Mathematical analysis of electromagnetic scattering by dielectric nanoparticles with high refractive indices

TL;DR

This paper provides a rigorous mathematical analysis of electromagnetic scattering by high-refractive-index dielectric nanoparticles, revealing their resonant behaviors and magnetic responses, supported by explicit calculations and numerical validation.

Contribution

It introduces a mathematical framework for analyzing dielectric resonances and magnetic responses in nanoparticles, including new estimates, asymptotic expansions, and a multipole radiation model.

Findings

High refractive index nanoparticles exhibit strong magnetic responses at resonant frequencies.

Resonant modes can be characterized by asymptotic expansions in particle size and contrast.

Nanoparticles behave like combined electric and magnetic dipoles at subwavelength resonances.

Abstract

In this work, we are concerned with the mathematical modeling of the electromagnetic (EM) scattering by arbitrarily shaped non-magnetic nanoparticles with high refractive indices. When illuminated by visible light, such particles can exhibit a very strong isotropic magnetic response, resulting from the coupling of the incident wave with the circular displacement currents of the EM fields. The main aim of this work is to mathematically illustrate this phenomenon. We shall first introduce the EM scattering resolvent and the concept of dielectric subwavelength resonances. Then we derive the a priori estimates for the subwavelength resonances and the associated resonant modes. We also show the existence of resonances and obtain their asymptotic expansions in terms of the small particle size and the high contrast parameter. After that, we investigate the enhancement of the scattering amplitude and the cross sections when the resonances occur. In doing so, we develop a novel multipole radiation framework that directly separates the electric and magnetic multipole moments and allows us to clearly see their orders of magnitude and blow--up rates. We prove that at the dielectric subwavelength resonant frequencies, the nanoparticles with high refractive indices behave like the sum of the electric dipole and the resonant magnetic dipole. Some explicit calculations and numerical experiments are also provided to validate our general results and formulas.