Collective excitations in perovskite cesium lead bromide (CsPbBr3) nanocrystals have garnered significant attention due to their ability to facilitate rapid and coherent emission, surpassing that of standard isolated nanocrystals. In 1954, Dicke formulated the theoretical framework for superradiance and superfluorescence, describing the correlated spontaneous emission of closely packed quantum emitters. Self-assembled perovskite superlattices have exhibited collective emergent phenomena characterized by fast, narrow, and coherent emission. However, the physical parameters that restrict this phenomenon are not yet fully understood, as all experiments demonstrating superfluorescence have utilized diffraction-limited laser sources and a broad temperature range. Here we show for the first time cathodoluminescence superfluorescence.

To address these limitations, we present a novel approach to investigating correlated phenomena with nanometer spatial resolution by utilizing a pulsed free electron beam to trigger superfluorescence in lead-halide perovskite superlattices. This method extends the limits of observation and offers new opportunities to study collective correlations with scanning electron microscope (SEM) resolution. We use a cathodoluminescence ultra-fast SEM (CL-USEM)

By controlling the electron-beam spot size, we observe two distinct emission regimes: (1) Defocused electron beams covering a large area (micron-scale) yield emission similar to conventional spontaneous emission in lead halide quantum dots (QDs). This emission arises from uncoupled and uncorrelated QDs that are independently excited due to the low electron density, resulting in interactions that are too distant to occur. (2) Focused pulsed beams on the superlattices within smaller areas (tens of nanometers scale) exhibit spectrally narrow, red-shifted, and fast emission, indicating superfluorescence from coherently coupled QDs. The transition from uncoupled to superfluorescent emission occurs when the electron density sufficiently excites several closely spaced QDs, well below the emitted wavelength.

Our findings provide insights into the fundamental properties of superfluorescence in perovskite cesium lead bromide nanocrystals and highlight the importance of understanding the dependencies on temperature, superlattice homogeneity, and coherence of the exciting source. This knowledge opens up possibilities for harnessing and optimizing collective optical phenomena in quantum materials for advanced applications in optoelectronics and quantum information processing.