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 ${\mathrm{FWHM}}_x=(0.44\pm 0.02)\lambda$ and ${\mathrm{FWHM}}_y=(0.52\pm 0.02)\lambda$ and the (depth of focus) $\mathrm{DOF}=(0.75\pm 0.02)\lambda$, 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° 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.

© 2013 Optical Society of America

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