Omnidirectional mirrors are multilayer periodic structures that reflect light in a wide range of wavelengths for all angles of incidence and for all polarizations of the incident light. These structures can be made using the porous silicon technology: the layers are electrochemically etched to have different porosities, corresponding to different refractive indices. Since the porosity depends on the current density of the electrochemical etching process, the layers may have any refractive index value from 1.2 to 2.7. In opposition to classical evaporated thin films where the refractive indices are fixed, this offers a new degree of freedom in the design of such structures. In this work, the four main parameters of a basic omnidirectional mirror (refractive index of the two kinds of layers and their optical thicknesses) are analyzed to find the condition to obtain the maximum width of the omnidirectional reflection band. The analysis is made using the framwork of the Photonic Band Gap materials and considering the omnidirectional mirrors as 1-D Photonic Crystals.
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