Special Issue of Silicon Photonics [Scanning the Issue]

Silicon, well established as an electronics platform, was first seriously explored as an optical waveguide platform in 1987 by Soref and Bennett. Light at fiber-optic communication wavelengths can pass through crystalline silicon with very high transmission. Silicon waveguides can be made by etching into the thin crystalline silicon layer on a silicon-on-insulator wafer. Because of the high refractive index of silicon compared to the surrounding oxide, these waveguides can bend with radii less than five micrometers and make compact splitters/combiners, filters, and polarization elements. High-speed optical modulators can be made by rapidly changing free electron and hole densities in the silicon in the optical path, and photodetectors can be made by epitaxially growing germanium on the silicon, both using electrically connected p-n junctions. The only element missing to complete an optical integration platform is an integrated light source, rendered very difficult because of the indirect bandgap of silicon and germanium. While to date, no practical electrically pumped light emitter has been deployed based on silicon itself, multiple hybrid and heterogeneous solutions have been reported.