Ralf Vogelgesang¹, Klaus Kern^1,2

Award-winning Poster at the SPP4 in Amsterdam, June 2009

¹Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany
²École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland

Purpose
Experimentally, apertureless scanning near field optical microscopy (aSNOM) has matured in recent years, offering routine subwavelength lateral resolution. The quantitative description of recorded images, however, stills poses challenges due to the simultaneous excitation of probe tip and sample. We aim at developing an analytical scheme that is capable of disentangling the interferometrically superposed signal components at the detector. The ultimate goal is an intuitive interpretation of recorded aSNOM images firmly based in rigorous scattering theory.

Methods
To reduce nearfield coupling in the tip-sample system, we consider a cross-polarization scheme that places an analyzer in the scattered beam orthogonal to the excitation polarization. Experimentally, we obtain with this approach nearfield optical images of plasmonic nanostructures that very closely resemble the expected eigenmode patterns (see Fig. 1) of the bare sample [1-3]. Using proper conditions, nearly no influence of the probing tip is perceptible, and the cross-polarization scheme seemingly isolates the sample signal.

Results
We have developed an analytical model for the aSNOM signal based on the reciprocity theorem. It leads to an overlap integral over a plane in the gap between sample and probe. Anharmonic modulation and demodulation due to probe tip motion are easily incorporated. In the case of cross-polarized aSNOM, the model explains the experimental conditions under which the best images are recorded. It simplifies significantly for rotationally symmetric probe geometries and its value can be further approximated to obtain the conditions for which aSNOM records the local field component normal to the substrate surface.

References
[1] T. Zentgraf, J. Dorfmüller, C. Rockstuhl, C. Etrich, R. Vogelgesang, K. Kern, T. Pertsch, F. Lederer, and H. Giessen, Opt. Lett. 33, 848-850 (2008)
[2] R. Vogelgesang, J. Dorfmüller, R. Esteban, R.T. Weitz, A. Dmitriev, and K. Kern. phys. stat. sol. (b), 245, p. 2255-2260 (2008)
[2] R. Esteban, R. Vogelgesang, J. Dorfmüller, A. Dmitriev, C. Rockstuhl, C. Etrich, and K. Kern, Nano Lett. 8, 3155-3159 (2008)