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Abstract
A novel and effective simultaneous recording method, to the best of our knowledge, is proposed for improving the diffraction efficiency and uniformity of full-color holographic optical elements (HOE) using the Bayfol HX102 photopolymer. To improve the diffraction efficiency of a full-color HOE, it is important to find the optimal recording beam intensity taking into account the initial and late responses of the medium. The range of optimal beam intensity for recording full-color HOE can be found experimentally by analyzing the inhibition period and response characteristics of the recording medium for three wavelengths. Through this method, a full-color HOE with an average diffraction efficiency of about 56.81% and a standard deviation of about 1.7% was implemented in a single layer photopolymer.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
There are various types of holographic recording media, such as photopolymers, silver halides, photoresists, photorefractive crystals, and dichromated gelatin [1]. Photopolymers are preferred for holographic recording because the post-treatment process after HOE recording is relatively simple. The post-processing of photopolymers can be carried out with UV exposure and does not require complex chemical treatment. Thus, photopolymers have been used as a recording medium for various holographic applications such as optical filters [2], optical elements [3], polarizing elements [4–6], and head-mounted display (HMD) [7–16]. Photopolymers [17,18] are mainly composed of polymer binders, monomers, photo-initiators, and dyes. And photopolymers mainly exhibit two different response characteristics during holographic recording. Initially, refractive index modulation does not occur in the photopolymer even after the dye absorbs light that can interact with the photo-initiator. Hence, no response is observed in the initial state. After a period of time (inhibition period), actual refractive index modulation occurs in the photopolymer as a result of the photochemical reaction induced by the photo-initiator. Here, the inhibition period refers to the time between the moment of exposure and the occurrence of the refractive index modulation of the medium. The initial response time of the medium tends to be shorter when it is illuminated by a specific wavelength for which it has high absorption or when it is subjected to a high intensity recording beam. Due to this property of the photopolymer, the characteristics of the inhibition period [19–21] may vary depending on the wavelength and the intensity of the recording beam. Most of the photopolymers have the property of an inhibition period, but not all photopolymers. The Bayfol HX102 photopolymer [22,23], used here, contains three photosensitive dyes which have different sensitivity to red (633 nm), green (532 nm), and blue (473 nm) wavelengths. In the implementation of full-color HOE for HMD applications, since the sensitivity of the recording medium is different for each wavelength, more complicated recording conditions are required compared to single wavelength recording in order to obtain uniform and high diffraction efficiency. Thus, wavelength multiplexing and multi-layer materials [24,25] were applied in the initial full-color hologram implementation. Thereafter, a single-layer of silver halide [26] and a photopolymer [27] were used for the full-color implementation. Piao et al. [11] proposed a sequential exposure recording method based on time-scheduled iterative exposure. This technique improves the diffraction efficiency of the reflective and transmissive type of full-color HOE [28]. Here, the reflective full-color HOE with an average diffraction efficiency of about 31% was implemented based on time-scheduled iterative exposures using the Bayfol HX photopolymer [11]. Using the same photopolymer, the average diffraction efficiency of about 12.2% (in case of simultaneous exposure) and 20.6% (in the case of sequential exposure) were achieved by reflective full-color HOE [29]. However, the medium in the sequential exposure method needs repeated exposure for a short time based on the recording order of each wavelength. The long total recording time and the complicated recording process are the disadvantages of this method. To overcome these problems and to improve the diffraction efficiency of full-color HOE, we analyzed the inhibitory properties of the initial response and the optical characteristics of the late response of the recording medium for each wavelength. Based on this analysis, we propose a new method using the inhibition characteristics of the medium for each wavelength. This paper introduces a new technique based on the three wavelengths simultaneous recording method. This approach utilizes the inhibition period characteristics of the recording medium to improve the diffraction efficiency and color uniformity of full-color HOE. The experiments reveal that the diffraction efficiency and uniformity of full-color HOE have been improved.
