Influence of high-refractive-index oxide cores on optical properties of metal nanoshells

We perform computationally a systematic study of the optical properties of nanoscale shell-core metal-oxide particles, where a dielectric core (vacuum, SiO₂, ZrO₂, or TiO₂) is surrounded by a metal shell (Ag, Au, or Cu). We give a detailed discussion of the observed features in the optical spectra. The calculations are done using Mie theory and the four-flux method. The optical spectra are dominated by the localized surface plasmon resonance (SPR) excitations induced by the metal shell. We find that the symmetric dipole SPR modes can be redshifted up to 1500 nm by decreasing the shell thickness down to 1 nm with a high-refractive-index core. However, this shift comes with a severe loss in the sharpness of the SPR peak as both the decrease of the shell thickness and the high-refractive-index core dampen and broaden the peak. Thus, only shifts up to 500-1000 nm are practical if good selectivity and high extinction are required, as is the case for many near-infrared absorption applications. The choice of core material was found to cause shifts of a few hundred nanometers.