Single-metal-cluster local imaging: polarized scattered electric field calculation compared to the field’s modulus and phase observed in the optical near-field

Simultaneous topographical and near-field optical imaging have been performed on single gold particles of diameters close to 12 nm. The optical source is a linearly polarized laser diode operating at Delta *l=780 nm away from the plasmon resonance of the particles. The experimental optical image is recorded with an apertureless scanning near-field optical microscope (ASNOM) operating in transmission mode. It is compared to the components of the polarized scattered electric field around a single cluster calculated using Mie formalism. We show that the tip used in the experiments is sensitive to the axial component of the scattered field, thus allowing us to obtain the amplitude and the phase of the local field. Our derivation brings out new information, usually shaded when applied to an ensemble of particles. In particular, the dipole model widely used to describe the scattered field by a spherical particle is not suitable to describe the three components of the scattered field in the near zone. Our results are of interest for fundamental studies of the optical properties of single metal clusters and the control of local phenomena such as enhancement, extinction, etc.