Silicon nanocrystals fabricated by Si+-ion implantation (400 keV, fluences from 4 to 6 x 10(17) cm(-2)) of fused silica form interesting active planar optical waveguides. The nanocrystals emit orange-red photoluminescence (PL) (under UV-blue excitation) and define a region of high refractive index that guides part of the PL along the layer. Light from external light sources can also be coupled into the waveguides (directly to the polished edge facet or from the surface by applying a quartz prism coupler). In both cases the optical emission from the sample facet exhibits narrow (10-20 nm full-with-at-half-maximum) polarisation-resolved transverse electric and transverse magnetic modes instead of the usual broad nanocrystal spectra. This effect is explained by our theoretical model, which identifies the microcavity-like peaks as leaky modes propagating along the waveguide/substrate boundary (not the usual modes guided inside the nanocrystal plane due to its graded index profile). The unconventional properties of this relatively easy-to-make all-silicon structure may be interesting for future photonic devices and sensors. (c) 2005 Elsevier B.V. All rights reserved.