Spherical SiO2 colloids with two different diameters (175 nm, 340 nm) were doped with erbium at different concentrations. The spheres show sharply peaked photoluminescence centered at 1.535 µm, due to intra-4f transitions in Er3+. From measurements of the Er decay rate for different Er concentrations the decay rate of isolated Er ions (i.e., in absence of concentration quenching) was determined for the two colloid diameters. The data were compared to spontaneous emission rates derived from calculations of the local optical density of states in the colloids. The calculation predicts a large difference in the spontaneous emission rate for both colloid sizes (61 vs 40 s–1), in perfect agreement with the measured data.

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Journal Appl. Phys. Lett.
de Dood, M.J.A, Slooff, L.H, Moroz, A, van Blaaderen, A, & Polman, A. (2001). Modified spontaneous emission in erbium-doped SiO2 spherical colloids. Appl. Phys. Lett., 79, 3585–3587.