Abstract
We aim here to present a novel Fresnel zone plate- based element which is referred to as multi-twisted zone plate. This element is a zone plate whose phase structure is multi-spirally shifted. It is made of the binary combination of two radial phase shifted spiral zone plate. Given the produced structure, two various-charged vortices are generated prior to the focal plane and following it. Focusing and defocusing behaviors of the element are surveyed theoretically and verified by experiments. Depending on the total charge, a variety of light-arm beams is generated. Further studies elucidate that the charge of the prefocus-formed vortex gradually transforms to the charge of the post focus-formed vortex. Simulation predictions are verified by experiment in order to explore its feasibility.
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