The rise of the communication high-way, the increased speed of computers, the diffusion of multimedia systems and the use of intelligent control systems are a few examples where a need for photonic applications has emerged. In this field the III-V and II-VI semiconductor compounds have bee the materials to develop photonic applications up to now. Silicon has not bee considered until the early nineties when Canham demonstrated that a Si based material was able to emit visible light at room temperature with quantum efficiencies approaching 10%. This materials is porous Si which is obtained by electrochemical dissolution of Si. Following this discovery a large effort has been renewed to develop alternative Si based materials where visible light emission is predictable and observable. The driving idea is to realise a material where confinement, passivation and short-range crystallinity are all resent. Indeed all these three items play a crucial role in porous Si.