2. Proposed method to improve the diffraction efficiency of a full-color HOE
According to Kogelink's coupled-wave theory [30], the most important factor contributing to diffraction efficiency is the refractive index modulation value (Δn) of the recording medium. The number of recording wavelengths, recording beam intensity, exposure energy, and recording angle are major external factors that can affect the refractive index modulation of the medium. And, the diffraction efficiency of the recorded reflective diffraction grating is calculated by the following Eq. (1).
where Id is the intensity of the diffraction beam, and It is the intensity of the transmission beam. The absorption and reflection of the medium and the glass substrate are not considered. The photosensitizer component of the photopolymer absorbs light. Therefore, after the recording, the photopolymer is exposed to a halogen (or UV) lamp to remove the photosensitizer component remaining in the medium. The Bragg angle shift of the diffraction grating is caused by the shrinkage of the recording medium due to the refractive index modulation. To compensate for this, the maximum diffraction efficiency is measured at a position where the Bragg angle is corrected by rotating the HOE mounted on the rotation motor stage.Due to the limitation of the amount of the reactive substance in the photopolymer, it is mandatory to homogeneous control the response characteristics of the recording medium. So that, it uniformly reacts at three wavelengths during the same exposure time to enhance the diffraction efficiency of full-color HOE. By matching the inhibition periods of the three wavelengths, the response characteristics of recording media having different sensitivities for each wavelength can be significantly improved. For the improvement of the diffraction efficiency of full-color HOE, the presented method has 3-steps based on the analysis of the inhibition period characteristics.
The proposed method consists of three steps as shown in Fig. 1.
Step 1: The basic optical characteristics of the recording medium for each wavelength are first studied. The diffraction efficiencies and inhibition period characteristics for different recording angles and beam intensities are measured for each wavelength. The recording of the diffraction grating is repeated while increasing or decreasing the beam intensity by a predetermined ratio of about 25–50% to study the optical properties of the recording medium. Recording conditions for which the inhibition period is less than 1 s or the maximum diffraction efficiency for each wavelength is less than 90% are excluded as it cannot improve the diffraction efficiency of the full-color HOE. The recording beam intensity is considered as very high for the inhibition period of less than 1 s. It is quite challenging to uniformly improve the diffraction efficiency for each wavelength because the actual refractive modulation is saturated within a few seconds when recording full-color HOE using a recording beam intensity having an inhibition period of less than 1 s for each wavelength. The probability of improving the average diffraction efficiency of a full-color HOE increase, when recording conditions with a higher diffraction efficiency for each wavelength are applied. Therefore, it is worth improving the average diffraction efficiency of full-color HOE, except for the recording beam intensity where the diffraction efficiency for each wavelength is less than 90%.
Step 2: In this step, the optimal intensity range is determined by analyzing and overlapping characteristic graphs of the inhibition period based on the data measured in step 1. This helps to find the conditions where the diffraction beam intensity increases even after exposure to the average saturation energy of each wavelength and the maximum diffraction efficiency is 90% or more. Here, the variation of the inhibition period with the recording beam intensity for each wavelength is plotted.
Step 3: Full-color HOE recording is performed by applying an arbitrary beam intensity within the optimal intensity range obtained in Step 2. The diffraction efficiencies of the full-color HOE for each wavelength are measured and compared to find the wavelength corresponding to the highest diffraction efficiency. If the difference in diffraction efficiency of each wavelength is 5% or more, only the intensity of the wavelength corresponding to the highest diffraction efficiency is reduced by about 5–15%, and then a full-color HOE recording process is repeated. This is done to find the optimal intensity for recording a full-color HOE where the difference between the maximum and minimum diffraction efficiencies for each wavelength is less than 5%. In other words, more uniform diffraction efficiency can be achieved for the three wavelengths by fine tuning the recording beam based on the optimal intensity range.
