EVANESCENT LIGHT-SCATTERING - THE VALIDITY OF THE DIPOLE APPROXIMATION

In near-field optics the very concept of dipole is often used to repre sent either an elementary source or a scattering center. The most simp le and widely used example is that of a small spherical particle whose polarizability is assumed to conform to the Clausius-Mossotti relatio n. While in conventional, far-field optics this approximation is known to be valid provided that the object is much smaller than the wavelen gth, its extension to near-field optics requires some precautions. Ind eed, in the case of the scattering, by a spherical object, of an evane scent field generated, for instance, by total internal reflection or b y a surface polariton, the strong-field gradient may increase the cont ribution to the polarizability of multipoles higher than the dipole. S uch high-order multipoles are seldom considered in near-field optics b ecause they complicate considerably any scattering calculation. In thi s paper we derive, for a spherical particle, the contributions of mult ipole orders up to the hexadecapole. This serves to illustrate the rel ative 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.