Generation of THz Vortex Beams and Interferometric Determination of Their Topological Charge

In this article, we developed and demonstrated the efficacy of 3-D printed spiral phase plates (SPPs) for the generation of vortex terahertz (THz) beams with angular momenta
$\ell = \pm 1$
and
$\ell = \pm 2$
. The vortex beam generation was confirmed by means of frequency-domain transmission spectroscopy. The topological charge
$\ell$
of the vortex beams was determined via phase-sensitive Mach–Zehnder interferometry, which enabled the superposition of a vortex beam with
$\ell$
and its conjugate beam with
$-\ell$
. The resulting interference patterns were found to be consistent with the expected intensity distributions for a given
$\ell$
, which provides strong confirmation of the spatial phase structure of the generated vortex beams in the THz regime.