Coherence effects in systems of Dipolar Bi-Spheres

Abstract

Abstract We study the behavior of the Negative Polarization Branch (NPB), using a scattering system which consists of two dipolar scatterers separated by a fixed distance and freely floating in space. For such a system, a resonance spectrum is obtained if one plots the scattering cross-section as a function of the polarizability. The excited resonances correspond with specific oscillation modes of the electric (and magnetic) dipole moments and arise due to the interaction between the dipoles. We show that the NPB can be generated if the system is put into the right resonance mode, but can also be suppressed if placed in a so called longitudinal mode. The effect of a magnetic permeability different from one on the NPB will also be considered. 1 Introduction Coherence effects in or close to the backscattering direction have been the focus of attention of many researchers involved in the theoretical and experimental analysis of the propagation of electromagnetic waves in random dense media where multiple scattering is important. On one hand we have the enhancement of the scattered intensity in the backward direction (EBS) and on the other hand we have the polarimetric opposition effects. Based on observations, two types of opposition effects are usually distinguished. The first one, often called the polarization opposition effect (POE) which appears as a narrow asymmetric branch and the second one, often referred to as negative polarization branch (NPB) and appears as a wide symmetric branch around the backward direction in the linear polarization coefficient (LPC) [1,2]. It is considered that the coherent backscattering mechanism is responsible for the EBS, the POE and the NPB [3]. However, it is assumed that the coherent backscattering is not the only contribution to the NPB. These phenomena have been observed in experiments related to the scattering of unpolarized electromagnetic radiation by astronomical objects [1] and have also been reproduced in laboratory experiments with both volume and surface geometries [2,3]. In this work we present a study of the previously cited NPB using a model constituted by two dipoles separated by a fixed distance. We will call this system a Dipolar Bi-sphere (DBS). This simple model has already been introduced by other authors to study coupling resonances [4] and the EBS [5]. The aim of this contribution is to study the effect of coupling resonances of the DBS on the coherence effects [6], for both non-magnetic (μ = 1) and magnetic particles (μ ≠ 1). The reason why we introduce the magnetic permeability in our study is because of the high interest for metamaterials during the last six years. While only very recently those materials have been seen to operate in the visible region [7] and only for slab geometries, an early theoretical study by Kerker et al. [8] showed that small spheres with μ ≠ 1 have some very interesting scattering properties. Herein lays the interest of studying scattering systems with magnetic properties.