We present a quantum-electrodynamical formalism to study the spontaneous emission from dipoles embedded in a non-absorbing and lossless multilayer dielectric structure. In this model the electromagnetic field is quantized by a proper choice of a complete and orthonormal set of classical modes and the analytical expressions for the emission rates are obtained within the framework of perturbation theory. We apply our model to investigate the 1.54 µm transition of Er 3+-doped SiO 2 thin layers acting as active material in planar slot waveguides in polycrystalline silicon. The theoretical results show that a strong reduction of the radiative lifetime does occur in the slot waveguide. Furthermore, by using the theoretical analysis together with photoluminescence measurements, we estimate also the radiative efficiency which is found to be only slightly reduced with respect to the value for Er 3+ in a bulk of SiO 2. These …