Exact electromagnetic multipole expansion using elementary current multipoles

Abstract: Multipole expansion plays an important role in the description of electromagnetic scatterers, allowing them to be accurately characterized by a small set of expansion coefficients. However, to describe electromagnetic excitations inside a scatterer, the current density in it should be decomposed into current multipoles. Such current expansion includes nonradiating current configurations that are absent in the classical field-based expansion. Unfortunately, the use of current multipoles has so far been limited by the absence of an exact and general expression for the current multipole moments beyond their point-multipole approximation. Here, for the first time to our knowledge, we derive such an expression and present the exact mapping relations between the classical and current multipole moments. We use our theory to calculate the scattering and extinction cross sections for large, wavelength-scale, optical scatterers supporting multipole excitations up to the sixth order. Our results show perfect agreement with the Mie theory. The expressions are valid for electromagnetic scatterers of arbitrary sizes and shapes without restrictions on the multipole orders. They describe all possible current configurations, including those that do not produce any fields in the far-field zone. Current multipoles complement the existing theory for electromagnetic multipole expansion, while their minimalistic and universal character makes them a convenient tool for characterizing and designing diverse electromagnetic scattering systems of arbitrary complexity.