Dynamic generation of multiplexed vortex beams by a space-time-coding metasurface

Dynamic generation of multimode vortex waves carrying orbital angular momentum (OAM) utilizing programmable metasurfaces has attracted considerable attention. Yet, it is still a challenge to achieve multiplexed vortex waves with an arbitrary customized mode combination, stemming fundamentally from the discrete control over phase exhibited by current programmable metasurfaces, which are typically constrained to a limited 1-bit or 2-bit discrete resolution. In this paper, we propose, to our knowledge, a new strategy for dynamic generation of multiplexed vortex beams based on a space-time-coding metasurface, capable of quasi-continuous complex-amplitude modulation for harmonic waves. As a proof of concept, a metasurface prototype for generating multiplexed vortex beams with the customized mode composition and power allocation is established based on the transmissive space-time-coding meta-atoms regulated by the field programmable gate array controller. The mode purity of the vortex beams with a single OAM mode of +1, +2, and +3 generated by the metasurface is as high as over 0.93. The generated multiplexed vortex beams carrying (+1, +2, +3) OAM modes with a power ratio of 1:1:1, (+1, +2, +3) modes with a power ratio of 1:2:3, and (−2, −1, +1, +2) modes with a power ratio of 1:2:2:1 are further verified effectively. The proposed space-time-coding metasurface has great potential for OAM multiplexing communication systems.