Modeling spontaneous and stimulated emission in GeSn waveguides on silicon

GeSn alloys grown on Si are attractive a possible gain medium for Si-based infrared lasers. GeSn waveguides have recently been shown to exhibit lasing at low temperatures, but room temperature lasing has not been achieved. In this work, we analyze the experimental results of optical emission from n-type GeSn (5-6% Sn) waveguides under optical pumping at room temperature. The data indicate that lasing is not achieved but that optical gain leads to a nonlinear increase in output power with respect to pump power. We have developed a theoretical model for the optical gain in the GeSn material and the emission spectrum and power dependence of the waveguides that includes both spontaneous and stimulated emission. When compared to our experimental results, it shows that stimulated emission plays a role in the waveguide emission, and that the it was just below the lasing threshold. We validated this model by reproducing experimental data from the literature where lasing was observed at low temperature in undoped GeSn (12% Sn) waveguides, and our model’s predictions for the lasing threshold temperature and spectral properties of the emission agree with the experimental data.