  We demonstrate the rapid readout of terahertz (THz) orbital angular momentum
(OAM) beams using an atomic-vapour based imaging technique. OAM modes with both
azimuthal and radial indices are created using phase-only transmission plates.
The beams undergo terahertz to optical conversion in an atomic vapour, before
being imaged in the far field using an optical CCD camera. In addition to the
spatial intensity profile, we also observe the self-interferogram of the beams
by imaging through a tilted lens, allowing the sign and magnitude of the
azimuthal index to be read out directly. Using this technique, we can reliably
read out the OAM mode of low-intensity beams with high fidelity in 10 ms. Such
a demonstration is expected to have far-reaching consequences for proposed
applications of terahertz OAM beams in communications and microscopy.
