[en] Two multilayer structures made of nanoporous silicon layers are designed, fabricated and characterized. The layers that form the structures are characterized by spectroscopic ellipsometry to determine their refractive index and etch rate. The first structure is a periodic structure that consists of the repetition of two layers with different refractive indices and thicknesses. The second structure is formed by two different periodic structures stacked together, being their bandgaps centered at different wavelengths and with common ranges of high reflectivity. The reflectivity spectra for different incidence angles of the periodic and the stacked structures are measured and the existence of an omnidirectional bandgap is analyzed. A model of the stacked structure is realized and its simulated results are compared with the measured reflectivity spectra