Dependence of the focal spot parameters on the relief height of the amplitude zone plate

In this paper amplitude Fresnel zone plates (ZPs) with silver and chromium relief on silica glass are theoretically and experimentally investigated. Study of the focal spot characteristics versus the height of chromium and silver ring of ZP was carried out. It showed that the height of the relief of 70 nm allows to achieve an optimum ratio of the width and the intensity of the focal spot. Also, the results of a numerical experiment on focusing radiation with the help of a fabricated ZP, the relief height of which was 70 nm are presented. Using a scanning near-field optical microscope (SNOM) with a small-aperture metal probe, we show that a chromium ZP with relief of height equals to 70 nm, diameter of 15 µm and focal length of λ = 532 nm focuses a linearly polarized Gaussian beam into an ellipse with the Cartesian axis diameters FWHM
x
= 0.42λ and FWHM
y
= 0.64λ.