Analysis of image formation in a near-field scanning optical microscope: Effects of multiple scattering

We analyze how the interaction between the probe and the field near the sample surface affects the characteristics of the images produced by a near-field scanning optical microscope. The numerical calculation is carried out using the finite-difference time-domain method, and a three-dimensional model that includes a probe of arbitrary shape is considered. Assuming an aperture-type probe (collection-mode near-field microscope), we compare the images produced from the same surface under both the constant-height-scanning and constant-distance-scanning modes. Our results indicate that the probe-sample interaction is critical in defining the characteristics of the generated images. We also investigated image characteristics as function of the polarization state of the illuminating light. When the effects of multiple scattering by the probe tip are taken into account, the calculated intensity image approximates the geometric profile of the sample surface. The dependences of the detected image intensity with the scanning mode utilized, the polarization state of the illuminating light and the sample size are also investigated.