Surface plasma resonance and magneto-optical enhancement in composites containing multicore-shell structured nanoparticles

Abstract

We have formulated the conditions in which dipolar surface plasma resonance is excited by light waves in composites containing nanometer-sized, $n$-fold multicore-shell structured particles (referred henceforth as ``nano-onions'') dispersed in matrices. The nano-onions have ellipsoidal shape and an arbitrary layer number $n$, which contain shell(s) (or core) of a metal having Drude type free electrons responsible for the surface plasma oscillation. By solving a quasistatic potential boundary problem in a nano-onion that is exposed to an external static electric field, we derived the effective dielectric permittivity tensor, including off-diagonal elements, for the composites, based on the Maxwell Garnett theory. The results were utilized not only to derive the resonance conditions but also to formulate the surface charge densities on the metal surfaces, from which we determined the symmetry of the dipolar surface plasmon polaritons excited in the metal shells. Calculations made on the composites containing model nano-onions of spherical shape having $n$-fold core-shell structure of sodium and a dielectric revealed the following results: (1) The surface plasmon resonance occurs at $n$ eigenfrequencies, similar to the mechanical oscillation in $n$-fold coupled oscillators; (2) at these eigenfrequencies, the composite causes resonant peaks of light extinction coefficient, and (3) the magneto-optical Kerr effect induced by a static external magnetic field is remarkably enhanced at the resonance frequencies. The magneto-optical enhancement is augmented by hypothetically reducing the dielectric loss in Na, thus increasing the quality factor $Q$ of the surface plasmon resonance. The validity limit in our calculations based on the effective medium approximation by the Maxwell Garnett theory is discussed, comparing with the calculations made by Sinzig and Quinten [Appl. Phys. A 58, 157 (1994)] based on a rigorous Mie scattering theory treatment.