Electromagnetic properties of resonant magnetoplasmonic core-shell nanostructures

We present a numerical model we have created and verified to characterize the frequency dependence of the effective magnetic permeability and permittivity of a core-shell (CS) nanostructure composed of a magnetic core and a plasmonic shell with well-controlled dimensions for different geometries and polarizations. Two principal ingredients in our model are as follows: (i) we consider two-dimensional (or cross sections of infinite three-dimensional parallel, infinitely long, identical, cylinders, where the properties and characteristics are invariant along the perpendicular cross sectional plane) three-phase heterostructure, and (ii) while strictly valid only in a dc situation, our analysis can be extended to treat electric fields that oscillate with time provided that the wavelengths associated with the fields are much larger than the microstructure dimension in order that the homogeneous (effective medium) representation of the composite structure makes sense. Such nanostructures simultaneously possess both magnetic gyromagnetic resonance and plasmonic resonance (PLR) resonances. To illustrate the effects of shape anisotropy of the CS structure, we analyze several possible shell shapes involving sharp edges and tips. Geometric parameters of the CS nanostructures and excitation polarized parallel and perpendicular to the antenna axis permit to finely tune the PLR. Changing the internal geometry of the nanostructure not only shifts its resonance frequencies but can also strongly modify the relative magnitudes of the electric field enhancement, independently of nanoparticle shape. The model sets the foundation of quantitatively determining the spatial confinement of the electric field in regions approximate to 20 nm in linear dimension. Because of its resonant nature, we found nanolocalized terahertz fields corresponding to large electric field enhancement two orders of magnitude higher in amplitude than the excitation optical field. The simulations in this paper are important because magnetoplasmonic CS nanostructures are currently being explored as candidates for resonant optical nanoantennas for biosensing applications. (c) 2011 American Institute of Physics. [doi: 10.1063/1.3527007]