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. 1. Introduction Axicons [1] are known to be suitable for generating a diffraction-free laser Bessel beam in a definite range of the optical axis [2, 3]. Such beams continue to attract researchers’ interest. In [4] a coreless silica fiber of diameter 30？μm and thickness 3？μm, combined with a lens of radius 70？μm was used for generating a Bessel beam of diameter 20？μm maintained over 500？μm distance at wavelength ？μm. In [5] the FDTD-method was used to model a 2D photonic crystal composed of an axicon-shaped rectangular array of dielectric rods: axicon base, 20a; height, 10a; refractive index of the rods, ; radius of the rods, 0.22a; wavelength, , where a is the period of the rod array. A diverging Bessel beam of diameter λ at half-maximum intensity was shown to be generated at a distance of a. A surface plasmon wave in the form of concentric rings described by the first-order Bessel function was reported in [6] in experiments using a radially polarized laser beam ( ？nm), a conical axicon, and an immersion microlens with numerical aperture 1.25 found in a silver film of thickness 50？nm (permittivity ). The central axial ring diameter was 278？nm and the thickness was . The surface plasmon pattern was observed with a near-field microscope Veeco Aurora 3 with a 50–100？nm resolution. In a similar work [7], a scheme including a radially polarized beam of a He-Ne laser ( ？nm), an axicon, and an immersion lens with in a 44？nm thick Au film ( ) was used to form a surface plasmon with a central focal spot of diameter λ. The plasmon was observed with the aid of a latex ball 175？nm in diameter. Focusing the laser light in the
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