3. Experiments and results
3.1 Variation of diffraction efficiency with recording angle and beam intensity
Three lasers were used as light sources for recording the reflective full-color HOE. The lasers used are as follows: 473 nm (Cobolt-50 mW), 532 nm (Cobolt-150 mW), and 633 nm (JDSU-21 mW). Figure 2 shows the optical recording system for a full-color HOE. A combination of two dichroic filters and one mirror is used to provide a single beam path for the three laser beams. The red/green/blue recording beams are completely co-linear to be precisely aligned optical components. To independently control the beam intensity and intensity ratio for each wavelength, a half-wave plate (HWP), an attenuator, and a polarizing beam splitter (PBS) are introduced into the laser beam path. In addition, the intensity and intensity ratio is controlled in the S polarized state by introducing a broadband HWP into the beam path of the light exiting the PBS. An electrical shutter [31–33] and an optical detector (Newport optical power meter 1830-C) are used to measure the change in diffraction beam intensity in real-time without being disturbed by the probe beam [34,35] while the diffraction grating is recorded on the photopolymer (Bayfol HX102). The shutter method was applied to measure the intensity change of the diffracted beam according to every recording condition and to analyze the optical characteristics of the recording medium. The diffraction efficiency is calculated by measuring the intensity of the diffracted and transmitted beam after HOE recording and UV (or halogen lamp) exposure. In other words, the diffraction efficiency is not measured in real-time, but it is calculated from the diffracted and transmitted beams measured in non-real time. The diameter of the full-color HOE is about 18 mm, which is controlled by using two irises. After the full-color HOE recording, the photopolymer is exposed for 5 minutes to light from a halogen lamp (OLYMPUS LG-PS2 100 W), which was placed at a distance of 5 cm from the photopolymer. The light from the halogen lamp reacts with the residual photosensitizer and fixes the plane wave diffraction grating of the full-color HOE.
Figure 3(a) illustrates the change in diffraction efficiency in reflection symmetric (θR=θS=θ/2) and asymmetric (θR=0°, θS=θ) recording structures for the photopolymer with the recording angle. The diffraction efficiency is computed using the measured intensity of the diffracted and transmitted beam after HOE recording and UV (or halogen lamp) exposure. The diffraction efficiency measurement of HOE is recorded by the optimal recording time measured using the shutter method to the continuous exposure recording method. Two steps are applied to confirm the correlation between the recording angle and the diffraction efficiency. Firstly, a 532 nm wavelength laser was set as the recording beam to represent the three wavelengths in the basic optical experiment. Secondly, we applied a continuous exposure time of 7 s at the green wavelength R (reference beam) = S (signal beam) =1 mW/cm2 beam intensity based on the optimal exposure time obtained through the shutter method. The diffraction efficiencies of the recorded HOE in the symmetric structures were found to be slightly higher than that of the HOE in the asymmetric structures. However, the trend of the variation of the diffraction efficiency with the recording angle was similar for both structures. In addition, the highest diffraction efficiencies were measured at a recording angle of 30° for both structures. Here, the reflection symmetry and asymmetry angles of 30° represent the recording angles of the two beams in the air (θR=θS=15°and θR=0°, θS=30°). These results confirm that the angle of the recording beam influences the diffraction efficiency and refractive index modulation of the medium. Based on the results of Fig. 3(a), in the subsequent full-color HOE experiments, the recording angle was fixed at 30° of reflection symmetry. Figure 3(b) shows the variation of diffraction efficiencies of the medium with the recording beam intensity, when the intensity of both R (reference beam) and S (signal beam) are approximately 0.1, 0.125, 0.25, 0.5, and 1 mW/cm2.
Fig. 3. Diffraction efficiency as a function of (a) recording angle (for 532 nm) for symmetric and asymmetric structures, and (b) recording beam intensity (for 473 nm/ 532 nm/ 633 nm).
The results show that the diffraction efficiency curves of the 532 nm and 633 nm wavelengths are almost similar. Diffraction efficiencies of approximately 93–96% are measured for the 532 nm and 633 nm wavelengths, irrespective of the change in the recording beam intensity. For the beam of 473 nm wavelength, the diffraction efficiency sharply decreases to less than 90% when the recording beam intensity is less than 0.25 mW/cm2. These results show that applying a beam intensity of less than 0.25 mW/cm2 at 473 nm wavelength is disadvantageous for improving the diffraction efficiency of the reflective full-color HOE.
3.2 Measurement of the inhibition period for three wavelengths
In these experiments, the exposure time for recording the HOE and the detection period for measuring the diffracted beam intensity are applied differently according to the recording beam intensity of each wavelength. The electrical shutter was applied instead of a probe beam to measure the real-time diffraction beam intensity during HOE recording because the Bayfol HX102 photopolymer reacts in all three wavelength ranges. The inhibition period and the diffraction beam intensity of each wavelength were measured in a period of 1–2 s depending on the recording beam intensity. The closing time of the periodic shutter in the signal beam path for measuring the diffracted beam was fixed at 0.45 seconds.
