  An orbital angular momentum (OAM) detection approach at microwave band is
proposed. A transmittance function is exploited to model a transmissive
metasurface. Then the metasurface is designed to convert an OAM wave to
multiple waves, only one of which is gaussian. The radiation direction of the
gaussian wave is distinguishable according to the order of incident OAM.
Consequently, by locating the gaussian wave, the incident OAM can be
conveniently determined. We use a simple field source to simulate the incident
OAM wave in fullwave simulation. It largely simplifies the simulation process
when an incident wave carrying OAM is needed. Both numerical and full-wave
simulation results are provided to validate our design and they show a good
agreement with each other. Then, the metasurface is optimized for high
directivity. Our work can provide an efficient and effective way for OAM
detection in radio communications.
