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Abstract
The low light-extraction efficiency of scintillators is due to total internal reflection and has led to the extensive use of photonic crystals to improve the light output. However, in some applications, photonic crystals cannot be fabricated directly on scintillators. Here, we demonstrate a promising method to improve the light output of scintillators by using a buffer layer coated with photonic crystals and then fixed to the scintillator. Through both numerical simulations and experiments, we investigate how the refractive indexes of the buffer layer and photonic crystal affect the light output from scintillators. The experimental results indicate that the light output of (Lu,Y)2SiO5:Ce scintillators is enhanced 1.9 times by using a sapphire buffer layer coated with an array of polystyrene nanospheres. This method can be used to improve the detection efficiency of radiation-detection systems when photonic crystals cannot be fabricated directly on the scintillator.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Introduction
Scintillators absorb the energy of high-energy particles and emit visible or near-visible scintillation radiation, which allows them to play an essential role in radiation-detection systems [1,2] used in high-energy physics [3–5], dark matter detection [6–8], and nuclear medical imaging [9–11]. The light output of scintillators, which is determined by the internal quantum efficiency and external quantum efficiency (also called the light-extraction efficiency), is a crucial factor affecting the detection efficiency and sensitivity of radiation-detection systems [12–14].
The refractive index of scintillators is usually high, leading to a low light-extraction efficiency because of total internal reflection. Given a scintillator refractive index ns=2, the approximate light-extraction efficiency is only 13%, which is calculated according to the first hit of the photon on flat surface of the scintillator with air coupling [15]. As a result, the scintillator light output is limited due to the low light-extraction efficiency, leading to low detection efficiency of radiation-detection systems.
In recent years, photonic crystals (PCs) have been widely used to improve the light-extraction efficiency of scintillators [16–20]. For instance, two-dimensional PCs fabricated by nanoimprinting have been used to improve the energy resolution of radiation-detection systems [21]. In previous work [22], PCs fabricated by hot embossing were used to improve the light-extraction efficiency of plastic scintillators. Methods to fabricate PCs have developed rapidly and now include nano-imprint lithography [23], electron-beam lithography [24], X-ray interference lithography [25], and self-assembly [26].
However, in some practical applications, PCs cannot be fabricated directly on scintillators. For example, in nuclear medical imaging, thin scintillators are used to improve the spatial resolution [27] and may be easily damaged by fabricating PCs directly on them. In high-energy physics, the large scintillators used to detect and identify radioactive sources [28] are too heavy to be spin-coated, so PCs cannot be fabricated directly on these scintillators. In these systems, the detection efficiency is limited due to the low light-extraction efficiency. Therefore, a different method is needed to improve the light extraction from scintillators when PCs cannot be fabricated directly on the scintillator.
To improve the light output of scintillators in systems that preclude the direct fabrication of PCs on the scintillator, we demonstrate herein a promising method that uses buffer layers coated with PCs. The PCs are first fabricated on a thin buffer layer, and then the buffer layer coated with PCs is packaged with the scintillator. In this way, the light extraction from scintillators can be improved by PCs fabricate ex situ. Because the PCs are not directly fabricated on the scintillators, the thickness and refractive index of the buffer layer must be accurately controlled to ensure the maximum enhancement of light extraction. In this work, we demonstrate how to enhance scintillator light extraction by using buffer layers coated with PCs. Numerical simulations and experiments are used to investigate how the refractive indexes of the buffer layer and PCs affect the light-extraction efficiency. The scintillator used in this work is (Lu,Y)2SiO5:Ce (LYSO), and the PCs are fabricated from self-assembled arrays of polystyrene (PS) nanospheres.
2. Experimental methods and numerical simulations
The LYSO scintillators were polished into 20 mm × 10 mm × 1 mm rectangular prisms. To study how the refractive index of the buffer layer affects the light output, we used silica and sapphire as buffer layer (polished into 20 mm × 10 mm × 0.1 mm rectangular prisms). The buffer layer should be sufficiently thin to prevent the scintillation light from escaping from the edge of the buffer layer. The refractive index of silica and sapphire are 1.46 and 1.76, respectively.
