Time-resolved luminescence measurements on silicon nanocrystal waveguides obtained by thermal annealing of plasma-enhanced chemical-vapor-deposited thin layers of silicon-rich oxide have revealed fast recombination dynamics related to population inversion which leads to net optical gain. Variable stripe length measurements performed on the fast emission signal have shown an exponential growth of the amplified spontaneous emission with net gain values of about 10 cm(-1). The fast emission component is strongly dependent on the pumping length for fixed excitation intensity. In addition, both the fast component intensity and its temporal decay revealed threshold behavior as a function of the incident pump intensity.