Gradient-Based Electromagnetic Inversion for Metasurface Design Using Circuit Models

A gradient-based optimization algorithm that is capable of directly designing a metasurface at the circuit parameter level for a desired field (amplitude and phase) pattern or a desired power (phaseless) pattern on some region of interest (ROI) external to the metasurface boundary is presented. Specifically, the inversion algorithm designs the microwave admittance profile of each subwavelength element of the metasurface when a three-layer admittance model is assumed. To this end, a forward model that maps the admittance profile of each layer of the metasurface to the desired field on the ROI is developed. Then, for the inverse design problem, a data misfit cost functional is defined and minimized over the unknown admittance profile using analytically derived gradients and step lengths. The developed inversion algorithm is then utilized to design metasurfaces capable of beam forming in the far-field zone.