Experimental statistics of near-field intensity distributions at nanostructured surfaces

Scanning near-field optical microscopy is a technique in which the resolution is primarily determined by the size of a probe and not by the wavelength of illumination as in classical (far-field) microscopy. However, the relationship between a sample and its near-field optical image is usually rather complex. Typical factors responsible, at least partially, for such a complexity are the conditions of illumination and detection, sample characteristics (e.g. roughness and dielectric constant) and optical properties of the probe. Theoretical and experimental works conducted to improve our understanding of the relation between the object and the image have been reported (Greffet & Carminati, 1997). Recently, with the help of a photon scanning tunnelling microscope we have carried out an extensive study of the resultant near-field intensity distributions due to the elastic (in the plane) scattering of surface plasmon polaritons (SPPs) at metal film surfaces. We have also directly observed (in similar experimental conditions) localized dipolar excitations in silver colloid fractals (Bozhevolnyi et al., 1998). In both cases, the studied phenomena are intimately related to the regime of multiple light scattering, in which the interference effects are rather complicated and therefore a proper interpretation of them was far from being trivial. Thus, even though a certain understanding of many features inherent to the subwavelength light interference phenomena was gained (Bozhevolnyi & Coello, 1998; Bozhevolnyi et al., 1998; Coello & Bozhevolnyi, 1999), it is clear 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 experimental statistics of near-field optical images exhibiting localized optical excitations (related to the regime of multiple scattering of light). We investigated optical images obtained with SPPs excited at different light wavelengths and scattered at different film surfaces, and with different polarizations and wavelengths of light scattered by silver colloid fractal structures. 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 theoretical studies reported by other authors (Sanchez-Gil & Garcia-Ramos, 1998).