Spectral Overlap Limits Optical Gain in Quantum Heterostructure Photonic Devices

Population inversion in quantum heterostructures underpins light generation in a wide range of photonic devices, including quantum cascade lasers and intersubband emitters. It is commonly assumed that increasing carrier density enhances inversion and optical gain; however, this trend can reverse at elevated densities due to many‐body interactions. Here, we investigate this behavior using a controlled doping series in terahertz quantum cascade lasers combined with self‐consistent nonequilibrium Green's function simulations. We show that Coulomb‐induced quasiparticle broadening increases with carrier density and competes directly with the intrinsic energy separation of the active states, reducing the spectral selectivity of optical transitions. We introduce a dimensionless spectral‐overlap parameter, Γ avg /Δ, where Γ avg represents the interaction‐induced optical linewidth and Δ is the interlevel energy separation, and demonstrate that gain across distinct device architectures follows a common trend with a maximum near Γ avg /Δ ≈ 1. Beyond this point, spectral overlap suppresses inversion and limits optical gain. These results reveal a density‐driven crossover in which many‐body broadening sets a fundamental spectral limit on optical gain in quantum‐confined photonic systems. The Γ avg /Δ parameter provides a universal design metric for identifying inversion limits and optimizing performance in quantum heterostructure light‐emitting devices.