Scintillators, capable of converting X-/γ-ray to light, find widespread applications in medical diagnostics, industrial product inspection, security screening, and high-energy physics [1], [2]. Despite the significant success, the growing need of peeking deep into matters in different modalities calls for the revolution of current scintillators. Apart from the ongoing goal of reducing radiation dosage, entailing scintillators with mechanical flexibility and ultrahigh spatiotemporal resolution is now at the forefront [3], [4]. For example, if the scintillator is made flexible, three-dimensional (3D) imaging of curved objects can be enabled without the use of multi-angle scanning and algorithmic image reconstruction [5]. Besides, flexible scintillators perfectly fitting the arc-shaped detector of cone-beam computed tomography (CT) can effectively eliminate the notorious vignetting effect thereof. Moreover, in modern radiotherapy, the precise irradiation of target tissue without impairing its surroundings relies on real-time monitoring of location-dependent hard X-/γ-ray dose rate, which is, however, beyond the reach of current scintillators possessing mediocre spatiotemporal resolution [6]. This incapability partially stems from a longstanding dilemma that when scintillators are thickened to sufficiently absorb hard X-/γ-ray (e.g., milli- or centi-scale), the fidelity of the extracted optical signal (e.g., intensity and spatial resolution) drops as a result of optical attenuation and crosstalk. Such a phenomenon is associated with isotropic scintillation emission, self-absorption along the propagation path, and lateral light scattering, which is known as an inherent drawback for scintillator detectors [7].

To inhibit deleterious optical attenuation and crosstalk, it is necessary to carefully control the propagation of light within the scintillator. Unlike the X-ray-to-light conversion, which is primarily determined by the chemical composition and crystal structure, light propagation varies depending on the forms of the scintillator, whether it is in the form of a film, particle–matrix composite, single crystal, or pixelated array. The principles of light management are therefore specific to the material forms. Recent advancements in optical engineering, summarized in Table S1 (online), aim to improve the performance of scintillators. In brief, for non-pixelated scintillators such as film and particle–matrix composite, optical engineering approaches are all targeted at minimizing the light scattering by eliminating optical heterogeneity occurring at the scattering sources (pores, grain boundaries, particle/matrix interfaces, etc.) [8]. As such, the ceiling of these non-pixelated scintillators is always their bulk single crystal. Nonetheless, even in bulk single crystals, substantial optical loss and crosstalk arising from the isotropic light emission is still present. In this context, scintillators targeting high-resolution hard X-/γ-ray imaging are routinely entailed with a photonic structure, viz., pixelating and isolating the scintillator possessing a larger refractive index than that of the surroundings according to the total reflection principle (Fig. 1a). In this conventional design, certain portions of the scintillation light with an incidence angle larger than the critical angle are confined in the pixel scintillator, whereas the rest of the light escaping from the pixel scintillator cannot be managed and reused. Even so, light propagation confined in the pixel scintillator still suffers from light scattering and self-absorption, leading to additional optical crosstalk and loss. It seems that in an elaborate pixelated scintillator, the tradeoff between radiation absorption (thick scintillator) and scintillating property (thin scintillator) still has not been totally addressed yet, imposing a fundamental performance ceiling on the current hard X-/γ-ray scintillators. Another limitation of these conventionally pixelated scintillators is their mechanical rigidity, which limits their conformability to the arc-shaped detector of cone-beam CT systems and hence causes the vignetting effect. Up to now, there are no reports regarding the pixelated flexible scintillator.

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Fig. 1

Now writing in Nature Photonics, Yi et al. [9] break the limitations by integrating the scintillator with a tailor-made organic double-tapered optical fibre array (Fig. 1b–d), which not only makes the scintillator flexible but also improves the scintillation light usage. Instead of confining light within the scintillator, this brand-new paradigm blazes a new trail, that is, deliberately guiding the scintillating light from the scintillator itself to its nearby fibre by evanescent wave coupling. Benefiting from the non-absorbing and optically homogenous (light scattering-free) nature of the fibre, along with the double-tapered structure to further manage the light trajectory [10], the double-tapered fibre provides preferential and low-loss optical paths toward the underlying pixel detector (Fig. 1d). In their design, the tapered fibre adopts a core-cladding configuration, with refractive indices of 1.52 and 1.41 for the core and cladding segments, respectively. Then, CsPbBr3 nanocrystals with a refractive index of 1.82 are filled as an inverted tapered scintillator. Once irradiated, isotropic light emission takes place in the CsPbBr3 as same as the case in a typical scintillator. However, out of the ordinary, the light incident on the cladding at an incident angle within the escape-cone angle (θ) directly enters the fibre and propagates in terms of total internal reflection. In parallel, light trapped in CsPbBr3 can also re-enter the fibre via two processes in addition to scattering: (i) given the inverted tapered geometry of CsPbBr3, the incident angle at the cladding (φ) interface would gradually decrease to the escape-cone angle, thus the trapped photons re-enter into the fibre, as indicated by the blue arrow; (ii) due to the small stokes shift, CsPbBr3 reabsorbs the trapped photons and re-emits photons isotopically for multiple times. Such a photon recycling process randomizes the emission angle and opens avenues for the efficient collection of secondary photons [11], as indicated by the black arrow (Fig. 1d), laying the foundation of high-quality radiation imaging.

