Near-Field Diffraction from a Binary Microaxicon

We study binary axicons of period 4, 6, and 8 μm fabricated by photolithography with a 1 μm resolution, 500 nm depth, and 4 mm diameter. Near-field diffraction focal spots varying in diameter from 3.5λ to 4.5λ (for the axicon of period μm) and from 5λ to 8λ (for the axicon with μm) are experimentally found on the optical axis at a distance of up to 40 μm from the axicon for the wavelength μm. The first focal spot is found at distance 2 μm ( μm), with the period of the focal spots being 2 μm ( μm) and 4 μm ( μm). Diffraction of linearly polarized plane and diverging waves is simulated using FullWAVE (RSoft) and a proprietary program BOR-FDTD, which implement finite-difference schemes to solve three-dimensional Maxwell's equations in the Cartesian and cylindrical coordinates. The numerically simulated values for diameters of the near-field focal spots for the axicon of period μm are in good agreement with the experimental values.