An optimized shutter closing time of 0.45 seconds is applied in the optical recording system considering the response speed of the optical detector and the communication speed between the shutter and computer. As mentioned earlier, all HOEs for diffraction efficiency measurements are recorded using the optimal recording time measured and the shutter method to the continuous exposure recording method, so the shutter closing time of 0.45 seconds does not affect the diffraction efficiency of the HOE at all. Based on our experience, there is no difference in the diffraction efficiency of HOE as a result of many previous tests using the shutter method and continuous exposure recording method. The HOE recording system has been implemented to detect only the diffracted beam intensity while the shutter is closed according to the detection cycle. Figure 4 shows the results of the relation between the inhibition period and the diffraction beam intensity for different recording beam intensities at three wavelengths. For all three wavelengths, the inhibition periods were measured when the reference beam intensity and the signal beam intensity were 0.1, 0.25, 0.5, and 1 mW/cm2, respectively. The inhibition periods at a wavelength of 473 nm were measured to be about 30 s, 10 s, 4 s, and 3 s for recording beam intensities of 0.1, 0.25, 0.5, and 1 mW/cm2, respectively. At the same recording beam intensities, the inhibition periods at wavelengths of 532 nm and 633 nm were measured to be approximately 14 s, 6 s, 2 s, and 1 s and 10 s, 4 s, 2 s, and 1 s, respectively. The inhibition dosages were measured to be about 4–6 mJ/cm2, 2–3 mJ/cm2, and 2 mJ/cm2 at wavelengths of 473 nm, 532 nm, and 633 nm, respectively. However, it can be seen that after the inhibition period, the response of the diffracted beam intensity due to the refractive index modulation is faster at 532 nm than at 473 nm and 633 nm. When the intensities of the recording beams at the wavelengths of 473 nm and 532 nm were 0.5 and 1 mW/cm2, respectively, the diffraction beam intensities showed the saturation and reduction characteristics at an exposure energy of approximately 16 mJ/cm2. However, at 633 nm wavelength, the diffraction beam intensity increased at the exposure energy of approximately 16 mJ/cm2 for the full range of recording beam intensity. In contrast, the diffraction beam intensity showed a continuously increasing trend around the exposure energy of approximately 16 mJ/cm2 at the recording intensities of 0.1 and 0.25 mW/cm2 for the three wavelengths. Therefore, in the case of the Bayfol HX102 photopolymer, the intensity ranges of the recording beams causes the diffraction beam intensity to increase or saturate around the average exposure energy of approximately 14–16 mJ/cm2 at 473 nm, 532 nm, and 633 nm wavelengths were 0.1–0.5 mW/cm2, 0.1–0.5 mW/cm2, and 0.1–1 mW/cm2, respectively. In summary, the inhibition periods were shortest at the wavelength of 633 nm, and the response after the inhibition periods were fastest at the wavelength of 532 nm. Also, as the recording beam intensity increased, the inhibition periods for the 532 nm and 633 nm wavelengths became similar. The inhibition periods were the longest at the 473 nm wavelength.
Fig. 4. The diffraction beam intensity as a function of exposure time for the three wavelengths, when recording beam intensity is (a) R = S=0.1 mW/cm2. (b) R = S=0.25 W/cm2. (c) R = S=0.5 mW/ cm2. (d) R = S=1 mW/cm2.
Hence, the intensity of the recording beams of all three wavelengths can be in the range from 0.1 to 0.5 mW/cm2 for achieving optimal diffraction efficiency. However, it can be seen from the experimental results of the Fig. 3(b) that excluding the recording beam intensities of less than 0.25 mW/cm2 at 473 nm is advantageous in improving the diffraction efficiency of full-color HOE.
3.3 Decision of the optimal beam intensity range for three wavelengths
By analyzing the characteristics of the inhibition period of the medium for the three wavelengths, the optimum intensity range for each wavelength can be found in more detail. Table 1 shows the measurement data of the inhibition period for the three wavelengths at different recording beam intensity. Figure 5 shows the variation of the inhibition period with the recording beam intensity for the three wavelengths. The inhibition period varies based on the recording wavelength and intensity of the recording beam. It has been demonstrated that the inhibition period characteristic is not a linear function in [20] and its data measured for each wavelength through optical experiments is close to an exponential fitting. As shown in Fig. 5, an exponential fitting best fits the measured data of the inhibition period for the three wavelengths. The black dotted box corresponding to the inhibition periods of 4–10 seconds on the Y-axis and the recording beam intensities of 0.1–0.5 mW/cm2 on the X-axis shows the optimal intensity range of the three wavelengths found by the proposed method. More precisely, the beam intensities on the X-axis for each wavelength simultaneously intersecting the inhibition periods of the Y-axis within the black dotted box are determined as the optimal intensity range.
