Low-temperature crystallization of a-Si is important for display and Silicon-On- Insulator (SOT) technologies. We present optical characterization (Raman scattering and photoluminescence) of H2 and O2 plasma enhanced crystallization of a-Si:H films. H2 plasma treatment is shown to be the most efficient, leading to larger grain sizes, and both H2 and O2 plasma lead to visible photoluminescence (PL). Recently, the PL of re-crystallized a-Si films has been explained in terms of quantum confinement . The mean size of the crystallites in our re-crystallized films is determined by Raman scattering for different treatments parameters. No correlation between size and the photon energy of the visible emission is found. However, we can clearly distinguish between the PL from purely amorphous and re-crystallized a-Si:H films: Their PL temperature dependence and spectra are very different. The origin of the visible PL in re-crystallized thin Si films is discussed.