Low-Toxicity Photopolymer for Reflection Holography - ACS Applied

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Low-Toxicity Photopolymer for Reflection Holography D. Cody,† S. Gribbin,† E. Mihaylova,‡ and I. Naydenova*,† †

Centre for Industrial and Engineering Optics, Dublin Institute of Technology, Dublin 8, Ireland Department of Mathematics, Informatics and Physics, Agricultural University, Plovdiv, Bulgaria

‡

ABSTRACT: A novel composition for a low-toxicity, water-soluble, holographic photopolymer capable of recording bright reflection gratings with diffraction efficiency of up to 50% is reported. The unique combination of two chemical components, namely, a chain transfer agent and a free radical scavenger, is reported to enhance the holographic recording ability of a diacetone acrylamide (DA)-based photopolymer in reflection mode by 3-fold. Characterization of the dependence of diffraction efficiency of the reflection gratings on spatial frequency, recording intensity, exposure energy, and recording wavelength has been carried out for the new low-toxicity material. The use of UV postexposure as a method of improving the stability of the photopolymer-based reflection holograms is reported. The ability of the modified DA photopolymer to record bright Denisyuk holograms which are viewable in different lighting conditions is demonstrated. KEYWORDS: low-toxicity photopolymer, holography, reflection holography, photopolymer, high efficiency holographic volume grating, Denisyuk of the DA photopolymer has been further boosted to 5.5 × 10−3 by doping with BEA-type zeolite nanoparticles.16 Ortuno et al. have developed a low-toxicity material called “Biophotopol”, which uses the low-toxicity material sodium acrylate (NaAO) as the substitute monomer for AA.17 An achievable diffraction efficiency of 90% in 300 μm thick layers in transmission mode has recently been reported for this material.18 Olivares-Perez et al. are currently developing several environmentally compatible holographic recording materials using PVA matrices sensitized with different additives including dihydrated copper chloride (CuCl2(2H2O)),19 Nopal Cactus extract,20 and Quail Albumen.21 While transmission mode holography is widely used for the development of many holographic applications22−27 as well as for characterization and comparison of different holographic recording media,28−35 reflection mode holography offers one distinct advantage: the ability to reconstruct the holographic image with white light. The recording of high diffraction efficiency reflection holograms in photopolymer media is considered by many holographers to be a challenging task. This is due to increased demands on the material in relation to the spatial resolution required for high spatial frequency recording, the dimensional stability of the material, and also the stability of the holographic recording setup itself. Traditionally, reflection mode recording was used for display holography.34,35 Due to the enhancement of holographic recording media including photopolymers for reflection mode recording, more applications are being explored. Three-

1. INTRODUCTION In the past decade, holography has become an area of great interest due to its wide and varied range of applications such as holographic digital microscopy,1 holographic tweezers,2 holographic solar concentrators,3,4 sensors,5−9 data storage,10,11 and holographic interferometry.12 For many of these applications the material used to record the holograms is the key component, which is why significant research effort is dedicated to the development of holographic recording materials with improved properties. Photopolymers are characterized by a relatively high sensitivity, large dynamic range, self-processing nature, and relatively low cost, which makes them an attractive holographic recording material. A large majority of the investigated water-soluble holographic photopolymers are acrylamide (AA) based due to their ability to readily record high diffraction efficiency holograms. A drawback of these materials is the carcinogenic and toxic nature of AA in its monomer form. In recent years, as holographic applications are becoming more and more of a reality, there has been a move in the research community toward photopolymer materials that not only perform well holographically but also are low-toxicity and environmentally compatible, in order to reduce the potential occupational and environmental hazards involved in large-scale material development and device fabrication. The development, cytotoxicity evaluation, and transmission mode characterization of a photopolymer composition which uses the nontoxic monomer diacetone acrylamide (DA) as a replacement for AA have previously been reported.13−15 The DA photopolymer has been shown to achieve diffraction efficiency values greater than 90% in 70 μm thick layers in transmission mode with refractive index modulation (Δn) values of 3.5 × 10−3.13 The achievable Δn © XXXX American Chemical Society

