Subwavelength focusing of laser light using zone plates with silver and chromium rings

In this paper using a scanning near-field optical microscope (SNOM) with a small-aperture metal tip, we show that a chromium ZP with relief of height equal to 30 nm and focal length of one wavelength focuses a linearly polarized Gaussian beam into an ellipse with the Cartesian axis diameters FWHM
_x
= 0.42λ and FWHM
_y
= 0.64λ where λ is the incident wave-length.