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We investigate volume holographic recording in a photopolymerizable nanoparticle-polymer (NPC) composite film that employs radical-mediated thiol-yne step-growth photopolymerizations. Because each alkyne functional group can react consecutively with two thiol functional groups in thiol-yne photopolymerizatins, the thiol-yne based NPC system dispersed with inorganic nanoparticles has the potentiality to overcome the drawback of low crosslinking densities but to retain the advantage of low shrinkage that is possible by use of thiol-ene photopolymerizations. We show that a thiol-yne based NPC film dispersed with 25 vol.% SiO2 nanoparticles and 15 wt.% single functional co-monomer gives the saturated refractive index change as large as 0.008 and the material recording sensitivity as high as 2005 cm/J at a recording and readout wavelength of 532 nm. We find that while the shrinkage of a volume hologram recorded in a thiol-yne based NPC dispersed with organic nanopartices can be as low as 0.5%, it is approximately 1% with the dispersion of SiO2 nanoparticles due to the plasticizing effect of the doped co-monomer. On the other hand, the thermal stability is improved better with the dispersion of SiO2 nanoparticles. We also demonstrate shift-multiplexed holographic storage of 80 digital data pages in a thiol-yne based NPC film with high readout fidelity.
© 2014 Optical Society of America
1. Introduction
Photopolymerizable nanoparticle-polymer comoposites (NPCs) for holographic applications generally consist of a photopolymer host that is uniformly dispersed with inorganic or organic nanoparticles [1]. Because nanoparticles can be assembled under holographic exposure by means of the so-called “holographic assembly of nanoparticles in polymer” [2,3], this holographic assembling technique provides volume holograms with high contrast in NPCs [4]. In order to efficiently perform the holographic assembly, the chain-growth free radical polymerizations are preferably used for host photopolymers since the rapid crosslinking reaction of chain-growing polymer in the bright illuminated regions facilitate the mutual diffusion and the phase separation of nanoparticles toward the darker regions [2,3,5–7]. Moreover, the inclusion of nanoparticles in such photopolymers forms rigid nanocomposite structures with increased glass transition temperature and therefore leads to the improvement of their thermal stability [8]. So far, volume holographic recording in NPCs dispersed with various types of nanoparticles were reported [4, 9–23], in which employed photopolymers were mostly (meth)acrylate photopolymers capable of chain-growth free radical photopolymerizations.
Although NPCs using chain-growth (meth)acrylate photopolymers provide high thermal stability due to their high crosslinking network formation, the most serious issue of using chain-growth (meth)acrylate photopolymers in holographic applications is volume (bulk) shrinkage of holograms taken place during recording. This happens because van der Waals distances between monomer molecules are converted to covalent bonds upon polymerization so that the resulting polymerization-induced microscopic free volume loss exhibits 22.5 ml shrinkage/mol C=C polymerized for chain-growth (meth)acrylate photopolymers and causes macroscopic volume shrinkage [24, 25]. Indeed, although the dispersion of nanoparticles in photopolymer substantively reduced shrinkage of recorded holograms [4, 9, 11, 12, 18, 22], such reduced shrinkage of a few % was still larger than 0.5%, the minimum requirement value for holographic data storage applications [26].
In order to mitigate shrinkage with NPCs further, we recently proposed the use of radical-mediated thiol-ene step-growth photopolymerizations [27] for NPCs. Thiol-ene systems, [28, 29] well-known for their uses in UV-curable adhensives and dental restorations, use step-growth polymerizations [see Fig. 1(a)] [30, 31] that proceed by a step-growth radical addition mechanism via sequential propagation of a thiyl radical (RS·) through a vinyl (an ene) monomer (R1CH=CH2) and the subsequent chain transfer of a generated thioether radical (R1C·H-CH2-SR) to a thiol (RSH), regenerating a thiyl radical and forming a thioether (R1CH2-CH2-SR). Shrinkage occurring before the gelation can be readily accommodated by the liquid mixture of oligomers [24]; the thiol-ene polymerizations proceed very rapidly but will not reach the gel point until high functional group conversions, resulting in 12–15 ml shrinkage/mol C=C polymerized [25], which represents a notable shrinkage and stress reduction when compared to that for (meth)acrylate systems as mentioned above. Other advantages of thiol-ene polymerizations include low toxicity and the absence of oxygen inhibition.
