Er doped Si nanoclusters (nc) represent one of the most promising approaches recently developed to overcome the intrinsic low efficiency of silicon as a light emitter. This system has been synthesized by annealing treatments of Er-doped SiOx films and its optical properties have been studied in detail. Si nc act as efficient sensitizers for the Er luminescence, producing a strong enhancement of the photoluminescence (PL) signal at 1.54 µm. Figure 1 shows that a further strong enhancement (more than two orders of magnitude) of the RT Er emission has been observed in silicon-on-insulator photonic crystal (PhC) waveguides containing a thin layer of Er-doped Si nc, when the involved transition 4I13/2→ 4I15/2 is in resonance with an appropriate mode of the PhC slab [1].