In situ characterization of optical tips using single fluorescent nanobeads

Aperture-type near-field scanning optical microscopy (NSOM) can be used to image single dipolar emitters at a spatial resolution beyond the diffraction limit. In addition, such imaging provides the intrinsic possibility to determine the three-dimensional orientation of the emitter due to the complexity of the tip's electromagnetic field. However, this determination necessitates the use of an appropriate analytical model for the tip field and a knowledge of crucial experimental parameters like aperture diameter, feedback distance, and the polarization direction of the incident light. A frequently cited model is the Bethe-Bouwkamp solution for a circular, sub-wavelength hole in a metallic screen illuminated by a plane wave. However, this model is unable to even qualitatively explain the experimental images of fluorescent nanobeads obtained with an aperture-type NSOM. We therefore present a simple, analytical model that fits all experimental data quantitatively and provides a realistic representation of the tip's electric field. We also propose the use of small fluorescent nanobeads in an experimental scheme for the in situ characterization of aperture diameter, feedback distance, and polarization direction of the incident light. (C) 2004 Elsevier B.V. All rights reserved.