We present a direct comparison of the optical gain in InGaAs quantum wells (QWs) and transition-metal dichalcogenide (TMD) monolayers (MLs), revealing a clear advantage of the latter in both material and modal gain. A central objective of this work is to quantify this gain advantage and to identify its microscopic origin using a state-of-the-art theoretical framework that combines first-principles electronic-structure calculations with many-body theory for the excited carriers. The superior gain performance of TMD MLs arises from their strongly enhanced interband Coulomb interaction. At the same time, the presence of nearby side valleys in the conduction band renders TMDs less favorable for maintaining excited carriers in optically active states. In contrast, the electronic band structure of InGaAs/GaAs QWs is inherently well suited for achieving population inversion. As a result, we find comparable transparency carrier densities in both systems, while the magnitude of the gain can be substantially larger in TMD MLs. Our analysis further shows that encapsulation of a ${\mathrm{WS}}_2$ ML in hBN mitigates the carrier drain into side valleys, thereby reducing the transparency carrier density compared with a ${\mathrm{WS}}_2$ ML on ${\mathrm{SiO}}_2$. A previously underexplored aspect is the Henry $\left(\alpha \right)$ factor, which governs the laser linewidth, chirp, and feedback tolerance. While InGaAs QWs exhibit unfavorably large $\alpha$ values near the transparency carrier density, ${\mathrm{WS}}_2$ encapsulated in hBN shows weakly negative values, indicating more favorable laser performance. Our results uncover a previously unrecognized potential of TMD MLs for laser applications and provide microscopically founded guidance for material choice, dielectric engineering, and device integration of ML-based gain media.

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• Excitons
• Gain properties of lasers
• 2-dimensional systems
• Lasers
• Transition metal dichalcogenides
• GW method
• Hierarchical equations of motion
• Many-body techniques