In this work, we present the waveguiding properties of a set of mono-dispersed silicon nanocrystal Si-nc superlattices. The measured modal refractive indices (obtained by the m-line technique) of different samples can only be explained by assuming a negative "form birefringence" of about 1% originating from the particular structure of the active core, i.e., a periodical set of parallel planes of two different materials. By means of a simple model for the ordinary and extraordinary refractive indices in this kind of structures, by using the information from TEM images and the nominal growth parameters, and by considering as free parameters the upper size of the diameter of Si-nc, we are able to fit the m-line measurements in a unique way. We also report gain measurements on this set of samples, showing that, under high power pulsed excitation, positive optical gain is observed in samples that provide both good light confinement inside the waveguide and the right Si-nc dimension. © 2004 Elsevier B.V. All rights reserved.