Received: May 9, 2016 Accepted: June 28, 2016

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DOI: 10.1021/acsami.6b05528 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Chemical Compositions of Tested DA-Based Photopolymers [all values in mol/L]

CTA0_ FRS0

CTA0_ FRS1

CTA1_ FRS1

CTA2_ FRS1

CTA3_ FRS1

CTA2_ FRS0

CTA2_ FRS1_EB

polyvinyl alcohol (10 wt %/vol) triethanolamine diacetone acrylamide N,N′-methylenebis(acrylamide) glycerol (FRS) citric acid (CTA) Methylene Blue dye (0.11 wt %/vol) Erythrosin B dye (0.11 wt %/vol)

1.73 0.58 0.23 0.05 5.29 × 10−4 -

1.67 0.56 0.22 0.05 0.51 5.09 × 10−4 -

1.61 0.54 0.21 0.05 0.96 0.02 4.91 × 10−4 -

1.61 0.54 0.21 0.05 0.96 0.04 4.91 × 10−4 -

1.61 0.54 0.21 0.05 0.96 0.06 4.91 × 10−4 -

1.73 0.58 0.23 0.05 0.04 5.29 × 10−4 -

1.61 0.54 0.21 0.05 0.96 0.04 1.88 × 10−4

from bright to dark fringe areas was suggested by Naydenova et al.61 Diffraction efficiency up to 30% at 4600 L/mm was achieved using this method. Here, an investigation into the influence of two chemical components on the holographic recording ability of a DA-based photopolymer in the reflection regime of recording is reported: citric acid which acts as a CTA and glycerol which acts as a free radical scavenger (FRS).14,62−66 The CTA restricts the growth of long polymer chains within the bright fringes during holographic recording by termination of a growing polymer chain and transfer of the free radical so that it can initiate a new polymer chain. This restriction of chain length is beneficial for obtaining high spatial resolution of the material required for obtaining high diffraction efficiency reflection holograms. The function of the FRS is to terminate active polymer chains, thus deactivating mobile short polymer chains diffusing from bright to dark fringe regions and restricting the extension of growing polymer chains into unilluminated regions of the grating. Glycerol has also previously been added to the DA photopolymer to boost diffusion of the relatively large DA monomer molecules during holographic recording, due to its nature as a plasticizer.14 To the best of the authors’ knowledge, this is the first time these two chemical components have been combined for the purpose of improved high spatial frequency holographic recording in photopolymer media. Here, the reflection mode holographic recording ability of the low-toxicity DA photopolymer has been characterized for the first time. A long-term study of reflection holograms recorded in the modified photopolymer composition has been carried out in order to verify the stability of the hologram, as it is expected that diffusion of short polymer chains formed by the CTA could cause deterioration of the diffraction efficiency.67 The ability of the modified photopolymer composition to record bright, easily viewable Denisyuk holograms has also been investigated. Denisyuk reflection holograms are an attractive option for many applications including sensors, anticounterfeit, and security holograms, as they are viewable in white light and can be mass produced via a relatively straightforward and stable recording geometry.

