Near-field diffraction from quasi-periodic structures in the THz regime

Near-field diffraction from periodic structures leads to self-imaging (the Talbot effect), while dedicated quasi-periodic structures are used to generate orbital angular momentum in THz beams. However, the image stack taken in regular steps of increasing distance behind the grating suffers from experimental effects such as inhomogeneous laser power, parasitic interferences, or misalignment. We circumvent these flaws by evaluating the image stacks in the Fourier domain to identify the Talbot distance. We show how a Fourier transform also allows the identification of the topological charge. We report results on linear, ring and fork gratings.