Fig. 5. The inhibition period as a function of the recording beam intensity for the three wavelengths.
Table 1. The measurement data of the inhibition period for different recording beam intensity.
The 473 nm wavelength beam with an intensity of less than 0.25 mW/cm2 is excluded to improve the diffraction efficiency of the full-color HOE. Due to the diffraction efficiency sharply decreases to less than 90% at 473 nm wavelength beam when the recording beam intensity is less than 0.25 mW/cm2. Because recording conditions with low diffraction efficiency at a single wavelength reduce the average diffraction efficiency even in full-color HOE recording. The thin black line within the black dotted box in Fig. 5 represents the more detailed optimum intensity range of about 0.25–0.45 mW/cm2, 0.125–0.25 mW/cm2, and 0.1–0.2 mW/cm2 at wavelengths of 473 nm, 532 nm, and 633 nm, respectively. Accurately controlling the differences in the sensitivity of the recording medium with respect to each wavelength is one of the ways to improve and optimize the diffraction efficiency. Hence, to improve the diffraction efficiency of a full-color HOE, it is important to find the optimal recording beam intensity taking into account the initial and late responses of the medium.
Therefore, the optimal recording beam intensity range can be found through the analysis of the characteristic graph of the inhibition period. In addition, the optimum intensity can be determined by fine-tuning the intensity of the wavelength having the maximum diffraction efficiency based on the optimum recording beam intensity range.
4. Results and discussions
Conventionally, one wavelength must be set due to the driving characteristics of the optical detector. To measure the diffracted beam of full-color HOE, when three wavelengths were simultaneously exposed, and the 532 nm wavelength is arbitrarily applied as the set wavelength of the detector. Figure 6 represents the diffracted beam intensity characteristics of a full-color HOE as a function of exposure time measured with the optical detector set to the 532 nm wavelength. Figure 6 shows the experimental results of applying the middle and highest recording beam intensity within the entire recording range in Table 2 to determine the exposure time for optimal full-color HOE recording.
Fig. 6. The diffraction beam intensity of full-color HOE as a function of exposure time for (a) the middle recording beam intensity and (b) the highest recording beam intensity.
Table 2. Average diffraction efficiency of full-color HOE for different recording beam intensity.
The recording beam intensities are expressed in the order of physical beam intensity. The middle values in the center of Table 2 represent the beam intensity within the optimal recording range. This range is obtained through the inhibition period characteristic analysis for each wavelength. The highest recording beam intensity indicates the physically highest beam intensity applied in the experiment.
The reason for applying the middle recording beam intensity is to experimentally determine the optimal continuous exposure time during the recording beam within the optimum recording beam intensity range. Moreover, the reason for applying the highest recording beam intensity is to experimentally reveal that the diffraction efficiency of full-color HOE does not improve even if the recording beam intensity is high in contrast to the monochromatic HOE diffraction efficiency characteristic, as shown in Fig. 3(b). Based on the optical experiment results, the exposure time for the subsequent optimal full-color HOE recording can be set to approximately 60 seconds. In order to measure the change of the diffracted beam intensity in real time, the detection period of the diffracted beam was applied differently according to the intensity of the recording beam. The diffraction efficiency was higher at the middle recording beam intensity than at the highest recording beam intensity in the entire recording range because the refractive index modulation value (Δn) was saturated around the exposure energy of 57 mJ/cm2 for the middle recording beam intensity as shown in Fig. 6(a).
