Subwave length focusing of laser light using a chromium zone plate

We study in which way the parameters of a focal spot generated by a chromium zone plate 15-µm in diameter synthesized by sputtering on a glass substrate and having a focal length equal to the incident wavelength of λ = 532 nm depend on the microrelief height. It is shown numerically that an optimal microrelief height of the zone plate is 70 nm. With these parameters, the minimal size of the focal spot is achieved. Using a scanning near field optical microscope the said zone plate is shown to focus a linearly polarized Gaussian beam into an elliptical focal spot having the full-width at half-maximum of FWHMx = 0.42λ and FWHMy = 0.64λ along the Cartesian axes.