Achieving wideband wavefront manipulation in asymmetric media by phase-engineered metasurfaces with near-unity transmission

Evaluation of aging infrastructure and imaging of biological matter are restricted by the bandwidth of electromagnetic wave manipulation in such asymmetric transmission scenarios. Here, we propose a theoretical design strategy for wideband gradient Huygens' metasurfaces that simultaneously achieve wideband impedance matching and wavefront transformation without increasing device complexity, compared to prior approaches. Our cascaded design strategy integrates dielectric spacers as a vital degree of freedom to harmonize and optimize resonance and propagation phases. We achieve an approximately 13% relative bandwidth in the X-band with near-unity transmission across 2π phase coverage, showcasing a substantial performance leap over existing solutions. Numerically and experimentally, we have demonstrated the wideband beam steering and energy focusing in asymmetric systems. This approach establishes a scalable framework for high-efficiency, wideband metasurfaces with potential applications in nondestructive testing, imaging, sensing, and next-generation wireless communications.