They can be used to create hybrid-mode horns and provide low cross-polarized radiation [5]. In this paper, we utilize a robust nature-inspired optimization technique, the genetic algorithm (GA), to design metasurfaces that have customized electromagnetic properties characterized by their surface impedances [6]. Design examples of soft and hybrid metasurfaces are presented, demonstrating the capabilities of this approach. A homogenization method has also been developed to match the optimized metasurfaces to homogeneous metamaterial slabs backed by a ground plane, which can significantly facilitate the modeling of metasurfaces when incorporated with horn antennas, for example.