Experimental statistics of near-field intensity distributions at nanostructured surfaces

Scanning near-field optical microscopy is a technique in which the resoluti on is primarily determined by the size of a probe and not by the wavelength of illumination as in classical (far-field) microscopy. However, the relat ionship between a sample and its near-field optical image is usually rather complex. Typical factors responsible, at least partially, for such a compl exity are the conditions of illumination and detection, sample characterist ics (e.g. roughness and dielectric constant) and optical properties of the probe. Theoretical and experimental works conducted to improve our understa nding of the relation between the object and the image have been reported ( Greffet & Carminati, 1997). Recently, with the help of a photon scanning tu nnelling microscope we have carried out an extensive study of the resultant near-field intensity distributions due to the elastic (in the plane) scatt ering of surface plasmon polaritons (SPPs) at metal film surfaces. We have also directly observed (in similar experimental conditions) localized dipol ar excitations in silver colloid fractals (Bozhevolnyi et al., 1998). In bo th cases, the studied phenomena are intimately related to the regime of mul tiple light scattering, in which the interference effects are rather compli cated and therefore a proper interpretation of them was far from being triv ial. Thus, even though a certain understanding of many features inherent to the subwavelength light interference phenomena was gained (Bozhevolnyi & C oello, 1998; Bozhevolnyi et al., 1998; Coello & Bozhevolnyi, 1999), it is c lear from the outcome of the investigations that more systematic studies in this context are still needed. A different and more powerful approach may be a statistical study of the recorded near-field intensity distributions. In this work, we report what we believe to be the first results on experime ntal statistics of near-field optical images exhibiting localized optical e xcitations (related to the regime of multiple scattering of light). We inve stigated optical images obtained with SPPs excited at different light wavel engths and scattered at different film surfaces, and with different polariz ations and wavelengths of light scattered by silver colloid fractal structu res. We have found significant differences in statistics between near-field intensity distributions taken at rough and smooth metal film surfaces and fractal structures. Finally, our predictions seem to be in agreement with t heoretical studies reported by other authors (Sanchez-Gil & Garcia-Ramos, 1 998).