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Abstract
The radiation resistance of liquid crystal diffraction gratings was investigated using a cobalt 60 gamma radiation source until the radiation reached a total dose of 1000 Mrad, corresponding to a very severe amount of radiation. The optical properties of gratings exposed to gamma-ray radiation were discussed with internal observations such as polarization optical microscopy and scanning electron microscopy. The radiation resistance of liquid crystal diffraction gratings for gamma-ray irradiation has been explained by associating the optical characteristic measurements for liquid crystal composite materials, including a glass substrate, by analyzing the internal volume grating structure. The radiation resistance for anisotropic diffraction in the HPDLC grating has been explained by the modulation of the LC orientation in the polymer network configuration affected by the gamma-ray irradiation.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical functional devices based on organic materials such as liquid crystals (LCs) and polymers have a wide range of applications [1–4]. Holographic polymer dispersed liquid crystal (HPDLC) gratings have attracted particular attention for the following reasons: having a self-organizing effect after subjugation to a one-time laser exposure process, fabricating a volume grating structure with an LC phase in fine grating pitch to obtain high diffraction efficiency, and providing a polarization function based on LC orientation. Thus, HPDLC gratings have achieved novel optical characteristics that can contribute to optical functional devices and have optical applications [5–7]. In recent years, the holographic memory for optically reconfigurable gate arrays (ORGAs) has attracted attention as a multi-context field-programmable gate array to realize fast and numerous reconfiguration contexts in the professional optical information processing field [8–12]. The ORGAs showed that the performance of parallel programmable gate array consisting of very large scale integration (VLSI) enabled the perfect avoidance of defect areas by using remaining areas effectively [13]. The hologram memory also demonstrated high resistance to the damage caused by disturbances from defects in the holographic memory [14, 15]. Therefore, ORGAs can withstand or overcome component defects, such as laser array, gate array, and holographic memory. This is useful in high-reliability applications such as space satellites and decommissioning of nuclear reactors under harsh radiation environments [16–22]. Therefore, we investigated the radiation resistance of VLSI hardware and materials in the holographic memory using a cobalt 60 gamma radiation source [23–25]. We conducted a diffraction efficiency measurement and polarizing optical microscopy (POM) for LC diffraction gratings irradiated with gamma rays up to 500 Mrad [25].
However, the effects of continuous irradiation on optical functional devices, using organic materials, require further discussion to explore the relation between the total ionizing dose of gamma rays and materials such as LC composites and substrate plates. It is also required to help compose holographic memory under severe radiation conditions [26–28]. Here, we report the effects of the radiation dose in gamma-ray irradiation fields by associating the optical characteristic measurements for LC composite materials, including a glass substrate, with internal volume grating structural analysis. Compared with the previous study, this experiment demonstrates stricter conditions to ensure reliability as the radiation reaches 1000 Mrad [25].
2. Experimental process for device fabrication and evaluation
2.1 Materials compositions
The LC and monomer mixture for anisotropic volume gratings comprised dipentaerythritol-hydroxyl penta-acrylate (DPHPA, Wako Chemical, Lot. No.16311-500) and nematic LC (RDP98487, DIC, Lot. No. 1202011). The monomer (DPHPA) has a refractive index of 1.49, and nematic LC (DIC RDP98487) has ordinary and extraordinary refractive indices of 1.53 and 1.71 at wavelength of 589 nm, respectively. The LC materials have a nematic to isotropic (N–I) transition temperature of 47.9 °C. They were mixed at a weight ratio of 60 wt.% and 40 wt.%, respectively. Xanthene dye (dibromofluorescein) and N-phenylglycine were used as a photoinitiator and coinitiator, respectively. They were introduced with a content ratio of 0.1 wt.% into the mixed material. Glass cells were assembled after radiation resistance tests were conducted on different glass substrates such as soda-lime glass and synthetic quartz glass plates with dimensions of 25 mm × 20 mm ×1.1 mm. Then, LC composites were injected and poured into the 10 µm air gap in the glass cells to compose the holographic memory.