Fig. 1 Radical-mediated step-growth polymerization mechanisms of (a) thiol-ene and (b) thiol-yne photopolymerization reactions [31].
Indeed, we could demonstrate one order-of-magnitude reduction of shrinkage with thiolene based NPCs dispersed with either hyperbranched polymer (HBP) nanoparticles or SiO2 nanoparticles [32–34]. For example, using multifunctional mercaptopropionate thiols and allyl ether enes with HBP nanoparticles, we showed shrinkage reduction of recorded holograms as low as 0.3% with the saturated refractive index change (Δnsat) and the material recording sensitivity (S) as large as 8 × 10−3 and 1014 cm/J, respectively, in the green [32]. Here S is defined as , where I0 is an average recording intensity, ℓ is the effective thickness, η is the diffraction efficiency, and τ is the induction time period. The reduced shrinkage was comparable to other low-shrinkage dry photopolymer systems such as those including a high content of inert binder components and using monomers capable of cationic ring-opening polymerization [35]. However, the thermal stability of recorded holograms lowered due to the use of organic HBP nanoparticles. Later, we showed the improved thermal stability by using a new thiol-ene combination of secondary dithiol and allyl triazine triene having the rigid structure together with SiO2 nanoparticles [33]. This thiol-ene based NPC system gave large Δnsat (1 × 10−2), high S (1615 cm/J) and low shrinkage (0.4%), satisfying the acceptable condition of ≥0.005, ≥500 cm/J and <0.5%, respectively, for holographic data storage [26]. Using this thiol-ene NPC system, we successfully demonstrated holographic shift multiplexing of more than one hundred 2D digital data pages [36]. It was shown that symbol-error rates (SERs) and signal-to-noise ratios (SNRs) were as low as 10−3 and larger than 2, respectively. Despite this success, the thiol-ene polymerizations typically result in, as compared to (meth)acrylate photopolymers, low crosslinking densities and thereby low glass transition temperatures, leading to the limited thermal stability of a recorded hologram. This is so because the conversion of a vinyl to a thioether provides the vinyl group as monofunctional in thiol-ene photopolymerizations.
In order to improve the thermal stability further with keeping the advantage (i.e., low shrinkage) of the thiol-ene photopolymerizations, we propose here the use of radical-mediated thiol-yne step-growth photopolymerizations [31, 37, 38] for volume holographic recording in NPCs. The thiol-yne photopolymerization mechanism [31] [Fig. 1(b)] is different from the thiol-ene photopolymeriztions [Fig. 1(a)] in such a way that each alkyne functional group can react consecutively with two thiol functional groups. A thiyl radical adds across the alkyne “triple bond” (R1C≡CH), forming a vinyl sulfide radical (R1C·=CH-SR). This radial abstracts a hydrogen atom from a thiol, regenerating a thiyl radical and forming a vinyl sulfide (R1CH=CH-SR). Then, a thiyl radical adds across the “double bond” of the vinyl sulfide, generating a dithioether radical. This process is followed by the abstraction of a hydrogen atom from a thiol by the dithioether radical, regenerating a thiyl radical and forming a dithioether. In this paper we show that the thiol-yne based NPC system with the dispersion of SiO2 nanoparticles gives Δnsat as large as 0.008 and S as high as 2220 cm/J at a recording and readout wavelength of 532 nm. We also show that while the shrinkage of a recorded hologram can be as low as 0.5% with the dispersion of HBP nanoparticles, the thermal stability is improved better than that of the thiol-ene based NPC system when SiO2 nanoparticles are used in the thiol-yne based NPC. Furthermore, we demonstrate shift-multiplexed holographic digital data page storage in the thiol-yne based NPC system with high readout fidelity.