Figure 1 shows a schematic diagram of the fabrication process. PS nanospheres are first self-assembled on a silicon wafer, following which an array of PS nanospheres is transferred in water onto the buffer layer. Finally, the buffer layer coated with an array of PS nanospheres is placed on the LYSO surface with a thin layer of silicone oil at the interface. A thin clip fixes the buffer layer to the scintillator. In the experiments, we used PS nanospheres with diameters of 300, 400, or 500 nm. For self-assembled arrays of PS nanospheres, the nanosphere diameter equals the PC period.
Fig. 1. Schematic diagram showing fabrication process including self-assembling PS nanospheres on the buffer layer and packaging the buffer layer onto a LYSO scintillator.
The emission spectra were measured by using a fiber spectrometer (PG2000-Pro-EX, Ideaoptics Co.) and exciting the sample at 23° with respect to the normal with UV radiation (365 nm). This geometry prevented UV radiation from entering directly into the fiber spectrometer.
The numerical simulations were based on a rigorous coupled-wave analysis (RCWA) with the emission wavelength set to 420 nm (i.e., the peak of the LYSO emission spectrum). All of the simulation results were polarization-averaged to compare with experiment results. To facilitate the numerical simulations, the coupling layer and the scintillator area were neglected. The lateral extent of the buffer layer and PC was set to infinity for the simulation.
3. Results and discussion
Figure 2 shows a schematic diagram of the extraction of scintillation light. The light-extraction process can be divided into two steps: First, scintillation light couples into the buffer layer. In this step, the difference between the buffer layer refractive index nb and the scintillator refractive index ns is the main factor determining the coupling efficiency. Second, scintillation light in the buffer layer is extracted to air through the PC.
Fig. 2. Schematic diagram showing light-extraction from scintillator. First, scintillation light is coupled into buffer layer. Second, the scintillation light is extracted through the PC.
Figure 3 shows scanning electron microscopy (SEM) images of the top surface of arrays of PS nanospheres. The nanospheres are self-assembled on the buffer layer and the nanosphere diameters are approximately 337, 400, and 500 nm in Figs. 3(a)–3(c), respectively.
Fig. 3. SEM images of top surfaces of PS-nanosphere arrays. The diameters of the PS nanospheres are (a) 337 nm, (b) 400 nm, and (c) 500 nm.
To analyze how the buffer layer and PC affect the light-extraction efficiency, we simulate the transmission of scintillation light from LYSO at different emission angles with different buffer layers and PCs (see results in Fig. 4). The total-internal-reflection angle is always 33.1° [asin(1/ns)], which is independent of nb. Thus, we can divide each figure into two regions with the total-internal-reflection angle as boundary. Transmission peaks appear beyond the total-internal-reflection angle, which is indicative of the PC affecting the light extraction. When nb is small, some of the transmission peaks disappear, which suggests that a fraction of the scintillation light from LYSO is totally reflected by the buffer layer. Once the transmission peaks appear, they remain independent of further change in nb.
Fig. 4. Calculated extraction of light from LYSO as a function of emission angle and buffer-layer index of refraction. Emission angle measured with respect to surface normal in Γ-Κ plane. The PC-array periods are (a) 300 nm, (b) 400 nm, (c) 500 nm, and (d) 600 nm.
As the period of the PC increases, the number of transmission peaks increases, and the height of the transmission peaks decreases, which suggests that the light-extraction efficiency depends on the PC. In previous work [17], the whispering gallery modes of the PC proved beneficial for extracting the scintillation light. As the period of the PC increases, the number of whispering gallery modes within the PC increases and the coupling efficiency decreases.
