Topological lasing based on direct bandgap Ge1-xSnx photonic crystals

Ge₁₋ₓSnₓ alloys have shown great promise as direct bandgap materials enabling the seamless integration of laser emitters into Si photonics. While the performance of photonic devices has steadily improved, achieving efficient, low-threshold lasing — particularly in the MIR and in group IV material systems — remains challenging, as traditional cavity designs often face limitations in terms of confinement and defect tolerance. Topological photonics offers a promising, yet largely unexplored, solution in this context by enabling symmetry-protected modes and robust light confinement — an approach that has not yet been applied to CMOS-compatible laser platforms. These advances pave the way for compact, scalable, and energy-efficient laser systems that can be used in quantum technologies, photonic integrated circuits, and other applications. Based on these advances, we report on the design based on FEM calculations, fabrication, and characterization of a one-dimensional topological photonic crystal laser made of a direct bandgap Ge1-xSnx alloy with 10% Sn content. The structure of our device employs a carefully engineered one-dimensional photonic crystal nanobeam based on the Su-Schrieffer-Heeger model, in which confined lasing modes emerge at the interface between regions with distinct topological phases. Preliminary results indicate that exploiting the properties granted by topology enables our device to achieve strong optical confinement and lasing, suggesting the potential for improved performance compared to lasers based on trivial cavities. The use of Ge1-xSnx as the gain medium enables a low-threshold laser in the order of tens of kWcm-2, allowing cryogenic operation above the liquid nitrogen temperature. This approach, which addresses long-standing challenges in robust light manipulation, is a critical step toward practical integration of topological photonics into scalable device platforms.