dimensional multiplexing of reflection holograms has been investigated by Matoba et al.36 and Jallapuram et al.37 as a potential method to improve the data storage density of photopolymer media. Reflection holograms find application as white light viewable sensors for analytes such as glucose,38 divalent metal ions,39 water vapor,6 and acetone.40 Holographic photopolymer-dispersed liquid crystal (HPDLC) media have been successfully used to record color-switchable reflection holograms.41,42 New methods of fabricating reflection holograms which will potentially allow for mass production are reported, including Aztec surface relief volume structures,43 novel recording geometries to produce rainbow-colored Bragg reflection gratings with continuously varying period,44 and holographic stereogram printing.45 The successful development of photopolymers for reflection holography in the research community is limited35,46−49 and practically nonexistent in regards to low-toxicity and environmentally compatible photopolymer formulations. The launch of commercial reflection photopolymers by Bayer,50 DuPont,51 and Polygrama52 has assisted research into reflection mode photopolymer-based applications. However, DuPont’s acrylatebased photopolymer products require protection from adverse effects caused by vapor inhalation and contact with skin.53 No information is readily available on the composition and toxicity of Bayer photopolymer products. Polygrama’s Darol photopolymer, while reported to have low toxicity, still requires a thermal postprocessing treatment step. Chain transfer agents (CTAs) have long been used to control the molecular weight and viscosity of acrylamide (AA)-based polymers in an attempt to improve their spatial resolution.54−56 The inclusion of CTAs in holographic photopolymer formulations was first proposed by Cole et al. as a means to control polymerization and prevent dark reaction, i.e., polymerization of material in unilluminated regions.57 Guo et al. expanded on the CTA role in holographic photopolymers, suggesting it as a method to promote short chain growth at higher spatial frequencies.49 They report an increase in diffraction efficiency from 12.6 to 18.07% at a spatial frequency of 4286 L/mm due to the incorporation of the CTA sodium formate (HCOONa). Fernández et al. report on the use of two different CTAs, namely, 4,4′-azobis(4-cyanopentanoic acid) (ACPA)58,59 and HCOONa,60 for reflection mode recording with an AA-based photopolymer. Although the inclusion of ACPA was shown to reduce the amount of polymerizationinduced shrinkage occurring in the layers, only small increases in diffraction efficiency were observed (maximum of 8.3% at a spatial frequency of 4553 L/mm), and no improvement in diffraction efficiency was observed with HCOONa. A method for improvement of the diffraction efficiency in reflection mode by control of the permeability of the binder matrix and thus achieving restriction of the diffusion of short polymer chains

2. MATERIALS AND METHODS 2.1. Materials. The different DA-based photopolymer compositions tested were prepared as described in Table 1. An amount of 0.5 mL of photopolymer solution was deposited onto glass slides (75 × 25 mm) and allowed to dry for 12−24 h in darkness under standard laboratory conditions (20−25 °C, 40−60% RH). Sample thickness was measured to be 80 ± 5 μm using a Micro XAM S/N 8038 white-light surface profiler. Sample thicknesses in the range of 40−160 μm were investigated, and 80 μm was selected due to maximum diffraction efficiency and layer uniformity at this thickness. The absorbance of the 80 μm thick Erythrosin B- and Methylene Blue-sensitized samples was determined to be 1.03 and 0.78 at 532 and 633 nm, respectively. B


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Figure 1. (a) Reflection mode holographic recording setup: P: polarizer, S: shutter, SF: spatial filter, C: collimator, BS: beam splitter, M: mirror. (b) Setup used for measurement of the diffraction efficiency of a recorded reflection grating. 2.2. Methods. The equation for volume, reflection holographic gratings used here to calculate the holographic recording conditions is given by eq 1

λ = 2nΛ sin θ

(1)

where λ is the recording wavelength outside of the material; n is the photopolymer refractive index (∼1.5); Λ is the grating period; and θ is the half of the angle between the two recording beams.12 Reflection gratings were recorded using a standard two-beam optical setup as shown in Figure 1(a). A HeNe laser and a Nd:YVO4 laser were used for all recordings at 633 and 532 nm, respectively. By varying the angle between the two beams, 2θ, the spatial frequency of the recorded grating was varied according to eq 1. A neutral density filter was used to control the recording intensity of the two beams. The recording intensity was adjusted to compensate for reflection losses calculated using the Fresnel reflection coefficient due to the oblique incident angle.68 The photopolymer layer was then slanted by a small angle (α = 5°) prior to recording in order to facilitate separation of the reflected and reconstructed beams during readout. A Newport 1830-C optical power meter was used to measure the intensity of the reconstructed reflection beam (Id) after recording had finished, as shown in Figure 1(b). The diffraction efficiency, η, is given by eq 2 η=

Id × 100 Io

Figure 2. η (%) of recorded reflection gratings for each of the DAbased photopolymer formulations described in Table 1.

