The authors report a systematic study of the lifetime of the 1.54 μm transition of Er3+-doped SiO2 thin film as active material in planar slot waveguides in polycrystalline silicon. The lifetime shows a strong reduction when compared with values measured in three other configurations. The experimental results, combined with a rigorous quantum-electrodynamical formalism, are consistent with a sizable increase in both the radiative and nonradiative decay rates of Er3+ transition in slot waveguide. The radiative efficiency is only slightly reduced with respect to Er3+ in the bulk oxide, this result being important for future realization of Si-compatible active optical devices.