  We investigate plasmon resonances in gold nanoparticles to enhance the
quantum yield of upconverting materials. For this purpose, we use a rate
equation model that describes the upconversion of trivalent erbium based
upconverters. Changes of the optical field acting on the upconverter and the
changes to the transition probabilities of the upconverter in the proximity of
a gold nanoparticle are calculated using Mie theory and exact electrodynamic
theory respectively. With this data, the influence on the luminescence of the
upconverter is determined using the rate equation model. The results show that
upconversion luminescence can be increased in the proximity of a spherical gold
nanoparticle due to the change in the optical field and the modification of the
transition rates.
