Resonant absorption and scattering in evanescent fields

The enhancement of optical resonances of small spherical particles in the c ase of excitation by evanescent waves is investigated. This enhancement is due to the large intensity gradient of the excitation and depends strongly on the decay length of the evanescent wave as well as on its polarisation. Resonances due to excitations of the bulk material are considered as well a s morphology-dependent resonances due to constructive interference of circu mpropagating waves in dielectric particles. In the important case, when the particle is in contact with a surface, where the evanescent wave is genera ted by total internal reflection, both types of resonances are attenuated, broadened, and redshifted due to multiple scattering processes. These effec ts have been studied by means of the numerical multiple multipole (MMP) met hod. It is found that even in this case strong enhancement of resonances du e to higher multipoles is retained, corresponding to scattering and absorpt ion spectra significantly different from those for plane-wave excitation. M ultiple scattering processes are found to become inefficient when the dista nce of the particle to the surface exceeds about the particle radius.