Unraveling the Nature of Near to Short‐Wave Infrared Stimulated Emission from PbS Nanocrystals under Vanishing Quantum Confinement

Abstract Colloidal semiconducting nanocrystals (NCs) present exciting opportunities to realize small footprint integrated lasers and amplifiers and recent reports indicated near‐to short wave infrared optically pumped lasing is possible using lead sulfide (PbS) NCs. However, it remains unclear what are the photo‐physical driving forces behind the efficient optical gain. In this work, quantitative and broadband ultrafast infrared spectroscopy is employed on PbS NCs ranging from a regime of strong to vanishing confinement. Modeling the experimental findings, the importance of spectral redshifts induced by structural reorganization is pinpointed, opposed to commonly observed multi‐exciton Coulomb‐type interactions in NCs. Through such a rather uncommon mechanism for optical gain in weakly confined NCs, observed near‐unity occupation gain thresholds are rationalized, next to a a sizable material gain and long gain lifetimes ‐ limited only by bulk defects. Typical shelling with CdS is not able to further improve the gain lifetime, due to an increase in inhomogeneous broadening canceling the needed excited state spectral shifts. These results indicate that a push to relaxed confinement is probably the best route forward to achieve continuous wave infrared lasing in the near future, yet care must be taken to preserve the beneficial role of structural relaxation effects on the surface.