Intensity and power flow symmetry of subwavelength focal spot

Using a plane-wave expansion method it is shown that the elliptical shape of the focal spot is determined by the longitudinal component of the electric field. However, the cross-section of the power flow has the shape of a circle because it is independent of the longitudinal component of the electric field. Using a near-field scanning optical microscope with metallic cantilever it is experimentally shown that a binary Fresnel zone plate with the focal length equal to the wavelength of the incident linearly polarized Gaussian beam forms an elliptical focal spot with diameters FWHMx = (0.44 ± 0.02)'lambda' and FWHMy = (0.52 ± 0.02)'lambda' and the depth of focus DOF = (0.75 ± 0.02)'lambda'. The comparison between the experiment and simulation confirms that the near-field scanning optical microscope measures the transverse intensity rather than the power flow and the total intensity. This is in agreement with the Bethe-Bowcamp law that states that a small-hole metallic cantilever measures only the transverse intensity.