the FRS and different concentration of the CTA: 0.02, 0.04, and 0.06 mol/L, respectively. A 3-fold increase in η was obtained for composition CTA2_FRS1, showing a clear optimum concentration of CTA of 0.04 mol/L. Further increase of the concentration of CTA to 0.06 mol/L does not yield any further improvement in η above 30%. Composition CTA2_FRS0 contains the best performing concentration of CTA (0.04 mol/L) and no FRS. In the absence of the FRS, gratings with η of 10% were obtained. These results indicate that both the FRS and CTA are required in order to achieve high η reflection gratings. It is important to note that the increased η obtained for composition CTA2_FRS1 is not due to increased monomer concentration; in fact, this composition contains the least amount of monomer as a percentage of total solid weight. 3.2. Dependence of η on Spatial Frequency and Recording Intensity. The influence of both spatial frequency and recording intensity on the diffraction efficiency of reflection gratings recorded with composition CTA2_FRS1 was then investigated. Gratings were recorded at spatial frequencies of 2500−4500 L/mm for recording intensities of 2−6 mW/cm2. Total exposure energy of 270 mJ/cm2 was used for all recordings, as this was identified as optimum for grating recording from a range of exposure energies from 90 to 360 mJ/cm2. The results of this study are shown in Figure 3. As seen clearly in Figure 3, higher recording intensities were observed to be optimum for maximum η at all tested spatial frequencies. This is due to the increased rate of polymerization at higher intensities, which facilitates the formation of the short polymer chains that are necessary to achieve large η in high spatial frequency gratings. However, the η of the recorded gratings is observed to decrease with increasing spatial frequency, regardless of the intensity used. Figure 3(b) shows clearly that the dependence of η on recording intensity is less

(2)

where Io is the incident intensity of the probe beam. For each measurement of η, the grating was rotated until the maximum value of Id was found. This was carried out in order to compensate for any swelling or shrinkage that may have occurred in the layer during recording, which may cause a small shift in the position of the Bragg angle, and also to allow for separation from the reflected beam, whose intensity is given by Ir, as shown in Figure 1(b).

3. RESULTS AND DISCUSSION 3.1. Optimization of the Photopolymer Composition. Reflection gratings were recorded initially at 633 nm using all Methylene Blue-sensitized photopolymer compositions described in Table 1 in order to determine the optimum formulation. Preliminary studies have shown that gratings with measurable η were successfully recorded at a spatial frequency of 3050 L/mm with a total recording intensity of 3.2 mW/cm2 and total exposure energy of 256 mJ/cm2; therefore, these recording conditions were used for an initial comparison of the different photopolymers. The η obtained for each of the photopolymer compositions tested is shown in Figure 2. The η values shown in Figure 2 were calculated using the mean η data of three independent experimental measurements. The error bars represent the standard deviation of the mean. For compositions CTA0_FRS0 and CTA0_ FRS1 containing no CTA, the same approximate value of η of 10 ± 2% was achieved; no improvement in η was observed with the inclusion of the FRS in composition CTA0_ FRS1. Compositions CTA1_ FRS1, CTA2_ FRS1, and CTA3_ FRS1 contain both C


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Figure 3. (a) η (%) vs spatial frequency (L/mm) for gratings recorded in composition CTA2_FRS1 with recording intensities of 2−6 mW/cm2. (b) η (%) vs recording intensity for gratings recorded at spatial frequencies of 2500, 3500, and 4500 L/mm. A total exposure energy of 270 mJ/cm2 was used for all recordings.

Figure 4. (a) η (%) vs exposure time (seconds) for gratings recorded in compositions CTA2_FRS1 and CTA2_FRS1_EB using recording wavelengths of 633 and 532 nm, respectively, at a spatial frequency of 3050 L/mm. (b) Absorbance vs wavelength (nm) for Methylene Blue and Erythrosin B dye solutions with identical concentrations. The dashed lines indicate the recording wavelength for each photosensitive dye.

3.3. Dependence of η on Recording Wavelength and Exposure Energy. The influence of both exposure energy and recording wavelength on the holographic recording ability of the CTA2_FRS1 composition was then investigated. Gratings were recorded at a spatial frequency of 3050 L/mm using recording wavelengths of 532 and 633 nm. A total recording intensity of 3 mW/cm2 and an exposure time of 120 s were used. Composition CTA2_FRS1_EB was used for the 532 nm tests; this is identical to CTA2_FRS1 except for the replacement of the Methylene Blue sensitizing dye with Erythrosin B. The results of this study are shown in Figure 4(a). A maximum diffraction efficiency of 48 ± 2% was obtained for an exposure time of 90 s using the 532 nm recording wavelength, compared to 28 ± 2% at 633 nm. For both the 532 and 633 nm data, an exposure time of 90 s corresponding to an exposure energy of 270 mJ/cm2 is observed to be optimum; increasing the exposure time above this does not further increase the grating η. This is comparable

