Group IV photonic devices for the mid-infrared

Group IV mid-infrared photonics is attracting more research interest lately. The main reason is a host of potential applications ranging from sensing, to medicine, to free space communications and infrared countermeasures. The field is, however, in its infancy and there are several serious challenges to be overcome before we see progress similar to that in the near-infrared silicon photonics. The first is to find suitable material platforms for the mid-infrared. In this paper we present experimental results for passive mid-infrared photonic devices realised in silicon-on-insulator, silicon-on-sapphire, and silicon on porous silicon. We also present relationships for the free-carrier induced electro-refraction and electro-absorption in silicon and germanium in the mid-infrared wavelength range. Electro-absorption modulation is calculated from impurity-doping spectra taken from the literature, and a Kramers-Kronig analysis of these spectra is used to predict electro-refraction modulation. We examine the wavelength dependence of electro-refraction and electro-absorption, finding that the predictions suggest longer-wave modulator designs will in many cases be different than those used in the telecom range.