Using first-principles calculations we investigate the influence of interface modi. cation and layer thicknesses on the optical properties of Si/SiO2 superlattices. Four interface models with different dangling-bond passivation are considered. The results demonstrate confinement effects not only for the fundamental band gaps but also for the optical properties. While for a large Si layer thickness of the Si/SiO2 superlattices the interface dependence is small, the calculations show a significant structure dependence for thin Si layers. (C) 2007 Elsevier Science. All rights reserved. (C) 2008 Elsevier B. V. All rights reserved.