After the exposure energy of 18 mJ/cm2 at the highest recording beam intensity as shown in Fig. 6(b), the diffracted beam intensity was lowered by the Bragg angle shift due to over-modulation and shrinkage of the recording medium. In other words, this phenomenon occurs because the intensity of the diffracted beam is measured in real-time in the process of deviating from the Bragg angle due to the shrinkage of the recording medium after the saturation exposure dose in the full-color HOE recording process. The exposure energies 57 mJ/cm2 and 18 mJ/cm2 in Figs. 6(a) and 6(b) are representing the calculated sum of the saturation exposure energies of the three wavelengths corresponding to exposure times of 60 s and 2 s, respectively. In order to understand the characteristics of the diffraction efficiency according to the change of the recording beam intensity for each wavelength, the average diffraction efficiencies of full-color HOE recorded in various recording beam intensity ranges are shown in Table 2 and Fig. 7. In Fig. 7, the recording beam intensity on the X-axis is expressed only in terms of the recording beam intensities of 633 nm wavelength shown in Table 2. As a result of the experiments, the average diffraction efficiencies of about 54.5% and 42.3% were obtained at the middle and highest beam intensities in the entire recording range, respectively.
Fig. 7. Variation of average diffraction efficiency of full-color HOE with the recording beam intensity. (The recording beam intensity on the X-axis is expressed only as a recording beam intensity of 633 nm wavelength).
The diffraction efficiencies were higher when the full-color HOE was recorded within the optimal intensity range indicated by the black dotted box in the graph of Fig. 5 determined using the proposed method. The optimal intensity range (indicated by the black dotted line) is for improving the color uniformity and diffraction efficiency of full-color HOE. The photopolymer layer has a limited amount of reactive material for each wavelength, such as monomers, dyes, and initiators. The recording conditions for uniform response of each wavelength are required while recording with three wavelengths. It has been confirmed through the experiments that the average diffraction efficiency of full-color HOE increases when the three wavelengths satisfy the recording conditions that it can react as uniformly as possible without being biased to a specific wavelength. In Table 2, the average diffraction efficiency of full-color HOE is measured to be 54.5% and 54.1%, respectively, within the optimal recording beam intensity range. And, when applying the recording beam intensity within the optimal intensity range to improve the diffraction efficiency of the full-color HOE, the diffraction efficiency was highest for 532 nm among the three wavelengths.
Therefore, only the recording beam intensity at 532 nm wavelength was fine adjusted for reducing the standard deviation of the diffraction efficiency for each wavelength. The exposure time for optimal full-color HOE recording was set to approximately 60 seconds based on the experimental results at the middle recording beam intensity shown in Fig. 6(a). Table 3 shows the average diffraction efficiency and standard deviation of the full-color HOE recorded with a finely tuned recording beam intensity.
Table 3. Average diffraction efficiency and standard deviation of the full-color HOE for the optimal recording beam intensity.
The difference between the maximum and minimum diffraction efficiency for the three wavelengths was found to be less than 5%, and the average diffraction efficiency and standard deviation were found to be approximately 56.81% and 1.7%, respectively. This result showed that the optimal recording beam intensity range of the medium for each wavelength can be found by the proposed method. Also, it is important to consider that the inhibition periods are not absolute values and may vary depending on the manufacturing date and storage conditions of the recording medium.
5. Conclusions
We proposed and experimentally verified a method to improve the diffraction efficiency and uniformity of full-color HOE for each wavelength. The optimal recording beam intensity range was determined to be 0.25–0.45 mW/cm2, 0.125–0.25 mW/cm2, and 0.1–0.2 mW/cm2 at 473 nm, 532 nm, and 633 nm wavelengths, respectively. An average diffraction efficiency of approximately 54.3% was measured in this intensity range. To further improve the diffraction efficiencies, the intensity of the 532 nm wavelength beam was finely adjusted from 0.125 mW/cm2 to 0.11 mW/cm2 and the intensities of 473 nm and 633 nm wavelengths were fixed at 0.25 mW/cm2 and 0.1 mW/cm2, respectively. By applying the optimal beam intensity found through the proposed method, the full-color HOE with difference of less than 5% between the maximum and minimum diffraction efficiency for each wavelength was implemented. The average diffraction efficiency and standard deviation of the full-color HOE recorded with the optimal beam intensity were about 56.81% and 1.7%, respectively. Finally, it was verified that the proposed analysis method of applying the inhibition period characteristics for each wavelength is stable and effective in improving the diffraction efficiency and uniformity of full-color HOE.
Funding
Korea government (2020-0-00929), (IITP-2020-0-01846), (NRF-2020R1A2C1101258).
Disclosures
The authors declare no conflicts of interest.
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