pronounced at higher spatial frequencies. The observed decrease in η with increasing spatial frequency is partly ascribed to the diffusion of short polymer chains out of the bright fringe regions, resulting in reduction of the refractive index modulation between the bright and dark fringe regions. This is a problem that becomes more significant as the fringe spacing is reduced with increasing spatial frequency and may be exacerbated by the presence of glycerol, which is a plasticizer, and therefore may increase diffusion of short DA polymer chains.14 Therefore, increasing the recording intensity will not significantly enhance η, as occurs at lower spatial frequencies. In general, a trade-off exists between achievable spatial frequency and diffraction efficiency of the recorded gratings due to the relative size of the polymer chains and the grating fringe spacing. While it is clear from Figure 2 that the CTA and FRS act to significantly enhance η of the DA photopolymer up to 3fold, there are still constraints on the achievable η in reflection mode. D


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ACS Applied Materials & Interfaces with the sensitivity of an AA-based reflection mode photopolymer, for which sensitivity in the range of 200−300 mJ/cm2 is reported.61 The η of the reflection gratings was observed to increase from 28% to 48% as the recording wavelength was decreased from 633 to 532 nm. This represents a 71% improvement in η. One possible explanation for this increase in η could be the fact that we probe at a shorter wavelength and, assuming that the Δn remains the same, the η measured at the lower wavelength is expected to be higher. We have estimated Δn for the reflection gratings with the highest value of η recorded at 633 and 532 nm. The Δn values for these gratings are 1.13 × 10−3 and 1.52 × 10−3, respectively, which equates to a 35% increase in Δn when the recording is performed at 532 nm. This implies that the probing at lower wavelength alone cannot be responsible for the observed improvement in η. Another contributory factor to the increased efficiency at 532 nm may be the difference in absorption of the two photosensitive dyes, Erythrosin B and Methylene Blue, which was measured with a PerkinElmer Lambda 900 UV/vis/NIR spectrometer. As seen in Figure 4(b), both dyes absorb at their respective recording wavelengths; however, the absorption of Erythrosin B is twice that at 532 nm than for Methylene Blue at 633 nm. Increased absorption of photons by the dye molecules in the photopolymer layer increases the volume of free radicals produced and therefore increases the extent of polymerization of the monomer molecules, which may result in enhanced η. 3.4. UV Post Recording Exposure and Hologram Lifetime Study. It is imperative that the recorded holograms are stable for practically all potential applications. It was observed that the reflection gratings recorded with the modified DA photopolymer are not stable, and the η is observed to degrade slowly over time. It is expected that this deterioration is linked to the diffusion of short polymer chains, the volume fraction of which has been boosted by the incorporation of the CTA, out of the polymerized regions within the grating.67 The short polymer chain diffusion is most likely facilitated to some extent by the presence of the FRS glycerol which also acts as a plasticizer. Lamination and dehydration of the samples with desiccant after recording did not prevent this effect. UV curing is a well-known technique to stabilize photopolymer-based holograms.58,69 It is likely that such UV exposure results in full polymerization of the photopolymer layer, including crosslinking of the short polymer chains, thereby preventing short chain diffusion into regions unilluminated during holographic recording. Here, a nanosecond pulsed 355 nm UV laser was used to UV expose the photopolymer layers directly after holographic recording. A range of exposure conditions were investigated, and an exposure time of 3 min with power per nanopulse of 150 mW/cm2 was observed to be optimum. Figure 5 shows an example of the normalized η of a recorded reflection grating (initial η = 20%) vs time after recording which has been exposed to UV light and a reference sample which was unexposed. In the case of the grating which received no UV treatment, a 50% drop in η is observed in the first 5 min. For the grating exposed to UV light after recording, η remains constant. An extended study of the gratings stability was carried out for one month after recording to investigate the lifetime of the holograms after UV fixing. An initial decrease of approximately 4% in η was observed to occur for all holograms in the first 24 h after UV-fixing. These holograms were not laminated or protected in any way and therefore were subject to external

