  We present design and experimental validation of the system for the
generation of the Orbital Angular Momentum (OAM) states using 3D-printed
low-loss metamaterial phase plates for application in the terahertz (THz)
wireless communications. By azimuthally varying the hole pattern density within
the phase plate, the local effective refractive index is varied, thus also
changing the local propagation constant in the azimuthal direction. The OAM of
any topological charge can be created by simply varying the thickness of the
phase plate. The phase plate with topological charge (m=1) is 3D printed and
the amplitude and the phase of the terahertz signal after passing the plate is
characterized using the THz-time domain imaging system. Finally, we present the
experimental setup and theoretical simulation on the multiplexing and
de-multiplexing of several different OAM states for applications in wireless
terahertz communication.
