This article reports the achievement of lasing in a direct-bandgap GeSn alloy grown on silicon (Si), marking a significant step towards monolithic integration of silicon-based photonic circuits with complementary metal-oxide semiconductor (CMOS) technology. The GeSn alloy, composed of germanium (Ge) and tin (Sn), is a group IV semiconductor that exhibits a direct bandgap, a property crucial for efficient light emission. The study demonstrates that GeSn can be grown on Si without introducing mechanical strain, a challenge in previous attempts to achieve direct bandgap materials in group IV systems. The research team, led by S. Wirths and D. Buca, grew GeSn layers on Ge-buffered Si(001) substrates using chemical vapor deposition. The layers were partially relaxed, with thicknesses ranging from 200 to 560 nm. The Sn concentration in the GeSn layers was determined using Rutherford backscattering spectrometry and X-ray diffraction reciprocal space mapping. The study found that a GeSn layer with approximately 12.6% Sn concentration exhibited a fundamental direct bandgap, with the Γ-valley located 28 meV below the indirect L-valley. This was confirmed through temperature-dependent photoluminescence (PL) measurements, which showed a significant increase in PL intensity with decreasing temperature, indicating a direct bandgap semiconductor. The team also demonstrated optical gain and lasing in the GeSn layer. By exciting the GeSn layer with a 1064 nm laser pulse, they observed a threshold in emitted intensity, indicating laser action. The lasing was achieved in a Fabry-Perot cavity formed by a 1 mm long waveguide, with a threshold excitation density of approximately 325 kW/cm². The study highlights the potential of GeSn as a direct bandgap material for monolithic integration with silicon-based photonic circuits, offering a path towards more efficient and compact optoelectronic devices. The results suggest that GeSn could be a promising candidate for future silicon-based photonic and optoelectronic applications, particularly in areas such as optical interconnects and trace gas sensing. The study also discusses the challenges in achieving direct bandgap GeSn, including low Sn solubility in Ge and large lattice mismatch, and proposes strategies to overcome these challenges. The research provides a foundation for further development of GeSn-based optoelectronic devices, demonstrating the potential of group IV semiconductors in silicon photonics.This article reports the achievement of lasing in a direct-bandgap GeSn alloy grown on silicon (Si), marking a significant step towards monolithic integration of silicon-based photonic circuits with complementary metal-oxide semiconductor (CMOS) technology. The GeSn alloy, composed of germanium (Ge) and tin (Sn), is a group IV semiconductor that exhibits a direct bandgap, a property crucial for efficient light emission. The study demonstrates that GeSn can be grown on Si without introducing mechanical strain, a challenge in previous attempts to achieve direct bandgap materials in group IV systems. The research team, led by S. Wirths and D. Buca, grew GeSn layers on Ge-buffered Si(001) substrates using chemical vapor deposition. The layers were partially relaxed, with thicknesses ranging from 200 to 560 nm. The Sn concentration in the GeSn layers was determined using Rutherford backscattering spectrometry and X-ray diffraction reciprocal space mapping. The study found that a GeSn layer with approximately 12.6% Sn concentration exhibited a fundamental direct bandgap, with the Γ-valley located 28 meV below the indirect L-valley. This was confirmed through temperature-dependent photoluminescence (PL) measurements, which showed a significant increase in PL intensity with decreasing temperature, indicating a direct bandgap semiconductor. The team also demonstrated optical gain and lasing in the GeSn layer. By exciting the GeSn layer with a 1064 nm laser pulse, they observed a threshold in emitted intensity, indicating laser action. The lasing was achieved in a Fabry-Perot cavity formed by a 1 mm long waveguide, with a threshold excitation density of approximately 325 kW/cm². The study highlights the potential of GeSn as a direct bandgap material for monolithic integration with silicon-based photonic circuits, offering a path towards more efficient and compact optoelectronic devices. The results suggest that GeSn could be a promising candidate for future silicon-based photonic and optoelectronic applications, particularly in areas such as optical interconnects and trace gas sensing. The study also discusses the challenges in achieving direct bandgap GeSn, including low Sn solubility in Ge and large lattice mismatch, and proposes strategies to overcome these challenges. The research provides a foundation for further development of GeSn-based optoelectronic devices, demonstrating the potential of group IV semiconductors in silicon photonics.