Stimulated Emission via Multi-Exciton Complexes in Colloidal 2D Materials

Two dimensional (2D) semiconductors or quantum wells grown by epitaxy have been intensely studied in the 1990’s. While these investigations showcased the huge potential of 2D excitons for opto-electronics, the need for cryogenic temperatures hampered many applications. In recent years, several new quantum well systems characterized by a low-screening dielectric environment, such as monolayers of CdSe or CsPbBr₃ have been developed. These materials feature a strongly enhanced exciton binding energy and sustain stable 2D excitons at room temperature. However, it remains unclear to what extend exciton-related transitions determine the opto-electronic properties – most notably stimulated emission – of these low-screening quantum wells, and how we can understand such processes for 2D excitons with in-plane mobility.<br/><br/>In this study, we address this issue taking high quality CdSe colloidal quantum wells (CQWs) as a model system for which we analyse stimulated emission using quantitative and polarization resolved pump-probe and luminescence spectroscopy. We give experimental proof that mobile excitons can form stable excitonic molecules – a two-exciton complex - in such quantum wells, even at room temperature. Next, we determined the absorption coefficient of the exciton → molecule transition at ≈10⁶ cm^-1, an exceptionally high number attesting to the huge potential of exciton-related transitions in low screening 2D materials for opto-electronics. Moreover, assuming stimulated emission through the molecule → exciton transition, we obtain for the first time a parameter-free description of the material gain spectrum of CdSe CQWs that matches the experimental gain spectrum and accounts for the high gain coefficients and the remarkable lack of gain saturation. The theoretical model is disruptively different from gain in 0D QDs and adds new insight into efficient optical gain development in 2D materials in general.<br/>Finally, we extend the study to (colloidal) perovskite quantum wells which show strong exciton-phonon coupling leading to a notion of 2D polarons, and show that the latter has a dramatic impact on the gain model and metrics, in particular leading to lower gain thresholds but smaller gain coefficients.