Spirally phase-shifted zone plate for generating and manipulating multiple spiral beams

A multi-purpose diffractive element with unique possibilities in generating a multi-spiral intensity pattern is reported. The element is created by the superposition of an inwardly and outwardly radial phase-shifted zone plate. It is demonstrated that a focused multi-spiral beam is acquired so the number of spiral arms, as well as spiral turns, is easily manipulated. Defocusing studies show that the focused spiral pattern preserves its shape along a rather long distance around the focus. As a novel effect, two vortices with different charges are formed at given distances before and after the focal plane, so the charge of the generated vortex before the focus is gradually transformed to that of the generated vortex after the focus. To elaborate the possibilities of the new element, experimental results verify the simulation predictions.