Controlling near-field focusing of a mesoscale binary phase plate in an optical radiation field with circular polarization

Binary Fresnel zone plates (ZP) are one of the most frequently used focusing elements of in-plane optical schemes in micro- and nanophotonics. With a decrease in the diameter and focal distance of the ZP to meso-wavelength sizes, the parameters of the focusing region begin to be significantly influenced by features of the ZP design (material, thickness, relief depth). The spatial structure of the focal spot formed in the near-field is investigated by the numerical finite elements (FEM) simulations of the transmission of a plane optical wave through a mesoscale binary phase ZP. We show that there is a range of optimal etching depths of the ZP ridges and optimal thicknesses of the plate substrate, at which the best focusing of the incident optical wave is realized in terms of the maximum field intensity and the minimum size of the focal spot. In addition, a concept of a super-focusing binary phase ZP with an immersion layer in the form of a truncated cone fabricated of ZP material is proposed, which makes it possible to focus the circularly polarized light wave into a subdiffraction region with a half-width of about l/2 n ( n is the ZP refractive index).