Coupling of Er ions to surface plasmons on Ag

${\mathrm{Er}}^{3+}$ ions located $100\mathrm{nm}$ beneath the surface of silica glass show an enhanced photoluminescence decay rate when the glass is covered with Ag. Correcting for concentration quenching effects, the decay rate is enhanced by 70%, compared to the case without Ag. The data are in agreement with a model that takes into account variations in local density of states and excitation of surface plasmons and lossy surface waves, resulting in direct evidence for the efficient generation of surface plasmons by excited ${\mathrm{Er}}^{3+}$ ions. Using the model, optimum conditions for coupling to surface plasmons are derived, which can be used to enhance the emission rate and quantum efficiency of a wide range of Er-doped materials.