Intensity and power flow symmetry of subwavelength focal spot
V.V. kotlyar, A.A. Kovalev, S.S. Stafeev

Full text of article: Russian language.

Abstract:
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 ±  0.0 2)l and FWHMy = ( 0.5 2 ±  0. 02)l and the depth of focus DOF = ( 0.7 5 ±  0.0 2)l. 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.

Key words:
subwavelength focusing, elliptical focal spot, Bethe-Bouwkamp law.

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