Modeling illumination-mode near-field optical microscopy of Au nanoparticles

We present a theoretical analysis of near-field scanning optical microscopy (NSOM) images of small Au particles made in the illumination mode. We model the metal-coated fiber tip as a thin disk consisting of a glass core and an aluminum coating. An external field locally illuminates the tip core. We solve for the local fields, including interactions between the tip and the Au particles, by use of the coupled dipole method and calculate the optical signal collected in the far field. We also determine the tip field, in the absence of the particle, for various tip sizes with different metal-coating thicknesses. Calculated tip fields and simulated images are compared with those obtained with the Bethe–Bouwkamp model, a commonly used simple model for the tip field. Calculated line scans of the NSOM images of Au particles depend strongly on the tip aperture size and metal-coating thickness. For blunt tips with a thick metal coating and sharp tips with a much thinner coating, our thin-disk model reproduces the key features of measured NSOM images. Line scans calculated with the Bethe–Bouwkamp model cannot describe the tip dependence of the experimental images. Tip fields obtained from the thin-disk model show significant enhancement beneath the metal coating and a broader field distribution perpendicular to the polarization. Tip fields obtained with the Bethe–Bouwkamp model do not show these effects. Differences in the line scans for these two models are correlated to the differences between the tip fields for the two models. These differences occur because only the disk model accounts for a finite metal coating.