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It is commonly assumed that increasing carrier density enhances inversion and optical gain; however, this trend can reverse at elevated densities due to many\u2010body interactions. Here, we investigate this behavior using a controlled doping series in terahertz quantum cascade lasers combined with self\u2010consistent nonequilibrium Green's function simulations. We show that Coulomb\u2010induced 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\u2010overlap parameter, \u0393\n                    <jats:sub>avg<\/jats:sub>\n                    \/\u0394, where \u0393\n                    <jats:sub>avg<\/jats:sub>\n                    represents the interaction\u2010induced optical linewidth and \u0394 is the interlevel energy separation, and demonstrate that gain across distinct device architectures follows a common trend with a maximum near \u0393\n                    <jats:sub>avg<\/jats:sub>\n                    \/\u0394\u00a0\u2248\u00a01. Beyond this point, spectral overlap suppresses inversion and limits optical gain. These results reveal a density\u2010driven crossover in which many\u2010body broadening sets a fundamental spectral limit on optical gain in quantum\u2010confined photonic systems. The \u0393\n                    <jats:sub>avg<\/jats:sub>\n                    \/\u0394 parameter provides a universal design metric for identifying inversion limits and optimizing performance in quantum heterostructure light\u2010emitting devices.\n                  <\/jats:p>","DOI":"10.1002\/nap2.70230","type":"journal-article","created":{"date-parts":[[2026,7,28]],"date-time":"2026-07-28T12:10:36Z","timestamp":1785240636000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Spectral Overlap Limits Optical Gain in Quantum Heterostructure Photonic Devices"],"prefix":"10.1002","volume":"15","author":[{"ORCID":"https:\/\/orcid.org\/0009-0002-7063-0860","authenticated-orcid":false,"given":"Nathalie","family":"Lander Gower","sequence":"first","affiliation":[{"name":"Faculty of Engineering and Institute of Nanotechnology and Advanced Materials Bar\u2010Ilan University  Ramat Gan Israel"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Muna","family":"Abedelrahman","sequence":"additional","affiliation":[{"name":"Department of Electrical and Electronic Engineering Ariel University  Ariel Israel"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Shiran","family":"Levy","sequence":"additional","affiliation":[{"name":"Faculty of Engineering and Institute of Nanotechnology and Advanced Materials Bar\u2010Ilan University  Ramat Gan Israel"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0001-6882-4642","authenticated-orcid":false,"given":"Mohammed","family":"Salih","sequence":"additional","affiliation":[{"name":"School of Electronic and Electrical Engineering University of Leeds  Leeds UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Maor","family":"Engel","sequence":"additional","affiliation":[{"name":"Department of Electrical and Electronic Engineering Ariel University  Ariel Israel"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0006-0931-5967","authenticated-orcid":false,"given":"Sumona","family":"Kumar","sequence":"additional","affiliation":[{"name":"School of Electronic and Electrical Engineering University of Leeds  Leeds UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5238-7409","authenticated-orcid":false,"given":"Sadhvikas J.","family":"Addamane","sequence":"additional","affiliation":[{"name":"Center for Integrated Nanotechnologies Sandia National Laboratories  Albuquerque New Mexico USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1987-4846","authenticated-orcid":false,"given":"A. 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