title: Analysis of the shape of a subwavelength focal spot for the linearly polarized light.
abstract: By decomposing a linearly polarized light field in terms of plane waves, the elliptic intensity distribution across the focal spot is shown to be determined by the E-vector's longitudinal component. Considering that the Poynting vector's projection onto the optical axis (power flux) is independent of the E-vector's longitudinal component, the power flux cross section has a circular form. Using a near-field scanning optical microscope (NSOM) with a small-aperture metal tip, we show that a glass zone plate (ZP) 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.
authors: Image Processing Systems Institute of the Russian Academy of Sciences, Samara 443001, Russia.
authors: Victor V
authors: VV
authors: Kotlyar
authors: Sergey S
authors: SS
authors: Stafeev
authors: Yikun
authors: Y
authors: Liu
authors: Liam
authors: L
authors: O'Faolain
authors: Alexey A
authors: AA
authors: Kovalev