Subwavelength elliptical focal spot generated by a binary zone plate

Using a near-field scanning optical microscope (NSOM) with a small-aperture metal tip, we show that a glass zone plate having a focal length of one wavelength focuses a linearly polarized Gaussian beam into a weak ellipse with the Cartesian axis diameters FWHM
_x
= (0.44 ± 0.02)λ and FWHM
_y
= (0.52 ± 0.02)λ and the depth of focus DOF = (0.75 ± 0.02)λ, where λ is the incident wavelength. The comparison of the experimental and simulation results suggests that NSOM with a hollow pyramidal aluminum-coated tip (with 70° apex and 100-nm-diameter aperture) measures the transverse intensity, rather than the power flux or the total intensity. The conclusion that the small-aperture metal tip measures the transverse intensity can be inferred from the Bethe-Bouwkamp theory.