Mapping near-field plasmonic interactions of silver particles with scanning near-field optical microscopy measurements

A scanning near-field optical microscope (SNOM) is a powerful tool to investigate optical effects that are smaller than Abbe’s limit. Its greatest strength is the simultaneous measurement of high-resolution topography and optical near-field data that can be correlated to each other. However, the resolution of an aperture SNOM is always limited by the probe. It is a technical challenge to fabricate small illumination tips with a well-defined aperture and high transmission. The aperture size and the coating homogeneity will define the optical resolution and the optical image whereas the tip size and shape influence the topographic accuracy. Although the technique has been developing for many years, the correlation between theoretically expected near-field data and measurement is still not convincing. To overcome this challenge, a new information depth is reached containing four optical images at the very same position. Different nanocluster samples with diverse distributions of silver particles are characterized via SNOM in illumination and collection mode. This will lead to topographical and optical images that can be used as an input for SNOM simulations with the aim of estimating optical artifacts. Including tip, particles, and substrate, our finite-element-method simulations are based on the realistic geometry. Correlating the high-precision SNOM measurement and the detailed simulation of a full line scan will enable us to draw conclusions regarding near-field enhancements caused by interacting particles.