Study of the features of PSTM images by means of a perturbative approach

We present a perturbative expression of the scattered near field. A comparison with a rigorous simulation using an integral method allows to determine its range of validity. The numerical results are then compared with near-field data reported by de Fornel et al. and a good qualitative agreement is observed. The perturbative expression of the scattered field provides an insight into the dependence of the near-field measurements upon the plane of incidence and the height of the tip. Finally, we show that it is possible to retrieve the exact profile from the near-field data with a subwavelength resolution.