Controlling the optical scattering of plasmonic nanoparticles using a thin dielectric layer

The effect of a thin dielectric film on the plasmonic behaviour of metal nanoparticles (MNPs) above a high refractive index substrate is explored. Using finite-difference time domain simulations, the optical properties of Ag nanoparticles are investigated as a function of film thickness, refractive index, and particle position within the film. We demonstrate that the addition of a film around a MNP at the air interface of a high-index substrate, where nair < nfilm < nsubstrate, will always increase the fraction of light coupled to the substrate (Fsubs). It is found that placement within a layer that does not conform to nair < nfilm < nsubstrate can lead to reduced enhancements in Fsubs. The principal application for this work is for light-trapping in thin-film solar cells. We show that the inclusion of a thin film can increase the fraction of radiation coupled into the substrate by up to 30 for solar wavelengths. Additional potential benefits of the film structure, such as greater tunability of scattering resonances, an increase in path length of light in the substrate, and some control over the emission pattern are demonstrated. MNPs in a film are found to produce a more finely structured emission pattern than particles at a simple interface, showing potential for this research to be applied to optical nanoantennae. © 2013 AIP Publishing LLC