Inverse design of ultrathin metamaterial absorber

Electromagnetic absorbers combining ultrathin profiles with robust absorptivity across wide incidence angles are essential for applications such as stealth technology, wireless communications, and quantum computing. Traditional designs, including Salisbury screens, typically require thicknesses of at least a quarter-wavelength (lambda/4), which limits their use in compact systems. While metamaterial absorbers (MMAs) can reduce thickness, their absorptivity generally decreases under oblique incidence conditions. Here, we introduce an adjoint optimization-based inverse design method that merges the ultrathin advantage of MMAs with the angle-insensitive characteristics of Salisbury screens. By leveraging the computational efficiency of the adjoint method, we systematically optimize absorber structures as thin as lambda/20. The optimized designs achieve absorption exceeding 90% at the target frequency of 7.5 GHz and demonstrate robust performance under oblique incidence, maintaining over 90% absorption up to 50{\deg}, approximately 80% at 60{\deg}, and around 70% at 70{\deg}. Comparative analysis against particle swarm optimization highlights the superior efficiency of the adjoint method, reducing computational effort by approximately 98%. This inverse design framework thus provides substantial improvements in both performance and computational cost, offering a promising approach for advanced electromagnetic absorber design.