Subwavelength 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 $15$-$\mu$m in diameter synthesized by sputtering on a glass substrate and having a focal length equal to the incident wavelength of $\lambda = 532$ 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 $\mathrm{FWHM}_x = 0.42\lambda$ and $\mathrm{FWHM}_y = 0.64\lambda$ along the Cartesian axes.