Germanium tin (GeSn) is a group IV semiconductor with a direct band-to-band transition below 0.8 eV. Nonequilibrium GeSn alloys up to 20% Sn content were realized with low temperature (160°C) molecular beam epitaxy. Photodetectors and light emitting diodes (LEDs) were realized from in situ doped pin junctions in GeSn on Ge virtual substrates. The detection wavelength for infrared radiation was extended to 2 μm with clear potential for further extension into the mid-infrared. GeSn LEDs with Sn content of up to 4% exhibit light emission from the direct band transition, although GeSn with low Sn content is an indirect semiconductor. The photon emission energies span the region between 0.81 and 0.65 eV. Optical characterization techniques such as ellipsometry, in situ reflectometry, and Raman spectroscopy were used to monitor the Sn incorporation in GeSn epitaxy.