Figure 5. η (%) vs time after recording (min) for composition CTA2_FRS1 samples which have been exposed to UV light after recording and one sample which received no treatment.

influences such as humidity and temperature, which may explain the observed small decrease. After this initial decrease, no further significant decrease in η was observed over a 28-day period. The fast decline in diffraction efficiency of the gratings which were not exposed to UV light after holographic recording is a strong proof that short polymer chain diffusion is an important factor and has to be suppressed. 3.5. Denisyuk Hologram Recording. The ability of the modified DA photopolymer composition to record reflection holograms using the Denisyuk reflection geometry was investigated. Holograms of a key were recorded using a 532 nm Nd:YAG laser. Photographs taken of the reflection holograms are shown in Figure 6(a) in ambient lighting

Figure 6. Denisyuk reflection hologram recorded of a key, illuminated in (a) ambient lighting conditions and (b) with a strong white light source.

conditions and (b) using an intense white light source. The hologram is bright and visible with both illumination techniques, demonstrating the modified low-toxicity photopolymer’s suitability for application in a range of areas including display holography, security and anticounterfeiting, and interactive white-light visible holographic sensors.

4. CONCLUSIONS In summary, the unique combination of two chemical components, namely, a chain transfer agent and a free radical scavenger, is reported to significantly enhance the holographic recording ability of a low-toxicity diacetone acrylamide-based E


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(7) Shi, J.; Hsiao, V. K. S.; Huang, T. J. Nanoporous Polymeric Transmission Gratings for High-Speed Humidity Sensing. Nanotechnology 2007, 18, 465501. (8) Blyth, J.; Millington, R. B.; Mayes, A. G.; Frears, E. R.; Lowe, C. R. Holographic Sensor for Water in Solvents. Anal. Chem. 1996, 68, 1089−1094. (9) Bianco, G.; Ferrara, M. A.; Borbone, F.; Zuppardi, F.; Roviello, A.; Striano, V.; Coppola, G. Volume Holographic Gratings as Optical Sensor for Heavy Metal in Bathing Waters, Proceedings of SPIE Conference 9506 on Optical Sensors, 5th May, 2015. (10) Guo, J.; Gleeson, M. R.; Sheridan, J. T. A Review of the Optimisation of Photopolymer Materials for Holographic Data Storage. Phys. Res. Int. 2012, 2012, 1−16. (11) Bruder, F. K.; Hagen, R.; Rölle, T.; Weiser, M. S.; Fäcke, T. From the Surface to Volume: Concepts for the Next Generation of Optical−Holographic Data-Storage Materials. Angew. Chem., Int. Ed. 2011, 50, 4552−4573. (12) Toal, V. Introduction to Holography; CRC Press, 2012. (13) Cody, D.; Naydenova, I.; Mihaylova, E. New Non-Toxic Holographic Photopolymer Material. J. Opt. 2012, 14, 015601. (14) Cody, D.; Naydenova, I.; Mihaylova, E. Effect of Glycerol on a Diacetone-Acrylamide based Holographic Photopolymer. Appl. Opt. 2013, 52, 489−494. (15) Cody, D.; Casey, A.; Naydenova, I.; Mihaylova, E. A Comparative Cytotoxic Evaluation of Acrylamide and Diacetone Acrylamide to Investigate their Suitability for Holographic Photopolymer Formulations. Int. J. Polym. Sci. 2013, 564319, 1−6. (16) Cody, D.; Mihaylova, E.; O’Neill, L.; Babeva, T.; Awala, H.; Retoux, R.; Mintova, S.; Naydenova, I. Effect of Zeolite Nanoparticles on the Optical Properties of Diacetone Acrylamide-based Photopolymer. Opt. Mater. 2014, 37, 181−187. (17) Ortuno, M.; Gallego, S.; Marquez, A.; Neipp, C.; Pascual, I.; Belendez, A. Biophotopol: A Sustainable Photopolymer for Holographic Data Storage Applications. Materials 2012, 5, 772−783. (18) Navarro-Fuster, V.; Ortuno, M.; Gallego, S.; Marquez, A.; Belendez, A.; Pascual, I. Biophotopol’s Energetic Sensitivity Improved in 300 μm Layers by Tuning the Recording Wavelength. Opt. Mater. 2016, 52, 111−115. (19) Olivares-Perez, A.; Hernandez-Garay, M. P.; Fuentes-Tapia, I.; Ibarra-Torres, J. C. Holograms in Polyvinyl Alcohol Photosensitized with CuCl2(2H2O). Opt. Eng. 2011, 50, 065801. (20) Olivares-Perez, A.; Toxqui-Lopez, S.; Padilla-Velasco, A. L. Nopal Cactus (Opuntia Ficus-Indica) as a Holographic Material. Materials 2012, 5, 2383−2402. (21) Olivares-Perez, A.; Ordonez-Padilla, M. J.; Toxqui-Lopez, S. Holograms in Albumins and Optical Properties Recorded in Real Time. Opt. Photonics J. 2015, 5, 177−192. (22) Akbari, H.; Naydenova, I.; Martin, S. Using Acrylamide based Photopolymers for Fabrication of Holographic Optical Elements in Solar Energy Applications. Appl. Opt. 2014, 53, 1343−1353. (23) Gambogi, W. J.; Gerstadt, W. A.; Mackara, S. R.; Weber, A. M. Holographic Transmission Elements using Improved Photopolymer Films, Proceedings of SPIE 1555, Computer and Optically Generated Holographic Optics; 4th in a Series, 256, 1991. (24) Sherif, H.; Naydenova, I.; Martin, S.; McGinn, C.; Toal, V. Characterisation of an Acrylamide-based Photopolymer for Data Storage utilizing Holographic Angular Multiplexing. J. Opt. A: Pure Appl. Opt. 2005, 7, 255−260. (25) Toishi, M.; Tanaka, T.; Watanabe, K.; Betsuyaku, K. Analysis of Photopolymer Media of Holographic Data Storage using Non-Local Polymerization Driven Diffusion Model, Jpn. J. Appl. Phys. 2007, 46, 3438. (26) Leite, E.; Naydenova, I.; Mintova, S.; Leclercq, L.; Toal, V. Photopolymerizable Nanocomposites for Holographic Recording and Sensor Application. Appl. Opt. 2010, 49, 3652−3660. (27) Tomita, Y.; Hata, E.; Momose, K.; Takayama, S.; Liu, X.; Chikama, K.; Klepp, J.; Pruner, C.; Fally, M. Photopolymerizable Nanocomposite Photonic Materials and their Holographic Applications in Light and Neutron Optics. J. Mod. Opt. 2016, 63, S1−S31.

