Generation of superhelical time-varying OAM beam with space-time-coding digital metasurface

The recently proposed extreme-ultraviolet (EUV) beams with time-varying orbital angular momentum (OAM) realized by high-harmonic generation (HHG) provides extraordinary tools for quantum excitation control and particle manipulation. However, such an approach is not easily scalable to other frequency regimes. Here, we design a space-time-coding digital metasurface operating in microwave frequencies to experimentally generate different modes of time-varying OAM. We also introduce a concept of superhelicity of time-varying OAM with a higher-order twist of the wavefront structure, which can be further realized by reflection phase profiles nonlinear in time through the metasurface. By developing a two-probe measurement technique, we dynamically map the OAM field pattern varying in time. The proposed superhelical time-varying OAM beam paves a way for particle manipulations and OAM multiplexing communications.