2.2 Holographic grating formation
Figure 1 shows an explanatory drawing of the experiment. An image of the laser interferometer layout and a block configuration diagram to explain the HPDLC grating formation by the laser interferometer are shown in Fig. 1(a) and (b). The light source is a green laser (Nd: YVO4) of 100 mW and a 532 nm wavelength. The laser beam collimated and linearly polarized perpendicularly to the grating vector was divided into two parallel beams, a reference beam and an object beam with the power density of 30 mW/cm2. The reference beam was incident on the sample at 30°. The temperature during device fabrication was set at 25 ℃ using a temperature controller with a Peltier element, and the laser interference exposure was performed for 180 seconds. Figure 1(c) shows a schematic illustration of an internal grating structure in the HPDLC sample formed by the periodically arranged LC-rich and cured polymer-rich phases. It is shown by previous studies that the LC molecules in LC-rich phase are aligned along the grating vector with the growth of the LC droplet formed by the phase separation induced in LC composites during a laser interference exposure [29–30]. The anisotropic diffraction of the HPDLC grating was measured by obtaining the polarization-azimuth dependence of the diffraction efficiency as a function of the incident polarization state with green laser light of 532 nm wavelength.
Fig. 1. Explanatory drawing of the experiment. (a) Layout of laser interferometer. (b) HPDLC grating formed by laser interferometer. (C) Internal grating structure of HPDLC sample.
We prepared two HPDLC gratings, samples A and B, under the same fabrication conditions for gamma-ray irradiation experiments.
2.3 Gamma-ray irradiation experiment using cobalt 60
We conducted this experiment using a cobalt 60 gamma radioisotope at the radiochemistry research laboratory at Shizuoka University. HPDLC gratings were mounted on a holder in a laboratory equipped with a cobalt 60 gamma-ray radiation source. The dose rate was 4.0 kGy/h and the total dose up to 10,000 kGy, which was comparable to 1000 Mrad gamma irradiation corresponding to a very strict experiment because radiation-resistant grade electronic memory such as ROM and RAM can be destroyed by 1 Mrad irradiation [21–23]. As a subsequent description of this paper, we use Mrad as the unit of gamma-ray irradiation since Mrad is usually used as the unit of irradiation experiment.
Here, we explain the transmittance of gamma rays irradiated to LC composites through glass plate substrates such as a soda-lime glass plate and a synthetic quartz glass plate for HPDLC gratings [24]. The transmittance (T) of gamma-ray through the glass substrate plate for HPDLC gratings is calculated using T = e-αρt [26]. The mass attenuation coefficient (α) was approximately 0.05–0.1 cm2/g for the cobalt 60 gamma radiation source at a photon energy of 1.2 MeV [28]. The transmittance is obtained as T = 0.97 when α = 0.1 cm2/g, density ρ = 2.5 g/cm3, and thickness t = 0.11 cm for the glass plate. Thus, confirming that most of the energy of gamma rays can be irradiated to the LC composite layer through a glass plate substrate.
Gamma rays irradiated samples A and B up to 500 Mrad while verifying their optical characteristics at 100 and 200 Mrad. After sample A was exposed to radiation up to 500 Mrad, its internal structure was evaluated using scanning electron microscopy (SEM). Sample B was exposed to radiation up to 1000 Mrad, and its internal structure was investigated using SEM.
2.4 Internal observation of volume grating structure
Polarizing optical microscopy (POM) (Olympus CX31-P) was used to observe the LC orientation conditions in the HPDLC grating composed of LC-rich and cured polymer-rich phases. We investigated the anisotropy of the LC orientation in gratings using POM under crossed Nicol conditions arranged by a polarizer and analyzer. We also observed the temperature dependence of internal fringe patterns of the HPDLC grating at different temperatures using a glass heater unit (BLAST BL-K05100A) because thermal modulation affects the LC alignment in HPDLC gratings more than the N–I transition temperature.
We used SEM (Hitachi S-4300, S-4800) to investigate the internal structure, including the internal morphology composed of a grating structure and an LC droplet configuration. The SEM observations were conducted in two methods: (i) the top region of the HPDLC grating was observed compared with POM observations after the top glass plate was removed from the sample, (ii) we observed the cross-section of the HPDLC grating after the sample was cut along the direction parallel to the grating vector using a glass cutter (Sankyo TC15) and kerosene as lubricating oil. We evaluated the LC droplet trace after nematic LC molecules were rinsed with methanol to observe the configuration of the trace of an LC droplet. Structural observations were conducted at different magnifications to investigate the grating configuration.
