Effects of photonic crystal structures on the imaging ... - OSA Publishing

Letter

Vol. 43, No. 22 / 15 November 2018 / Optics Letters

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Effects of photonic crystal structures on the imaging properties of a ZnO:Ga image converter MENGXUAN XU,1 LIANG CHEN,2,* BO LIU,3 ZHICHAO ZHU,3 FENG HUANG,4 1,2,5 AND XIAOPING OUYANG

WEI ZHENG,4

CHAOHUI HE,1

1

School of Nuclear Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China State Key Laboratory of Intense Pulsed Radiation Simulation and Effect, and Radiation Detection Research Center, Northwest Institute of Nuclear Technology, Xi’an 710024, China 3 Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, China 4 School of Materials, State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, China 5 e-mail: [email protected] *Corresponding author: [email protected] 2

Received 14 September 2018; revised 21 October 2018; accepted 21 October 2018; posted 22 October 2018 (Doc. ID 341629); published 14 November 2018

We investigate the effects of photonic crystal structures on radiation imaging properties of a ZnO:Ga image converter. The results show that photonic crystal structures can regulate luminescence distribution and spatial resolving power, which is caused by the light extraction and the defect scattering of photonic crystal structures. The present investigation confirms that photonic crystals can improve the imaging properties of existing image converters and proposes a new coupling mode between the photonic crystal image converter and back-end optical devices, which is beneficial to the application of photonic crystals in the field of radiation imaging. © 2018 Optical Society of America https://doi.org/10.1364/OL.43.005647

As a kind of scintillator material, ZnO has been known for its sub-nanosecond ultra-fast response [1], which makes it an ideal image converter material for use in transient radiation imaging diagnostic systems [2]. In our previous work [3], ZnO:Ga single crystals were used in transient radiation imaging and as an image converter to diagnose a pulse X-ray radiation field, and high temporal resolution results were successfully obtained. However, due to the low scintillation efficiency of ZnO, the imaging quality needs to be improved. The most effective and direct method of improving imaging quality is to increase the scintillation efficiency of ZnO, and work has been reported on the scintillation efficiency optimization of ZnO [4–8]. However, the luminescence mechanism of pure and doped ZnO remains unclear [9], and directly regulating a material’s luminescence performance is difficult, which increases the difficulty of obtaining a ZnO image converter that meets our requirements in the short term. Photonic crystals [10] have been widely used in the fields of microwaves [11], optical communication [12], and optoelectronics [13]. In the field of radiation detection, researchers have 0146-9592/18/225647-04 Journal © 2018 Optical Society of America

prepared photonic crystal structures on various scintillators through different processes to enhance the output of light and have obtained good results [14,15]. Therefore, if the photonic crystal structures can be prepared on the surface of the existing ZnO image converter, they are expected to help remedy the shortcoming of low scintillation efficiency and to improve the imaging quality. In previous work, we obtained large-area photonic crystal structures on the surface of the scintillator and achieved good results in terms of light extraction [16–18]. In the scintillator application, there is no requirement for signal position information; however, in the image converter application, the position information is highly demanded. Although photonic crystals are effective in improving the light extraction, the influence of photonic crystals on the imaging properties of an image converter is rarely reported. Therefore, the effect of photonic crystals on the imaging properties of the ZnO image converter must be studied before practical application. In this Letter, photonic crystal structures were prepared by the self-assembly method on the surface of a ZnO:Ga image converter, and the effects of photonic crystal structures on imaging properties were studied in order to improve the performance of ZnO:Ga image converters in transient radiation imaging systems. ZnO:Ga single crystal samples were prepared by the hydrothermal method [19–21], as shown in Fig. 1(a). The sample sizes were 20 mm × 20 mm × 1 mm and 20 mm × 20 mm × 0.5 mm, and the carrier concentration of both was 1 × 1019 cm−3 . For comparison, we divided the two surfaces of the sample into four different regions: A1, A2, B1, and B2, as shown in Fig. 1(b). Among the four regions, only the A2 region was covered with the photonic crystal structures. The photonic crystal structures were prepared by the selfassembly method [18], and they consisted of a monolayer of polystyrene (PS) spheres with diameters of 402 nm. On the PS spheres, a conformal layer of TiO2 with a thickness of 50 nm was prepared by an atomic layer deposition system

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Fig. 3. Schematics of imaging property measurements.

Fig. 1. (a) Photographs of the ZnO:Ga samples with different thicknesses; (b) schematic illustration of different regions and surfaces of the samples; (c) and (d) SEM images of the photonic crystal structures with low and high resolution, respectively.

