Statistics of local field intensity enhancements at nanostructured surfaces investigated with a near-field optical microscope

Near-field intensity distributions measured at nanostructured surfaces with a scanning near-field optical microscope are treated to determine the statistics of local field intensity enhancements. The probability density function (PDF) of the field intensity enhancement is calculated for surface-plasmon-polariton scattering by randomly rough surfaces of thin metal films and for light scattering in silver colloid fractals. We demonstrate that the statistics of the intensity distributions established in different scattering regimes, viz., single and multiple scattering, is distinctly different, and compare our results with the theoretical predictions available. We show that. in the regime of multiple scattering, the PDF decreases exponentially for large values of the intensity enhancement with the slope for surface-plasmon scattering being significantly different from that for light scattering in surface fractal structures. We also find that, in the latter case, the PDF slope is only weakly influenced by variations in the wavelength and polarization of incident radiation.