3. Results and discussion
3.1 Effect of gamma-ray on a glass plate substrate
Figure 2 shows a comparison in transmittance spectra for different glass substrates after irradiation of approximately 100 Mrad total-ionizing-dose. From the calculation result in previous section, the radiation energy of 97% is considered to reach the PDLC layer through the soda-lime and synthetic quartz glass plates for the gamma-ray. Figure 2(a) shows the transmittance of a soda-lime glass plate, while Fig. 2(b) shows that of a synthetic quartz glass plate. Figure 2(a) shows a significant decrease in the transmittance spectra in the bandwidth 400–800 nm after gamma irradiation. The transmittance decreases from 84% to 43% at a wavelength of 532 nm, corresponding to the green laser source. However, little change is seen in the transmittance spectra in Fig. 2(b) for before and after irradiation. The insets show a comparison between the appearance of these glass plates after gamma irradiation. Although the soda-lime glass plate in Fig. 2(a) changed to a brownish color, the synthetic quartz glass plate in Fig. 2(b) showed transparency.
Fig. 2. Comparison of transmittance spectra of glass plates before and after gamma-ray irradiation for (a) a soda-lime glass plate and (b) a synthetic quartz glass plate. The insets show the comparison in appearance of the glass plates after gamma irradiation.
In the synthetic quartz glass containing no impurities, there is almost no change in spectral transmittance, and transparency is maintained. The soda-lime glass contains trace impurities and we consider that the color variation of the soda-lime glass plate from transparent to a brownish color upon gamma irradiation is due to the presence of impurity such as iron, titanium, and nickel [27]. We used synthetic quartz glasses as a radiation-resistant glass substrate for the HPDLC grating to investigate the effect of radiation dose on LC composites in gamma-ray irradiation fields.
3.2 Anisotropic diffraction
Figure 3 shows changes in the diffraction efficiency with an increased radiation dose of gamma-rays for HPDLC gratings for samples A and B. The both samples of A and B were fabricated same laser interference conditions using the same materials compositions to see the difference due to individual differences of samples. Figure 3(a) and (b) show the polarization-azimuth dependence of diffraction efficiencies for samples A and B, respectively, and Fig. 3(c) shows diffraction efficiencies of the incident P-polarization light condition for the HPDLC grating. The diffraction efficiency is obtained as the ratio of diffraction intensity to incident polarized light intensity. Initial diffraction efficiencies of P-polarization for samples A and B were 78% each. The diffraction efficiency of P-polarization in sample A had a greater tendency to decrease than that of sample B, and it reached 49% under gamma-ray irradiation of 500 Mrad. The diffraction efficiency of sample B decreased gradually with an increase in the total-ionizing-dose and reached 69% after gamma-ray irradiation at 500 Mrad. The decrease in diffraction efficiency shown in Fig. 3 is due to LC-rich and cured polymer-rich phases inside the LC volume grating. The difference between the results of sample A and sample B seems to depend on the individual differences of structural modulation that occurs inside the LC-rich and cured polymer-rich phases of samples irradiated with gamma rays. Therefore, we discuss the observation and analyze the internal structure of the LC volume grating in the next section.
Fig. 3. Changes in diffraction efficiency with increasing radiation dose of gamma-ray. (a) and (b) show polarization-azimuth dependence of diffraction efficiencies, and (c) shows diffraction efficiencies for incident P-polarization light condition in HPDLC gratings formed as samples A and B.
