Resonant absorption and scattering in evanescent fields

The enhancement of optical resonances of small spherical particles in the case 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 as morphology-dependent resonances due to constructive interference of circumpropagating waves in dielectric particles. In the important case, when the particle is in contact with a surface, where the evanescent wave is generated by total internal reflection, both types of resonances are attenuated, broadened, and redshifted due to multiple scattering processes. These effects have been studied by means of the numerical multiple multipole (MMP) method. It is found that even in this case strong enhancement of resonances due to higher multipoles is retained, corresponding to scattering and absorption spectra significantly different from those for plane-wave excitation. Multiple scattering processes are found to become inefficient when the distance of the particle to the surface exceeds about the particle radius.