(SUNALE R-200) to ensure that the PS sphere structures were strong enough and to further enhance the extraction efficiency. Figures 1(c) and 1(d) show scanning electron microscope (SEM) images of the A2 region under different resolutions. The PS spheres form hexagonal periodic arrays, which are not completely ordered, and defects and overlaps still exist, which are inevitable in preparing large-area photonic crystals by the self-assembly method [22]. The X-ray excitation spectrum and the spectral integral results of two ZnO:Ga samples are shown in Figs. 2(a)–2(b). The signal intensity of the A2 (or B2) region was significantly higher than that of the A1 (or B1) region, and the spectral integral enhancement was in the range of 61%–87%, indicating the remarkable effect of the photonic crystal structures on light extraction enhancement. In addition, the spectral integral result of the 1 mm sample was slightly enhanced, compared with that of the 0.5 mm sample, which was caused by the increase of sensitive volume. The effect of photonic crystal structures on the imaging properties of the ZnO:Ga image converter was studied by measuring the luminescence distribution and the spatial resolving power in different regions under X-ray excitation. The setup of the experiment is shown in Fig. 3, where the X-ray

Fig. 2. X-ray excitation spectrum of (a) a 1 mm sample and (b) a 0.5 mm sample.

source (Moxtek, TUB000140, 40 kV, 200 μA) produces X-rays, and the produced X-rays pass through the aluminum sheet (thickness: 1 mm), which filters out the low-energy components. The ZnO:Ga sample was excited by the filtered X-ray, and the luminescence was recorded by the intensified chargecoupled device (ICCD) camera (Andor, DH34-18F-63). In the experiment, the luminescence distribution on the surface of the ZnO:Ga sample could be obtained when the ZnO:Ga sample was directly irradiated by the filtered X-ray; the spatial resolving power could be obtained when a tungsten resolution card (thickness: 10 mm) was placed in front of the ZnO:Ga sample [3], and the modulation transfer function (MTF) curves could be obtained by the edge-knife method. By comparing the results of the different regions, the effect of the photonic crystals on the imaging properties of a ZnO:Ga image converter could be evaluated. The luminescence distributions on both surfaces of the two ZnO:Ga samples are shown in Figs. 4(a)–4(d). In each figure, the sample surface can be divided into two regions according to the grayscale, and the boundary of the two regions is consistent

Fig. 4. Luminescence distribution on different surfaces of two samples: (a) 1 mm sample, Surface A; (b) 1 mm sample, Surface B; (c) 0.5 mm sample, Surface A; and (d) 0.5 mm sample, Surface B; grayscale range: 0–30,000.

Letter with the edge of the photonic crystal structures in Fig. 1(a). On both sides of the boundary, the higher grayscale side is the A2 (or B2) region, and the lower grayscale side is the A1 (or B1) region, indicating that the photonic crystal structures can enhance the signal intensity in radiation imaging. The enhancement is in the range of 59%–84%. In Figs. 4(a)–4(d), the luminescence distribution [3] of two samples can also be successfully obtained. In the 0.5 mm thick sample, the uniformity of the A2 (or B2) region is degraded relative to the A1 (or B1) region on both surfaces of the sample, indicating that the photonic crystal structures have a negative impact on the luminescence uniformity of the sample. The grayscale on A2 (or B2) in Figs. 4(c)–4(d) is significantly increased near the boundary; this is due to the imperfection caused by the preparation process (the self-assembly method), which makes the distribution of photonic crystals on the surface hard to control and further leads to the uneven distribution of luminescence. In contrast, the luminescence uniformity of the A2 (or B2) region of 1 mm is approximately 9%, which shows little change compared with the A1 (or B1) region. In particular, A2 has better uniformity of illumination relative to A1, which is because the photonic crystal structures increase the average grayscale, although the standard deviation of the surface grayscale is not improved to the same extent. These results show that the photonic crystal structures can regulate the luminescence uniformity of the image converter. We can improve the luminescence uniformity by improving the preparation process of the photonic crystal structures. The spatial resolving powers of two ZnO:Ga samples were evaluated. A 10 mm thick tungsten resolution card was placed in front of the ZnO:Ga samples, which caused the tungsten strips with a density of 2 lp/mm to be located at the junction of the two regions on the light output surface. The intuitive spatial resolving power results can be obtained by imaging the 2 lp/mm density tungsten strip area. The spatial resolving power of the two samples is shown in Figs. 5(a)–5(d), and the

Fig. 5. Spatial resolving power on different surfaces of two samples: (a) 1 mm sample, Surface A; (b) 1 mm sample, Surface B; (c) 0.5 mm sample, Surface A; (d) 0.5 mm sample, Surface B; grayscale range: 0–30,000.