3.3 Observation using a polarized microscope
Figure 4 shows fringe pattern images from sample A. These images were observed using a polarizing microscope at the crossed Nicol condition with a polarizer and analyzer. The grating vector of sample A, as shown in Fig. 1(c), was rotated 45° with respect to the polarizer, and then the periodic line-shaped brightness distribution induced by the difference in birefringence at LC-rich (bright region) and cured polymer-rich (dark region) phases in the HPDLC grating were observed. The insets are enlarged views of the area surrounded by a square near the center of the fringe pattern. The images from (a) to (d) correspond to the following experimental conditions in the radiation dose of gamma rays: 0, 100, 200, and 500 Mrad. Several diminished line-like areas are observed in Fig. 4(b) irradiated up to 100 Mrad. After that, as the radiation dose increases to 200 and 500 Mrad in Fig. 4(c) and (d), respectively, the line-shaped area further expands with the decrease in brightness.
Fig. 4. Images of fringe patterns observed at the crossed Nicol condition with polarizer and analyzer by a polarizing microscope for the HPDLC grating formed as sample A. The insets are magnified images of a square region in the fringe patterns. The images from (a) to (d) are observed at the initial condition (0 Mrad), 100 Mrad, 200 Mrad, and 500 Mrad in the radiation dose of gamma-ray.
Figure 5 shows fringe patterns observed using a polarizing microscope for the HPDLC grating formed from sample B. Images from (a) to (e) were observed under the following experimental conditions, with radiation dose of gamma-rays: 0, 100, 200, 500, and 1000 Mrad. The diminished line-like areas were almost undetectable in Fig. 5(b) and (c) corresponding to the gamma-ray irradiation of 100 and 200 Mrad. However, several diminished line-like areas were observed in Fig. 5(d) and (e) corresponding to the gamma-ray irradiation of 500 and 1000 Mrad. The spread of diminished line-like areas, which decreased in brightness due to the influence of gamma-ray irradiation, appear smaller in sample B than in sample A. The HPDLC grating formed as sample A appears to be more susceptible to defects than that of sample B at small radiation doses. The difference between the two samples may depend on the state of the polymer network formation in LC-rich phase due to individual differences during HPDLC grating fabrication.
Fig. 5. Fringe patterns observed at the crossed Nicol condition with a polarizer and analyzer using a polarizing microscope for the HPDLC grating formed in sample B. Images (a) to (e) are observed at the initial condition (0 Mrad), 100 Mrad, 200 Mrad, 500 Mrad, and 1000 Mrad for the radiation dose of gamma-ray.
However, the diminished line-like areas, which were induced due to a change in LC alignment of the fringe pattern appeared in samples A and B as the total-dose of gamma-ray irradiation increased. The diminished line-like areas were observed at specific fringe regions, and the LC orientation in that area changed to a lower orientation order than in other regions because the brightness of the area decreased and the configuration was observed as a dark line. It is assumed that there is a need to clarify the radiation resistance of the LC and the polymer in the HPDLC grating. Therefore, we evaluated the temperature dependence of the specific fringe regions including diminished line-like areas to find out if the N-I transition function as a basic characteristic of liquid crystal materials was affected under the gamma-ray irradiation.
Figures 6 and 7 show a comparison of temperature dependence in the fringe pattern for HPDLC gratings to investigate the effect of gamma-ray irradiation on LC molecules. Some results from Fig. 7 have been discussed in a previous paper [25]. However, we made a detailed observation based on the comparison using samples from before and after the gamma-ray irradiation. HPDLC gratings placed on the glass heater unit to control the temperature were investigated using POM. Internal fringe patterns, including diminished line-like areas below and above the N–I transition temperature, were evaluated. Based on the results of temperature dependence of internal fringe patterns, we can clarify if the N-I transition function as a basic characteristic of liquid crystal materials was affected under the gamma-ray irradiation because the LC alignment of HPDLC gratings changes rapidly at a higher temperature than that of the N–I transition temperature of the nematic LC. Figure 6 shows results without gamma-ray radiation, while Fig. 7 shows results with 500 Mrad gamma-ray radiation in sample A. Figures 6(a) and 7(a) show the image processed regions in the fringe. The regions rotated by 45° were processed into interference fringe images with continuous brightness and cross-sectional intensity distributions. Figures 6(b) and 7(b) show the comparison of fringe images at different temperatures of 25 °C (B1) and 60 °C (B2) corresponding to the nematic to isotropic transition temperatures. Figures 6(c) and 7(c) show the cross-sectional intensity distributions obtained from the fringe images. In Figs. 6(b) and (c), the fringe pattern and the intensity distribution are observed at room temperature at 25 °C, while the periodical intensity distribution forming interference fringes disappeared at 60 °C, above the N–I transition temperature. In Fig. 7(b) and (c), the brightness in diminished line-like areas in the right half was lower than that in the left-half at 25 °C. The periodical intensity distributions forming interference fringes were almost undetectable in images. The intensity distributions (B2) in Fig. 7(b) and (c) further decreased at 60 °C, above the N–I transition temperature, and the difference between the right and left areas were not visible in Fig. 7(c). However, when the temperature returned to 25 °C, the original intensity distribution was reproduced. From these results, irrespective of the 500 Mrad gamma-ray irradiation, LC molecules in LC-rich phases including diminished line-like areas in the HPDLC grating maintained the N–I transition function associated with temperature modulation. Diminished line-like areas appeared in the HPDLC grating with an increase in gamma-ray irradiation, which is connected to the droplet structure surrounding the LC molecule in the LC-rich phase.
