**4. Applications**

Due to its high chemical reactivity and rapid oxidation, porous silicon was being utilized for device isolation. By the end of the 1980s, porous silicon had also been used for other purposes like the realization of SOI substrates and the formation of a buffer layer in epitaxial growth of compound semiconductors on silicon substrates. However, it was only after the discovery of strong photoluminescence in porous silicon that the material attracted broad attention. Since then porous silicon has been used for the fabrication of gas sensors, humidity sensors, biosensors, light emitting structures, optical waveguides, distributed Bragg reflectors, Fabry-Pérot resonators, photonic crystals, flat panel displays, optical and acoustic filters, ultrasound generators, and many other devices. Even though optoelectronics has remained the main research area of porous silicon, recently, the material has found application in other areas like medicine, diagnostics, cosmetics, consumer care, and nutrition. In contrast to the conventional chip-based applications, these new areas rely on porous silicon powders and independent structures. In this section, we briefly discuss the application domains of porous silicon and the reader is encouraged to refer to the literature dedicated to porous silicon applications (for example, see [78]).
