Hybrid Inverse Design Framework for Microwave Ultra-broadband Achromatic Metalens

hybrid inverse design framework is proposed for efficient design of ultra-broadband achromatic metalenses with very simple resonant structures. The framework combines a data-driven inverse dispersion engineering approach along with an optimal random spatial multiplexing (ORSM) method. The two-stage framework is based on a design strategy that segments the entire operating band into multiple sub-bands and performs continuous achromatic design within each sub-band. As a result, the framework alleviates the phase dispersion requirements imposed on meta-atoms by conventional dispersion engineering for ultra-broadband operation, which typically necessitates complex and time-consuming meta-atom design. The inverse dispersion engineering, trained on a compact dataset of only 792 samples, accurately predicts meta-atom structures with desired phase and amplitude responses. Subsequently, the ORSM method optimizes the spatial arrangement of the inverse-designed meta-atoms, ultimately generating the final metalens layout. To validate the performance of the proposed framework, a reflective microwave ultra-broadband metalens is designed, simulated, and characterized. Experimental results demonstrate achromatic focusing with a focal length of 215 mm and an average focusing efficiency of 62.73% over the operating band of 8-16 GHz. This method could pave a new way for the development of achromatic meta-devices for wideband microwave applications.