  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.
