Optical constants of porous silicon films and multilayers determined by genetic algorithms

Porous silicon (PS) can be optically described as a homogeneous mixture of air, silicon, and, eventually, silicon dioxide, which explains the great tunability of its refractive index. This behavior makes porous silicon a very interesting material for the development of interference filters for use in optoelectronic applications. For the accurate design of such filters, the precise determination of the optical constants of PS is mandatory. However, since PS is a dispersive and absorbing material, determination of its complex refractive index is not trivial. In the present work, a genetic algorithm has been used to precisely determine the complex refractive index and thickness of thin films from their reflectance spectra in the visible wavelength range. This algorithm was applied to porous silicon. The precise spectral values of the refractive index obtained for PS layers of different characteristics have been used to design and fabricate interference filters with a predetermined optical behavior. In addition, the performance of the algorithm in two different working modes is discussed.