Scattering and extinction of evanescent waves by small particles

Expressions have been obtained for the total cross sections for extinction and scattering of evanescent waves by small spherical particles. Due to the different structure of p- and s-polarized waves, cross sections for both extinction and scattering of p-polarised evanescent waves exceed those for s-polarised waves, as long as the Mie coefficients for magnetic multipoles are smaller than those for electric multipoles for each multipolar order, i.e. for sufficiently small particles. The difference increases with the angle of incidence of the totally reflected wave. The definition of cross sections for evanescent-wave excitation with variable intensity over the cross-sectional area of the particle allows quantitative comparison with the case of plane-wave excitation. For example, within the dipole approximation the cross sections for plane-wave excitation lie between those for p- and s-polarised evanescent-wave excitation, respectively. Due to the inhomogeneity of the evanescent field higher multipole contributions are strongly enhanced as compared to plane-wave excitation and increase further with the angle of incidence, resulting in corresponding changes in the scattering and extinction spectra. These effects are demonstrated in the specific case of scattering of evanescent waves by small silver particles.