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Collective optical properties can emerge from an ordered ensemble of emitters due to interactions between the individual units. Superlattices of halide perovskite nanocrystals exhibit collective light emission, influenced by dipole-dipole interactions between simultaneously excited nanocrystals. This coupling changes both the emission energy and rate compared to the emission of uncoupled nanocrystals. We demonstrate how quantum confinement governs the nature of the coupling between the nanocrystals in the ensemble. The extent of confinement is modified by controlling the nanocrystal size or by compositional control over the Bohr radius. In superlattices made of weakly confined nanocrystals, the collective emission is red-shifted with a faster emission rate, showing the key characteristics of superfluorescence. In contrast, the collective emission of stronger quantum-confined nanocrystals is blue-shifted with a slower emission rate. Both types of collective emission exhibit correlative multiphoton emission bursts, showing distinct photon bunching emission statistics. The quantum confinement changes the preferred alignment of transition dipoles within the nanocrystal and switches the relative dipole orientation between neighbors, resulting in opposite collective optical behaviors. Our results extend these collective effects to relatively high temperatures and provide a better understanding of exciton interactions and collective emission phenomena at the solid state.

Scintillation materials convert high-energy radiation to optical light through a complex multistage process. The last stage of the process is spontaneous light emission, which usually governs and limits the scintillator emission rate and light yield. For decades, scintillator research focused on developing faster-emitting materials or external photonic coatings for improving light yields. Here, we experimentally demonstrate a fundamentally different approach: enhancing the scintillation rate and yield via the Purcell effect, utilizing optical environment engineering to boost spontaneous emission. This enhancement is universally applicable to any scintillating material and dopant when the material's nanoscale geometry is engineered. We design a thin multilayer nanophotonic scintillator, demonstrating Purcell-enhanced scintillation with 50% enhancement in emission rate and 80% enhancement in light yield. The emission is robust to fabrication disorder, further highlighting its potential for x-ray applications. Our results show prospects for bridging nanophotonics and scintillator science toward reduced radiation dosage and increased resolution for high-energy particle detection.

Perovskite nanocrystal superlattices (NC SLs), made from millions of ordered crystals, support collective optoelectronic phenomena. Coupled NC emitters are highly sensitive to the structural and spectral inhomogeneities of the NC ensemble. Free electrons in scanning electron microscopy (SEM) are used to probe the cathodoluminescence (CL) properties of CsPbBr 3 SLs with a ∼20 nm spatial resolution. Correlated CL-SEM measurements allow for simultaneous characterization of structural and spectral heterogeneities of the SLs. Hyperspectral CL mapping shows multipole emissive domains within a single SL. Consistently, the edges of the SLs are blue-shifted relative to the central domain by up to 65 meV. We discover a relation between NC building block colloidal softness and the extent of the CL shift. Residual uniaxial compressive strains accompanying SL formation are contributors to these emission shifts. Therefore, precise control over the colloidal softness of the NC building blocks is critical for SL engineering.