Following this concept, the authors set out to rationalize the tapered fibre design. Theoretical calculation based on geometrical optics indicates that the tapered fibre with an optically active surface extending along the fibre axis permits a relatively uniform and efficient light collection, differing vastly from the flat-cleaved fibres whose light collection efficiency decreases steeply along its depth. This agrees well with the experimental observation reported elsewhere [12], which is due to the fact that as the diameter increases along the depth of the tapered fibre, higher-order light propagation modes are progressively populated. Considering the benefit gained from fibre tapering, one may deem that a larger taper angle offers a higher light collection efficiency. In fact, increasing the taper angle leads to a side effect — a large-size bottom end that jeopardizes the spatial resolution of the imaging. By leveraging the diameter and length of the fibre, it is tailor-made into a double-tapered structure, with an upper taper angle of 7° for efficient light collection and a bottom taper angle of 4° to ensure high spatial resolution. Monte Carlo simulation by taking photon recycling of the perovskite into account shows that when such a double-tapered fibre array is embedded with CsPbBr3 having an internal quantum efficiency of 95%, more than 45% of the scintillation light can be collected and utilized. This results in orders of magnitude enhancement in light output with respect to CsPbBr3 scintillator without a fibre array, or simply with a flat-cleaved fibre array or single-tapered fibre array (Fig. 1e).

As a proof-of-concept, the authors realized the device fabrication through a high-precision 3D printing technique. It turns out that the fibre-enclosed scintillator exhibits a high light yield of 48,500 photons MeV−1, which doubles the record value reported for CsPbBr3 [13], again validating the enhanced luminescence output originating from the utilization of light escaped to the fibres. Besides, the scintillator (thickness: 150 μm) achieves a high spatial resolution of 22 lp mm−1, representing the state-of-the-art imaging capability of the perovskite array scintillators reported so far. Moreover, the spatial resolution remains as high as 5 lp mm−1 once the fibre-enclosed scintillator is thickened to 800 μm. It should be mentioned that such a remarkable resolution is 5 times higher than that with no fibres and far exceeds the commercialized pixelated CsI:Tl scintillator with a similar thickness [14]. More strikingly, with a further increase of thickness to 4 mm, the fibre-enclosed scintillator enables the visualization of gamma-ray spot patterns used in clinical medicine, which is in stark contrast to that without fibres (Fig. 1f).

Another key feature of this fibre-enclosed scintillator is that it can be made mechanically flexible. Notably, this is the first demonstration of a pixel-dense, flexible scintillator made to our knowledge, which is highly adaptable to different application scenarios, particularly for those requiring non-flat detectors. For instance, the scintillator detector can be reshaped into a hemispherical dome or the like. Considering that X-/γ-ray radiation is generally of an intrinsic cone-beam nature, only these curved detectors guarantee precise radiation detection by allowing the receiving of radiation with identical intensity over their active area (Fig. 1g). As exemplified by Yi et al. [9], compared to the fibre-enclosed planar scintillator, the hemispheric one offers over 3 times higher sensitivity than those of flat scintillators upon X-/γ-ray radiation. In fact, besides being perfect dosemeters, such flexible radiation detectors hold great promise in medical cone-beam computed tomography. Last but not least, the fibre-enclosed scintillator delivers almost unchanged radioluminescence intensity in the stability tests including changing the working temperature from −20 to 60 °C, continuous radiation exposure (30 Gyair) and four weeks’ storage. These results indicate that the fibre embedding technology does not have a detrimental effect on stability. In fact, the embedded fibres, made of PDMS and epoxy resin, which are commonly used in composite scintillators and as encapsulation materials for perovskite, may contribute to the favourable stability observed in the scintillator.

Overall, the pioneering design of a tapered-fibre-contained optical function device in conjunction with luminescent colloidal nanocrystals has not only extended the fabrication route for pixel-dense flexible scintillators but also provided fresh insights into the light management in scintillators. Inspired by such technological and theoretical achievements, research on further lifting the performance ceiling of scintillators via exploiting low-loss light propagation of the optical fibre will be flourishing soon. First and foremost, there is a need to comprehensively understand the interactions between photon recycling within the scintillator and photon outcoupling at the interface of the fibre. A valuable insight can be gained from perovskite light-emitting diodes, where photon recycling has been shown to contribute more than 70% of the emitted light through perovskite engineering and photonic structure texturing at the interface [15]. Therefore, extensive efforts are required to achieve comprehensive control over photon recycling and subsequent photon outcoupling in scintillators. Second, the challenges of large-area fabrication of these fibre-enclosed scintillators are waiting to be addressed. The current spin-coating technique used for perovskite deposition may not be scalable or suitable for large-scale production. It is anticipated that more scalable and patternable processes such as inject printing could be explored as alternatives. Third, the mechanical properties of inorganic scintillators and organic fibres differ from each other, and any detachment at the interfaces between the scintillator and fibre could significantly impact light propagation. Therefore, the stability of the integrated scintillator under bending conditions needs to be rigorously evaluated. Lastly, further validation of this pixelated and flexible fibre-enclosed scintillator in real-world applications is necessary. For instance, deploying this scintillator in cone-beam computed tomography and demonstrating its superior imaging performance compared to expensive pixelated inorganic ceramic/monocrystalline scintillators will be a revolutionary achievement.