photopolymer in reflection mode. A 3-fold improvement in diffraction efficiency is observed with the modified composition, with diffraction efficiencies of up to 50% obtained at a spatial frequency of 3050 L/mm. This clearly demonstrates the effectiveness of this new approach for enhancement of high spatial frequency holographic recording in photopolymer media. To the best of the authors’ knowledge, 50% is the highest reported value for diffraction efficiency of reflection holograms recorded at 3050 L/mm in a noncommercial, lowtoxicity, water-soluble holographic photopolymer. Characterization of the dependence of diffraction efficiency of the reflection gratings on spatial frequency, recording intensity, exposure energy, and recording wavelength has been carried out. At the upper studied spatial frequency limit of 4500 L/mm, η values of 15% were obtained. It has been demonstrated that UV exposure after holographic recording is a suitable technique for stabilization of the DA photopolymer reflection gratings and extends the grating lifetime, which is otherwise limited due to the diffusion of short polymer chains from the bright to the dark fringe regions of the recorded holographic grating. The ability of the modified DA photopolymer to record bright, easily viewable Denisyuk reflection holograms which are suitable for many applications including interactive holographic sensing, display holography, and as security features has been demonstrated.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Enterprise Irelandfunded project CF 2015 0076P. The authors also acknowledge the Irish Research Council Embark Programme and Dublin Institute of Technology’s Arnold F. Graves Postdoctoral Scholarship Scheme for funding this research.

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REFERENCES

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Low-Toxicity Photopolymer for Reflection Holography - ACS Applied

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