Influence of the probe--sample interaction on scanning near-field optical microscopic images in the far field

We have studied the influence of probe--sample interaction in a scanning near-field optical microscopy (SNOM) in the far field by using samples with a step structure. For a sample with a step height of \sim \lambda /4, the SNOM image contrast between the two sides of the step changes periodically at different scan heights. For a step height of \sim \lambda /2, the image contrast remains approximately the same. The probe--sample interaction determines the SNOM image contrast here. The influence of different refractive indices of the sample has been also analysed by using a simple theoretical model.