The effects of high-energy silicon (5 MeV, 7.5 MeV and 30 MeV) irradiations have been optically investigated by the dark-mode m-lines technique. In all cases, an optically isotropic homogeneous layer is created after a certain critical fluence that depends on ion and energy. The structure of the layer has been investigated by micro-Raman spectroscopy and RBS/channeling. The inner boundary of the layer separating the amorphous and crystalline regions moves into the crystal on increasing fluence. The results are discussed based on the occurrence of a sharp threshold in the electronic stopping power leading to the formation of overlapped latent (amorphous) tracks.