In contrast with the region beyond the total-internal-reflection angle, the transmittance of the scintillation light first increases and then decreases as nb increases. When nb = ns, the transmittance reaches its maximum, which indicates that the transmittance within the total-internal-reflection region is mainly determined by Snell’s law. The greater the difference nb − ns, the lower the transmittance. In addition, transmission dips appear within the total-internal-reflection angle, which means that a fraction of the scintillation light is reflected by the PC back into the LYSO. This light may be reflected back toward the buffer layer again by the bottom interface of LYSO, giving it another chance for extraction. As the period of PC array increases, the number of transmission dips increases, as does the magnitude of the transmission dips.
To directly observe a variation in light-extraction efficiency, we calculate the total extraction efficiency (see Fig. 5). The light extraction efficiency reaches a maximum for the PC-array period of 400 nm, which is close to the simulated wavelength of the scintillation light (420 nm). This result is consistent with the results of previous work [16], which showed maximum light-extraction efficiency when the period of the PC array is roughly equal to the wavelength of the scintillation light. Consistent with the discussion above, the light-extraction efficiency is best when nb = 1.8.
Fig. 5. Calculated total light-extraction efficiency for LYSO with various buffer layers coated with PC. The PC periods are given in the legend.
Figures 6(a)–6(c) show the emission spectra of LYSO with buffer layers of different materials coated with PCs. At first glance, the light output is improved for LYSO for all PC-coated buffer layers, which suggests that any buffer layer coated with a PC can enhance the scintillator light-extraction efficiency. Upon closer inspection, the sapphire buffer layer leads to better light-extraction efficiency than the silica buffer layer, which is consistent with the simulation results. The larger difference in refractive index between silica and LYSO lowers the coupling efficiency of light from scintillator to buffer layer, thereby reducing the light-extraction efficiency. For a PC array with a 400 nm period, the light output from LYSO with a sapphire buffer layer is maximal at 1.9 times the light output of LYSO alone. The profile of the emission-enhanced spectrum of LYSO with the sapphire buffer layer is consistent with that obtained with a silica buffer layer, which means that the profile of the emission-enhanced spectrum is determined by the PC period rather than by the refractive index of the buffer layer. In addition, a fraction of the scintillation light escapes from the side of the buffer layer in experiments, which is not considered in the numerical simulations. Consequently, the experimental light-extraction efficiency is less than in the simulation.
Fig. 6. Emission spectra of LYSO with different buffer layers coated with PCs and excited in the UV (365 nm). The PCs periods are (a) 337 nm, (b) 400 nm, and (c) 500 nm. (d) Angular distribution of relative light-emission efficiency at 420 nm from LYSO with different buffer layers coated with PCs with a period of 400 nm.
Figure 6(d) presents the emission at 420 nm for LYSO as a function of emission angle and for different buffer layers coated with PCs with a period of 400 nm. The normalized profiles of the angular-dependent emission of LYSO with different buffer layers are basically the same, which suggests that the introduction of the buffer layer and PCs does not affect the angular dependence of the emission. Note that, in experiments, the PCs consist of self-assembled arrays of PS spheres, which have imperfect periodicity over the area of the scintillators that slightly affects the angular dependence of the emission. However, the light output of LYSO with a buffer layer is slightly less than that of LYSO with no buffer layer because a fraction of the scintillation light escapes from the side of the buffer layer. Note that the buffer layer is only used when PCs cannot be fabricated directly on the scintillator.
4. Conclusion
In summary, we demonstrate in this work a promising method to improve the light output of scintillators on which PCs cannot be directly fabricated. The approach consists of using a buffer layer pre-coated with a PC and subsequently fixed onto the scintillator. Through numerical simulations and experiments, we investigate how the refractive indexes of the buffer layer and PC affect the light-extraction efficiency. The results reveal a 1.9 times enhancement in light extraction from LYSO with a sapphire buffer layer coated with self-assembled, 400-nm-diameter PS nanospheres.
The improved light extraction from scintillators should improve sensitivity and detection efficiency in radiation-detection systems. This method can be used to improve the light output of luminescence materials even when the PC microstructure cannot be directly fabricated on the luminescent material.
Funding
National Key Research and Development Program of China (2016YFA0301101); National Natural Science Foundation of China (11804252, 11975168).
Disclosures
The authors declare no conflicts of interest.
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