Active double-layered Huygens’ metasurface for time-varying OAM beam generation in the microwave regime

Time-varying orbital angular momentum (OAM) beams offer promising capabilities for applications in electromagnetic wave manipulation due to their unique potential in spatiotemporal differentiators, sub-/superluminal pulse control, and high-dimensional information transmission. However, realizing time-varying OAM beams in the microwave region remains challenging. Active metasurface technology provides a novel approach to generating time-varying OAM in the microwave frequency band, but existing solutions primarily rely on reflective or multilayer transmissive structures, which struggle to meet the diverse needs of integration and complex application scenarios. To address this, this paper proposes and demonstrates a transmissive metasurface generator based on active double-layered Huygens' units for generating time-varying OAM beams in the microwave frequency range. The device consists of eight independently controlled voltage regions, and by dynamically adjusting the voltages within these regions, it enables dynamic modulation of the incident polarization wave over a 2π phase range. At the Huygens' frequency of 8.5 GHz, both simulation and experimental results validate the feasibility and effectiveness of this structure in generating time-varying OAM beams. The findings of this study offer a new approach to achieving high integration, greater flexibility, and more diverse time-varying OAM beam generation, with potential applications in time-division multiplexing and information encryption.