Highly spectral dependent enhancement of upconversion emission with sputtered gold island films.

We report a five-fold overall enhancement of upconversion emission in NaYF(4) : Yb/Er nanocrystals when coupled with gold island films. Spectroscopic studies show that the enhancement factors are highly dependent on the exact spectral positions and excitation power density, with a largest enhancement factor of more than 12 observed at selected spectral positions, which may be attributed to different upconversion processes involved.