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

• Published in: Applied optics. Optical technology and biomedical optics Vol. 52; no. 3; p. 330
• Main Authors: Kotlyar, Victor V, Stafeev, Sergey S, Liu, Yikun, O'Faolain, Liam, Kovalev, Alexey A
• Format: Journal Article
• Language: English
• Published: United States 20.01.2013
• ISSN: 2155-3165
• DOI: 10.1364/AO.52.000330

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.