Fig. 6. Temperature dependence in fringe patterns for the HPDLC sample without gamma-ray radiation. (a) Observation region of the fringe. (b) Comparison of fringe images at different temperatures of 25 °C (B1) and 60 °C (B2) corresponding to below and above nematic to isotropic transition temperature. (c) Cross-sectional intensity distributions in fringe images of (b).
Fig. 7. Temperature dependence in fringe patterns for sample A irradiated by the 500 Mrad total ionizing dose. (a) Observation region in the fringe. (b) Comparison of fringe images at different temperatures of 25 °C (B1) and 60 °C (B2) corresponding to below and above nematic to isotropic transition temperature. (c) Cross-sectional intensity distributions in fringe images of (b).
3.4 SEM observation
Figure 8 shows SEM images with different magnifications in the upper surface after peeling the glass plate from HPDLC grating. A1 and A2 show images without the ionizing-dose irradiation in the initial fabrication condition, while B1 and B2 show images of another HPDLC grating formed as sample A and irradiated by the 500 Mrad gamma-rays. After LC molecules in the coalesced droplets of the LC-rich phase in the upper surface were rinsed by methanol, the cured polymer-rich and LC-rich trace phases were observed using SEM. As shown in A2 and B2, the periodical bright and dark lines are considered to correspond to cured polymer-rich and LC-rich droplet trace phases, respectively, because dense areas are bright and low-density areas are dark in SEM observations. The grainy aspects in the images are a result of peeling from the polymer layer forming domain. The configuration of the polymer network as the trace of LC droplets in images B1 and B2 is a dark layer showing a larger domain shape than that of A1 and A2. Thus, the polymer network density existing in the LC-rich droplet trace phase of B1 and B2 is lower than that of A1 and A2, while the density in strongly bonded polymer-rich phase doesn't show much difference between the two images. The decrease of the polymer network density seems to have occurred by the changes in the polymer network configuration in the LC-rich phase as shown by another SEM observation that follows.
Fig. 8. SEM images with different magnification in the upper surface after peeling the glass plate from HPDLC gratings: A1 and A2 show the images without the ionizing-dose irradiation in initial fabrication condition, while B1 and B2 show the images of another HPDLC grating formed as sample A irradiated by the 500 Mrad total-ionizing-dose for comparison.
