Orbital Angular Momentum in the near-field of a fork grating

Abstract Light beams that carry Orbital Angular Momentum (OAM) are explored in applications from microscopy to quantum communication. At the same time, a revival of the Talbot effect is seen in modern optics from atomic systems to x-ray phase contrast interferometry. In this paper, we link these two phenomena through a symbiotic interaction in the THz range by means of binary amplitude fork gratings. While the assessment of OAM in the far-field diffraction orders of a fork grating is well-established, here we show how the topological charge of the diffracted beam can be evidenced in the near-field by means of the Talbot effect. For this purpose, the evolution of the diffracted beam behind the fork grating is analysed in Fourier domain. We obtain the typical donut-shaped power distribution and isolate the inherent phase vortex using the Fourier phase retrieval method.