Analysis of the shape of a subwavelength focal spot for the linearly polarized light

Victor V. Kotlyar, Sergey S. Stafeev*, Yikun Liu, Liam O'Faolain, Alexey A. Kovalev

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

65 Citations (Scopus)


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 FWHMx = (0.44 +/- 0.02)lambda and FWHMy = (0.52 +/- 0.02)lambda and the (depth of focus) DOF = (0.75 +/- 0.02)lambda, where lambda is the incident wavelength. The comparison of the experimental and simulation results suggests that NSOM with a hollow pyramidal aluminum-coated tip (with 70 degrees 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. (C) 2013 Optical Society of America

Original languageEnglish
Pages (from-to)330-339
Number of pages10
JournalApplied Optics
Issue number3
Publication statusPublished - 20 Jan 2013


  • LENS
  • ZONE
  • BEAM


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