Investigation of focusing features of a spiral binary axicon

It is well known that polarisation transformations can occur not only under sharp focusing conditions, but also in the paraxial regime. We investigate such spatial polarisation transformations of three-dimensional distribution in the focal region in the paraxial regime due to the addition of a lens by a spiral binary diffraction axicon. Theoretical analysis demonstrates the possibility of such transformations. We demonstrate that, in these cases, a zero intensity is formed on the beam axis before the focal plane, and an intensity peak is formed on the beam axis after the passage of the focal plane. The experimental results obtained are in good agreement with the simulation results. We believe that results can be useful in the field of optical manipulation of nano-and microscale objects, as well as in the field of material processing.