Förster transfer and the local optical density of states in erbium-doped silica

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Optically excited erbium ions incorporated near the surface of a silica glass decay by spontaneous emission and---at high Er concentration---via F\orster energy transfer to quenching sites. By externally modifying the photonic microstructure we vary the local optical density of states (LDOS) in samples with different degrees of F\orster transfer. Changes in spontaneous emission rate are consistent with calculated variations in LDOS at $1.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, while F\orster energy transfer is not affected by the LDOS. The latter is attributed to the fact that F\orster transfer involves virtual photons with a broad energy spectrum, excluding a special role for the LDOS at the atom's transition frequency.