Evanescent light scattering: The validity of the dipole approximation

In near-field optics the very concept of dipole is often used to represent either an elementary source or a scattering center. The most simple and widely used example is that of a small spherical particle whose polarizability is assumed to conform to the Clausius-Mossotti relation. While in conventional, far-field optics this approximation is known to be valid provided that the object is much smaller than the wavelength, its extension to near-field optics requires some precautions. Indeed, in the case of the scattering, by a spherical object, of an evanescent field generated, for instance, by total internal reflection or by a surface polariton, the strong-field gradient may increase the contribution to the polarizability of multipoles higher than the dipole. Such high-order multipoles are seldom considered in near-field optics because they complicate considerably any scattering calculation. In this paper we derive, for a spherical particle, the contributions of multipole orders up to the hexadecapole. This serves to illustrate the relative importance of each order. Moreover, within the framework of the coupled dipole method, we study, self-consistently, the problem of the scattering of an evanescent held by the sphere. We show that, with an initial field decreasing exponentially, the dipole approximation can be misleading.