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red box is the tungsten strip area with 2 lp/mm density. In the red box, the difference in spatial resolving power of different regions on the sample surface can be directly distinguished via the naked eye. In Figs. 5(a) and 5(c), the tungsten strip area can be well resolved in the A1 region, but the areas are blurred in the A2 region, thus causing a significant deterioration in imaging quality. In comparison, in Figs. 5(b) and 5(d), the tungsten strip area can be well resolved in the regions of B1 and B2. Figures 6(a) and 6(b) show the vertical grayscale distribution of the tungsten strip area in Figs. 5(a)–5(d), and the tungsten strips are represented by black dashed lines in Figs. 6(a) and 6(b). The height of each curve represents the signal intensity at that position, and the fluctuation in the tungsten strip area shows the spatial resolving power of the relative region: the more intense the fluctuation, the higher the spatial resolving power. In Figs. 6(a) and 6(b), the fluctuation of the A2 region is significantly smaller than that of the other three regions of the same crystal, indicating that the A2 region has the weakest spatial resolving power among the four regions. Compared with the results of the A1, B1, and B2 regions, no significant difference was observed in the degree of fluctuation, indicating that the three are not too different in spatial resolving power. The MTF curves obtained by the knife-edge method are shown in Figs. 6(c) and 6(d); the spatial resolution can be evaluated by comparing the MTF10 value (the spatial frequency corresponding to an MTF of 0.1) of different regions. Obviously, the MTF10 value of A2 region is less than 1, while the MTF10 values of other regions are greater than 2, indicating that the A2 region has the weakest spatial resolution among the four regions. This is also consistent with the conclusion in Figs. 6(a) and 6(b). In terms of signal intensity, it can be seen in Figs. 6(a) and 6(b) that the grayscale of the B2 region is much larger than that of the A1 and B1 regions, which is consistent with the results shown in Figs. 5(a)–5(d). Therefore, among the four regions, the A2 region cannot meet the application requirements in the spatial resolution, and the other three regions can meet the requirements. Moreover, the signal intensity of the B2 region is greatly improved, compared with the A1 and A2 regions because of the factor of photonic crystal structures, so it can be considered that

Fig. 6. Vertical grayscale distribution in the tungsten strip area and MTF curves of two samples: (a) 1 mm sample, vertical grayscale distribution; (b) 0.5 mm sample, vertical grayscale distribution; (c) 1 mm sample, MTF curves and (d) 0.5 mm sample, MTF curves.

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the B2 region has better imaging quality among the four regions. In summary, when using the backside of the photonic crystal structure surfaces as the light output face, the signal intensity can be greatly enhanced and the spatial resolving power has barely changed, so as to improve the imaging quality. Therefore, in practical imaging applications, a non-structural surface can be used to couple with the back-end optical devices to achieve better results, and this coupling is different from commonly reported coupling modes of photonic crystals used in the field of radiation detection [22,23]. In this Letter, the photonic crystal structures are not luminescent, and the thickness (approximately 500 nm) is far below the sample thickness (0.5 and 1 mm). Therefore, the photonic crystal structure layer does not affect the X-ray energy deposition process and the scintillation process in ZnO:Ga. However, the photonic crystal structure layer will affect the light transmission process. Ideal photonic crystal structures will have a certain modulation effect on the photon distribution but, in fact, the defects of the photonic crystal structure are inescapable, which will cause random scattering of photons and have a negative impact on the results. If the photons are emitted from the structural surface, the photons must pass through the photonic crystal structure layer to be recorded by the ICCD camera. In contrast, if the photons are emitted from the nonstructural surface, the influence of the above factors will be greatly reduced because of the self-absorption of ZnO:Ga [24]. Thus, the spatial resolution is significantly better than that of the structural surface. From a practical perspective, certain requirements are implied for the photonic crystal structures because of the large size of the image converter. The self-assembly method can easily obtain photonic crystal structures on a large sample. Compared with methods such as interference lithography, nanoimprinting, and electron beam lithography, the self-assembly method is simple in process and low in cost; therefore, it is more suitable for photonic crystal structure preparation on the large-size image converter surface. Although the surface consistency of photonic crystals produced by the self-assembly process is insufficient and has a certain influence on the imaging properties, it can be overcome by optimizing the self-assembly process and changing the coupling mode. In summary, we fabricated photonic crystal structures by the self-assembly method on the surface of a ZnO:Ga image converter and studied the effects of photonic crystals on the imaging properties of the ZnO:Ga image converter. Our results indicated that photonic crystals can enhance the light output and improve the imaging properties of ZnO:Ga image converters when using the non-structural surface to emit photons. The results also provided suggestions for the new coupling mode between a photonic crystal image converter and back-end optical devices, which is of great significance to the application of weak luminescence fast response image converters in transient radiation imaging. Our results explored the application of

Letter photonic crystals in the field of radiation imaging, which is also useful for image converters of materials other than ZnO:Ga. Funding. National Natural Science Foundation of China (NSFC) (11505140, 11435010, 11605140, 11574230).

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