A Comparative Analysis of Intensity and Energy Flux in the Focus of the High-Numeric-Aperture Conventional and Spiral Zone Plates

Using simulation by means of the finite difference method in the time domain (FDTD), we demonstrated that focal spots of different shape with respect to both intensity and energy flux form upon tight focusing of a Gaussian linearly polarized beam containing an optical vortex by a Fresnel zone plate and upon focusing of a Gaussian beam by a spiral zone plate. The most significant differences are observed for the topological charge equal three. The energy flux upon focusing of a Gaussian beam by a Fresnel zone plate has a ring distribution, while the distribution of intensity and energy flux upon beam focusing by a spiral zone plate has three local maxima, which corresponds to the spiral-zone-plate order. The petal structure of intensity (and the energy flux) changes to a ring distribution at a certain distance from the focus.