Porous silicon possesses unique features that make it particularly useful in the design of optical biosensors. Chemists discovered these properties unexpectedly in the late fifties when they attempted to polish silicon wafers to form better electrical contacts in the first integrated electronic circuits. They used an electrolyte containing hydrofluoric acid, which left a spongelike nanocrystalline silicon structure on the wafer. 1 This roughened, chemically reactive surface makes the material a versatile substrate for biosensors.Electrochemical etching is now a standard way to fabricate nanostructured porous silicon. The morphology—and consequently the physical and chemical properties of the surface—can be precisely controlled by varying the composition of the electrolyte, impurities in the silicon, and the density of current used to etch the surface. Computer-controlled production can create silicon films with precise thickness and pore sizes that range from a few nanometers up to microns. Moreover, because the etching process is self-stopping, stacks of multiple layers each with a different porosity can be fabricated in a single run. The dielectric characteristics of each silicon layer, and in particular its refractive index, n, can be modulated between those of crystalline silicon (n= 3.54, porosity= 0) and air (n= 1, porosity= 100%). Surfaces with a variety of optical properties can be realized by alternating high and low porosity layers. Examples include Fabry–Perot interferometers, omnidirectional Bragg reflectors, optical filters based on microcavities, and even complicated quasi-periodic sequences (see Figure 1). Porous silicon also has features that make it …