Dual-type fractal spiral zone plate for generating sequence of square optical vortices.

We propose a technique for generating a sequence of co-axial zero axial irradiance with a so-called dual-type fractal spiral zone plate (DTFSZP). Based on the Fresnel diffraction theory, we simulated the focusing performance of this optical device. The results reveal that DTFSZP has the remarkable ability of generating a sequence of optical vortices with larger depth of focus and high lateral resolution. The central diffracted image rotates in the vicinity of the focal plane. Moreover, the focusing performance follows a modulo-4 transmutation rule. Such optics promises a complementary and versatile high-resolution non-destructive tool for particle manipulation and provides potential application in three-dimensional optical alignment systems.