Abstract One promising approach for the development of silicon-based-light-emitting devices is the epitaxial growth of Si nanostructures. In this context, we propose the lattice matched system CaF2/Si/CaF2 as a prototype of a well controlled and ordered Si-based system with known microscopic structure. We present here a combined theoretical and experimental investigation of ultra-thin silicon (111) layers embedded in CaF2. Our all electron calculation predicts the band gap opening and the presence of confined and interface states leading to a quasi-direct band gap. We have synthesized, by molecular beam epitaxy, Si/CaF2 multilayers which efficiently photoluminesce at room temperature. The photoluminescence spectra show a strong resemblance to those of porous silicon. There is a critical dependence of the photoluminescence efficiency on the thickness of the Si layers and a blue shift for decreasing Si layers thickness. Our results allow us to conclude unambiguously that quantum conf...