Transition between cooperative emission regimes in giant perovskite nanocrystals

Interactions between emitters can create cooperative effects that alter light emission. In superfluorescence (SF), excited dipoles couple coherently and radiate collectively, requiring low energetic disorder and strong temporal coherence. Conversely, amplified spontaneous emission results from stimulated amplification and does not require temporal coherence but, unlike SF, sufficient propagation for optical gain. Caesium lead halide perovskite nanocrystals exhibit both amplified spontaneous emission (in disordered films) and SF (in ordered assemblies); however, the connections between these regimes remain unclear. Here we demonstrate that temperature and excitation density can drive the transition between both regimes in a thin film of giant CsPbBr₃ perovskite nanocrystals. At temperatures below 45 K, excitonic SF was observed, whereas above a transition range between 45 K and 100 K, amplified spontaneous emission prevails but requires increased optical excitation and emitter density. Our results work out the different collective effects present in lead halide perovskites, providing fundamental insights into cooperative phenomena and guidance for the development of compact and bright (quantum) light sources.