Superfluorescent scintillation from coupled perovskite quantum dots

Scintillation, the process of converting high-energy radiation to detectable visible light, is pivotal in advanced technologies spanning from medical diagnostics to fundamental scientific research. Despite significant advancements toward faster and more efficient scintillators, there remains a fundamental limit arising from the intrinsic properties of scintillating materials. The scintillation process culminates in spontaneous emission of visible light, which is restricted in rate by the oscillator strength of individual emission centers. Here, we observe a novel collective emission phenomenon under X-ray excitation, breaking this limit and accelerating the emission. Our observation reveals that strong interactions between simultaneously excited coupled perovskite quantum dots can create collective radioluminescence. This effect is characterized by a spectral shift and an enhanced rate of emission, with an average lifetime of 230 ps, 14 times faster than their room temperature spontaneous emission. It has been established that such quantum dots exhibit superfluorescence under UV excitation. However, X-ray superfluorescence is inherently different, as each high-energy photon creates multiple synchronized excitation events, triggered by a photoelectron and resulting in even faster emission rates, a larger spectral shift, and a broader spectrum. This observation is consistent with a quantum-optical analysis explaining both the UV-driven and X-ray-driven effects. We use a Hanbury-Brown-Twiss g^(2) (τ) setup to analyze the temperature-dependent temporal response of these scintillators. Collective radioluminescence breaks the limit of scintillation lifetime based on spontaneous emission and could dramatically improve time-of-flight detector performance, introducing quantum enhancements to scintillation science.

💡 Research Summary

The paper reports the discovery of a collective radioluminescence phenomenon—superfluorescence—occurring in CsPbBr₃ perovskite quantum‑dot (QD) superlattices when excited by either ultraviolet (UV) light or hard X‑rays. By assembling monodisperse 8 nm QDs into three‑dimensional cubic close‑packed superlattices, the authors create a dense array of emitters whose dipole‑dipole interactions become significant at low temperature.

A two‑step theoretical framework is developed. First, Monte‑Carlo simulations model the cascade that follows absorption of an 8 keV X‑ray photon: a high‑energy photoelectron is generated, travels through the material, and creates multiple excitations in neighboring QDs with an average spacing of ~50 nm, yielding roughly two excited QDs per primary excitation. In contrast, a UV photon can only generate a single excitation because the QD bandgap (~2.45 eV) is far larger than the photon energy. Second, the emission dynamics of the excited QDs are described using a Lindblad master‑equation approach for a collection of two‑level systems coupled via dipole‑dipole interaction. The interaction strength J is set to 100 meV for UV‑driven cases and to a higher mean value of 150 meV with larger variance for X‑ray‑driven cases, reflecting the higher density of simultaneous excitations. This model predicts faster decay rates, larger red‑shifts, and broader spectra for X‑ray‑induced superfluorescence.

Experimentally, the authors probe the samples over a temperature range of 80–300 K using a Hanbury‑Brown‑Twiss (HBT) interferometer to obtain second‑order photon‑correlation functions g²(τ). Under X‑ray excitation at 80 K, the correlation data reveal two exponential components: a fast component with a lifetime of 0.24 ± 0.02 ns and a slower component of 0.74 ± 0.12 ns, corresponding respectively to the collective (red‑shifted) and uncoupled emission peaks observed in the spectra. At room temperature, only a single decay of 3.35 ± 0.07 ns is present, matching the spontaneous emission of isolated QDs. Thus, the collective emission is 14 × faster than the individual QD radiative lifetime.

Spectroscopic measurements show two distinct peaks. The uncoupled emission remains centered at ~2.45 eV (≈508 nm) for both excitation regimes. The collective emission appears as a red‑shifted band: under UV excitation the shift is modest (~60 meV), while under X‑ray excitation it can reach up to 320 meV. The X‑ray‑induced band is also noticeably broader, reflecting the larger variability in coupling strengths caused by multiple simultaneous excitations and structural disorder (domain boundaries, residual strain, angular misalignment). The relative weight of the collective component varies with temperature and sample quality, reaching up to 77 % in some X‑ray spectra at 80 K, whereas UV‑driven collective emission typically accounts for ~50 % of the total intensity.

The authors discuss the implications for scintillator technology. Conventional scintillators are limited by the oscillator strength of individual emitters, which caps the maximum emission rate. By harnessing superfluorescence, the radiative decay can be accelerated into the sub‑nanosecond regime, dramatically improving time‑of‑flight (TOF) resolution in positron‑emission tomography (PET), X‑ray computed tomography (CT), and high‑energy physics detectors where timing precision is paramount. Moreover, the work opens a new research direction—quantum‑enhanced scintillation—where collective quantum optical effects are deliberately engineered to surpass classical performance limits. Potential extensions include exploring other perovskite compositions, larger superlattice domains, and excitation with higher‑energy gamma photons to further increase excitation density and collective emission strength.

In summary, the paper provides the first experimental evidence that X‑ray excitation can trigger superfluorescence in perovskite QD superlattices, leading to a red‑shifted, spectrally broader, and dramatically faster emission component. The combined theoretical and experimental analysis convincingly attributes these effects to high‑density, synchronized excitations and enhanced dipole‑dipole coupling. This breakthrough suggests a pathway to ultrafast, bright scintillators with quantum‑level performance improvements, promising substantial impact across medical imaging, security scanning, and fundamental particle detection.