Next, we conducted another SEM observation to further investigate the internal configuration of the HPDLC grating for sample B after subjecting to a 1000 Mrad gamma-ray irradiation. Figure 9 shows cross-sectional SEM views of HPDLC gratings with different magnifications. In Fig. 9, A1 and A2 show the internal structure without ionizing-dose irradiation in the initial fabrication condition, while B1 and B2 show the HPDLC grating for sample B irradiated by a total-ionizing-dose of 1000 Mrad. For SEM, HPDLC gratings fabricated using a glass-cell sample filled with LC composites were cut parallel to the grating vector. The cross-sectional view of grating structures was observed under several magnifications after LC molecules in coalesced droplets of the LC-rich phase were rinsed with methanol. Compared with the images of A1 and A2 in Fig. 8 and A1 and A2 in Fig. 9 which were taken before the gamma-ray irradiation, the former surface seems to be smoother than that of latter one. This seems to be due to the difference in observation surface because the upper surface on a flat substrate was observed after peeling the glass plate substrate from HPDLC grating in the images of A1 and A2 in Fig. 8. In Fig. 9, diagonally arranged grating structures are observed. In close-up views of regions surrounded by squares, we can recognize the slanted volume grating structures formed by the solid and the trace configuration of the LC droplet, including void regions corresponding to the cured polymer and LC-rich droplet trace phases. The configuration consisting of trace LC droplets in the LC-rich phase looks different in A2 and B2. A network shape with finely distributed voids is observed in the LC-rich droplet trace region of A2, while a network shape with relatively large rugged voids is observed in B2. The shape and void size of the LC-rich droplet trace changed due to the effects of a 1000 Mrad gamma-ray irradiation. The changes in the polymer network configuration in the LC-rich phase could have been caused due to gamma-ray irradiation on the LC orientated droplets surrounded by the polymer network structure. Thus, the difference in polymer network configuration between A2 and B2 leads to the development of diminished line-like areas appearing in fringe patterns irradiated by the 1000 Mrad total-ionizing-dose. The deteriorated region observed as the diminished line-like area grew as a stringy texture. The growth of deteriorated region is caused by the change of the polymer network shape in the LC-rich phase. The internal grating structure in the HPDLC sample is formed by the periodically arranged LC-rich and cured polymer-rich phases induced by a laser interference pattern. It is considered that the defect area does not spread uniformly throughout the inside of the HPDLC grating but spreads linearly as a stringy texture because a specific polymer network region along the LC-rich phase is susceptible to deterioration due to gamma-ray irradiation.
Fig. 9. SEM cross-sectional views of HPDLC gratings with different magnification: A1 and A2 show the internal structure without the ionizing-dose irradiation in initial fabrication condition, while B1 and B2 show the HPDLC grating formed as sample B irradiated by the 1000 Mrad total-ionizing-dose.
4. Conclusions
We investigated the effect of a 1000 Mrad total-ionizing-dose of gamma-ray irradiation on volume gratings formed by LC composites to ensure the reliability of holographic memory. Two HPDLC gratings fabricated under the same conditions were used for gamma-ray irradiation experiments, and optical properties of LC volume gratings were measured for radiation doses of 0, 100, 200, 500, and 1000 Mrad, separately. The 78% initial diffraction efficiency for P-polarization decreased gradually with an increase in the total-ionizing-dose for both HPDLC grating samples. One sample reached 49% for gamma-ray irradiation of 500 Mrad, while another reached 68% after gamma-ray irradiation at 1000 Mrad. The two samples showed different decreasing trends, but the diffraction efficiency decreased with increasing radiation dose.
The internal structure was investigated to determine the reason for the decrease in the diffraction efficiency of HPDLC gratings. POM observation after the irradiation of the gamma-ray showed that diminished lines appeared as dark fringe patterns, and they comprised the LC-rich and cured polymer-rich layers as the total ionizing-dose of gamma-ray irradiation increased. The change in the polymer network shape in the LC-rich droplet trace layer observed by SEM affected the LC orientation in droplets surrounded by the polymer network structure. The decrease in diffraction efficiency of the HPDLC grating was related to the modulation of the LC orientation in the polymer network configuration affected by the gamma-ray irradiation. The anisotropic diffraction of the HPDLC grating was maintained at approximately 50% even for a bad sample under gamma-ray irradiation of 500 Mrad.
The gamma-ray irradiation was performed under very severe conditions and even radiation-resistant grade electronic memory such as ROM and RAM were destroyed by 1 Mrad irradiation [21–23]. Thus, the resulting holographic grating is expected to have radiation resistance for optical properties in gamma-ray irradiation fields.
Funding
JAEA Nuclear Energy S&T and Human Resource Development Project (JPJA19F19209710); Japan Society for the Promotion of Science (JSPS) (KAKENHI Grant Number 20K04632).
Disclosures
The authors declare no conflict of interest.
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