Antenna Inverse Design Using Mu-Near-Zero Isorefractive (MNZIR) Materials

Inverse design based on topology optimization relies on continuous interpolation between two materials, with the goal of finding an optimized structure to achieve a desired performance. This has worked well for two simple dielectrics, however, much of RF engineering makes use of very good conductors such as Copper. Unfortunately, from the inverse-design perspective, very good conductors are considered “extreme” as compared to traditional dielectrics due to their very large imaginary permittivity. In contrast, mu-near-zero isorefractive (MNZIR) materials possess a large real permittivity and a small real permeability. Both MNZIR and highly conductive materials exhibit similar behavior at their interfaces, meaning that they both display a reflection coefficient with near-unity magnitude and 180° phase. In inverse design, one needs to interpolate to intermediate values of material parameters. But the intermediate values of conductivity exhibit large losses, which is neither similar to a dielectric nor a good conductor. The intermediate losses potentially impede converging to the desired optimal performance. However, intermediate values of MNZIR are both lossless and can maintain a fixed refractive index, e.g., an index of unity. We use this concept to demonstrate the power of this interpolation to design an inverse-designed patch antenna on a PCB.