Modeling of lasing threshold in GeSn waveguides on silicon

Silicon photonic integrated circuits (PICs) could be used for many applications such as increasing internet routing bandwidth, enabling LIDAR for self-driving vehicles, and lab-on-a-chip biological and chemical sensors, but the creation of a Si-based laser has yet to be achieved. Si has an indirect band gap, so it cannot be used as a laser gain medium due to inefficient optical emission. GeSn alloys are grown on Si substrates, and GeSn has a band structure that allows for efficient optical emission in the infrared. Waveguides fabricated from GeSn allows have demonstrated lasing at cryogenic temperatures, but room temperature lasing has yet to be achieved in such devices.

In this work, we modeled the lasing threshold based on theoretical and empirical methods, and we explored its dependence on waveguide propagation loss, mirror reflection, Sn content, and recombination lifetime. The lasing threshold can be lowered through a variety of methods including material quality, device design, and fabrication techniques. Our results indicate that increasing the recombination lifetime through reduction of defects, which cause Shockley-Read-Hall nonradiative recombination, from 5 ns to 20 ns leads to a 70% reduction in the lasing threshold power. This indicates that room temperature lasing can be achieved through modest improvements in material quality. Further reductions in lasing threshold can be achieved through surface passivation and mirror design.