Optical effects of multipole interaction in aggregated structures: Rigorous scattering solution for a periodic chain of cylinders

Recent experimental studies on the optical properties of gold colloidal aggregates have found interesting characteristics, such as double absorption peaks and enhanced depolarized scattering, that are not explainable in terms of traditional mean-field theories. Based on the observation that the aggregate clusters have a locally chainlike structure of touching spheres, a simple model for the calculation of their absorption and scattering characteristics is proposed that consists of a randomly oriented chain of cylindrical particles under the excitation of an electromagnetic field. The consideration of cylindrical geometry considerably simplifies the mathematics while retaining the essential physics of capacitive coupling between the particles. In the optical frequency regime, a rigorous scattering solution for the model shows that, besides the single-particle Mie resonance, there exists a low-frequency collective excitation arising from the anisotropic, high-multipole interactions between the metal particles. It results in a second absorption peak whose peak position is controlled by the capacitive coupling between the adjacent particles. Both the polarized and the depolarized scattering also show accompanying increases at the collective excitation frequency. Good agreement is found between the calculated results and the experimental observations.