Diffraction of vortex Gaussian beams from a two-dimensional Raman-induced grating
In this paper, we study diffraction of a vortex Gaussian probe beam on a two-dimensional (2D) Raman-induced diffraction grating. Both near- and far-field diffraction of a vortex beam is considered. In the near field, quasi-Talbot images occur at specific distances from the grating, which corresponds to the classical Talbot length. Diffraction patterns in the Talbot planes are a periodic 2D array of ring-like vortex beamlets with topological charges (TCs) equal to the illuminating probe beam's charge. The lateral (off-axis) beamlets consist of several overlapping vortices with the TCs l = 1 and l =−1, and their centers (singular points) are offset relative to each other. It is shown that in the near field the TC is conserved, and the total diffraction field represents a single (global) vortex with an effective TC equal to the charge of the vortex probe beam. In the far field, diffraction patterns are also a 2D array of ring-like local vortices with a period depending on the z coordinate. Their TCs are equal to the charge of the probe field. It is shown that in a far field, the diffracted field's total TC is also equal to that of the probe field. We demonstrate that by choosing the pump field parameters, one can effectively control the intensity of diffraction orders.