Multipolar scattering of subwavelength interacting particles: Extraction of effective properties between transverse and longitudinal optical modes

Abstract

The general concern of this investigation is the extraction of the effective electromagnetic properties of agglomerates of randomly located particles, which are small compared to the wavelength. The focus is on the spectral window ranging from the transverse phonon to the longitudinal phonon frequencies, in which resonances may be excited. In this frequency domain, it is shown that limiting the problem to bare dipole-dipole interactions leads to inaccurate calculations of the electromagnetic fields, resulting in a doubtful extraction of the effective properties. We perform the complete electromagnetic calculation by taking into account the higher multipole orders that are activated when the agglomerates are illuminated. Several results, which are not usually observed outside the frequency range highlighted here, are revealed by means of extensive numerical simulations. In particular, we evidence large deviations compared to the predictions provided by the effective medium theories, while we find that the volume of the microstructures (each one with a different internal geometry) that are used to average the fields must be unusually large to avoid a bias in the determination of the effective properties. Furthermore, the evolution of the incoherent component of the fields between the two optical phonon modes is investigated.