  Silicon photonics in the near-Infra-Red, up to 1.6 um, is already one of key
technologies in optical data communications, particularly short-range. It is
also being prospected for applications in quantum computing, artificial
intelligence, optical signal processing, where complex photonic integration is
to be combined with large-volume fabrication. However, silicon photonics does
not yet cover a large portion of applications in the mid-IR. In the 2 to 5 um
wavelength range, environmental sensing, life sensing, and security all rely on
optical signatures of molecular vibrations to identify complex individual
chemical species. The markets for such analysis are huge and constantly
growing, with a push for sensitivity, specificity, compactness, low-power
operation and low cost. An all-group-IV, CMOS-compatible mid-IR integrated
photonic platform would be a key enabler in this wavelength range. As for other
wavelengths, such a platform should be complete with low-loss guided
interconnects, detectors, modulators, eventually, and most importantly
efficient and integrated light sources. This chapter reviews recent
developments in the fields of mid-IR silicon-compatible optically and
electrically pumped lasers, light emitting diodes and photodetectors based on
Ge, GeSn and SiGeSn alloys. It contains insights into the fundamentals of these
developments, including band structure modelling, material growth and
processing techniques.
