Elucidating the photophysics behind the stimulated emission processes in CsPbBr

\nThanks to their excellent photoluminescence quantum yields, their facile and low-cost production, and their processing versatility, CsPbBr3 perovskite nanocrystals (NCs) stand out as excellent candidates to implement light-emitting devices. Elucidating their stimulated emission mechanisms is fundamental to achieve much more efficient and versatile perovskite lasers. In particular, two questions remain open: why the Amplified Spontaneous Emission (ASE) band is significantly shifted from the fluorescence one, and why the former seems to suddenly emerge from, and coexist with, the latter. These characteristic features have led to a debate, which is not settled yet, on which is the mechanism behind the ASE band shift. In this communication, we try to settle this debate and address these questions through experimental ASE measurements combined with numerical simulations. We show that the ASE behaviour in CsPbBr3 NCs thin films stems from a combination of reabsorption, excited state absorption, excitation of differently polarized waveguide modes, and the coexistence of short- and long-lived localized single excitons. The results in this work help understanding the stimulated emission mechanisms in perovskites and provide insightful information on research avenues to increase the efficiency of the light-emitting devices based on these materials.\n