Evanescent-wave scattering in near-field optical microscopy

Extended Mie theory is used to investigate the scattering and extinction of evanescent waves by small spherical particles and aggregates of such parti cles. Metallic, dielectric and metal-coated dielectric particles are taken into consideration. In contrast to plane-wave excitation, p- and s-polarize d spectra differ in the case of evanescent waves due to the inherent asymme try of both polarizations. Furthermore, contributions from higher multipole s are strongly enhanced, compared with plane-wave excitation, and the enhan cement factors are polarization dependent. The corresponding changes in the scattering and extinction spectra are most pronounced in cases where highe r multipoles exhibit resonances in the spectral range considered. This appl ies, for example, to morphological resonances of dielectric particles with size parameters > 1. The effect of the surface, where the evanescent wave i s generated by total internal reflection, on the scattering and extinction spectra is investigated via numerical field calculations employing the mult iple multipole method. In an application to apertureless near-field optical microscopy, the variation of the scattered power is calculated when a sili con particle is scanned across a silver particle in the evanescent field.