(398l) Plasmonic Nanoshell Colloids With Reversible Magnetic Field Induced Optical Anisotropy: Synthesis, Characterization and Simulation

Small particles possessing a localized plasmon resonance in the visible and near-infrared spectral regions are promising for applications ranging from (bio)chemical sensors and drug delivery systems to photovoltaic devices and optical metamaterials. Recently there has been a growing interest in hierarchical structures and coatings based on such plasmonic building blocks which, due to electrodynamic interactions, display markedly different optical properties compared to those of isolated particles. Most structures produced using colloidal nanoparticles rely on the irreversible free or templated deposition or assembly in order to obtain plasmonic interactions. However, the addition of a superparamagnetic component to a plasmon resonant particle is an approach which permits the reversible adjustment of interactions and thereby the optical properties. Although the literature contains some reports of superparamagnetic and plasmon resonant structures, the influence of magnetic field on their optical behaviour is rarely considered. In this presentation we will describe the synthesis, characterisation and electrodynamic simulation of dispersions of magneto-optical nanocomposite particles produced using colloidal techniques. We utilize core particles with different superparamagnetic iron oxide loadings produced by self-assembly and emulsification techniques. These are then coated with gold or silver in order to form superparamagnetic nanoshells which possess visible and near-infrared resonances, the wavelengths of which depend on the core and shell dimensions. Using a modified spectroscopic ellipsometer we show that an applied magnetic field of moderate strength is sufficient to produce reversible and anisotropic linear optical response in dilute suspensions of such multifunctional particles. In particular, this response is significantly enhanced near the dipole resonance of the metal shell. Electrodynamic simulations based on a combination of the Lorenz-Mie, T-matrix and Maxwell Garnett effective medium approaches confirm that optical interactions between particles in chain-like configurations parallel with the direction of magnetic field are responsible for the field-induced optical changes observed experimentally. Finally, we attempt to use simulations to design synthetically realisable structures with even larger field-induced responses.