  Improving phosphor photoluminescence efficiency is a key parameter to boost
the performances of many optical devices. In this work, colloidal silver
nanocubes, homogeneously spread on a luminescent surface, have proved to help
both injecting and extracting light in and out of the photoluminescent layer
and hence contributed significantly to the enhancement of the fluorescence.
This approach has been applied to two materials: the well-known
Y$_3$Al$_5$O$_{12}$:Ce yellow phosphor and an optical quartz. The emission
efficiency, for sol-gel derived YAG:Ce layers, has increased of 80\% in the
presence of an optimal nanoparticle density -- whereas for quartz, a weakly
fluorescent material, the photoluminescence signal can be enhanced by a
200-fold factor. A physical analysis based on simulations shows that the
disorder is an important factor and that the surface density of Ag
nanoparticles is a crucial parameter.
