This book gives the synthesis of porous silicon multi-layers (PSL) by a simple route called electrochemical etching. The prepared multi-layers and their physical characterizations have been carried out. These multi-layers have been used to prepare distributed Bragg reflectors and planar microcavities for a potential sensing application. This book also deals with an optical study based on the refractive index deduced by experimental ellipsometric measurements as well as calculating and modelling manners such as the Bruggeman effective medium approximation.