Petal-like zone plate: long depth bifocal diffractive lens and star-like beam generator.

Abstract

This study introduces and examines the diffraction properties of a so-called petal-like zone plate, which comprises Fresnel zones analogous to petals. We show that the focusing behavior of this novel type of zone plate depends on the number of petals included in the element. For a small value of the petal frequency, we observe star-like diffraction patterns at the focal plane, whose number of star arms equals the petal frequency of the element when the frequency is an odd integer and is twice as large as the petal frequency when it is an even number. In addition, we have shown that the star-like pattern rotates when it passes through the focus. Moreover, it is demonstrated that the element acts as a long depth bifocal diffractive lens for a large value of the petal frequency. The spacing between the foci is simply controlled by a so-called shifting parameter. At the same time, an annular beam is observed in the middle of the line joining the two foci together. Consequently, an axial bottle-like beam is produced around the focus, whose size could be simply monitored. Simulation results are followed and verified by experimental works.