2. Experiments
2.1. Sample preparation
We employed a primary thiol monomer, trimethylolpropane tris(3-mercaptopropionate) (trithiol, Aldrich), and an yne monomer, 1, 7-octadiyne (diyne, Aldrich). These chemical structures are shown in Fig. 2. A stoichiometric composition of thiol and yne functional groups was used in the thiol-yne monomer formulation. We mixed the thiol-yne monomer formulation with SiO2 nanoparticles (average size of 13 nm) dissolved in methyl isobutyl ketone [9], together with a co-monomer, N-vinyl-2-pyrrolidone (NVP, Aldrich), as a plasticizer. As will be described in detail later, doping concentrations of NVP in the thiol-yne monomer mixture were determined by measuring their optimum values maximizing Δnsat at a given concentration of SiO2 nanoparticles; they were 10, 10, 10, 15 and 20 wt.% at SiO2 nanoparticle concentrations of 10, 15, 20, 25 and 30 vol.%, respectively. These combinations were used in our experiment unless otherwise stated. In addition, we added 2 wt.% titanocene organo-metallic complex (Irgacure 784, Ciba) in combination with 2.5 wt.% benzoyl peroxide (BzO2, Aldrich) to efficiently generate an initiating benzoyl oxy radical that abstracts a hydrogen atom from a thiol monomer thereby giving a thiyl radical in the green (532 nm) [39]. All reagents were used as received without further purification. Such syrup was cast on a glass plate and was dried in an oven to eliminate the solvent. We refer such a thiol-yne monomer-initiator combination with SiO2 nanoparticles to as sample I. To make film samples for optical measurements, we covered the syrup on a 10 μm spacer-loaded glass plate with another glass plate. For comparison we also prepared another stoichiometric thiol-yne monomer-initiator combination with organic HBP nanoparticles in the absence of NVP. We refer this combination to as sample II. Figure 3 shows spectral dependences of absorption coefficients before and after homogeneous exposure of incoherent green LED light for samples I and II with their optimum SiO2 and HBP nanoparticle concentrations of 25 and 20 vol.%, respectively, that maximize Δnsat. It can be seen that they are more or less 20 cm−1 at a wavelength of 532 nm before and after exposure for samples I and II, respectively. This means that their optically available thicknesses are much thicker than the required minimum media thickness of 500 μm [26].
Fig. 3 Spectral dependences of absorption coefficients (αs) for samples I and II before and after curing under green LED exposure.
2.2. Photopolymerization kinetics
We performed real-time spectroscopic studies by using a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700, ThermoFisher Scientific) with a liquid nitrogen cooled MCT detector to measure photo-induced polymerization reactions of thiol and yne functional groups in real time under ambient conditions. While the thiol functional group conversion was monitored with the S-H stretching absorption peak at 2570 cm−1, the yne functional group conversion was monitored with the ≡C-H stretching absorption peak at 2116 cm−1 [31]. Thiol and yne functional group conversions were calculated by taking the ratio of a change in absorption peak under curing to the absorption peaks before curing. Details of sample preparation and measurements for the real-time FTIR study were described in [21, 33].
Thiol and yne functional group conversions as a function of curing time were measured for the stoichiometric thiol-yne formulation without nanoparticles by using the real-time FTIR spectrometer. Figure 4 shows a parametric plot of measured thiol and yne conversions. It can be seen that the relative conversion ratio of thiol to yne functional groups is smaller than unity during exposure, indicating that the propagation of an yne monomer with a thiyl radical is faster than the chain transfer of a vinyl sulfide radical with an unreacted thiol. We speculate that a portion of yne monomers homopolymerize via reactions with vinyl sulfide radicals and/or initiating benzoyl oxy radicals that arise after decomposition of the complex of BzO2 with the photo-excited isomer of Irgacure 784 [39].
Fig. 4 Parametric dependences of thiol and yne functional group conversions without nanoparticle dispersion. The solid line corresponds to stoichiometric functional group conversion.
Parametric plots of thiol and yne functional group conversions for sample I are shown in Fig. 5(a), while those for sample II are shown in Fig. 5(b). It can be seen in Fig. 5(a) that the dispersion of SiO2 nanoparticles does not substantively influence on the reaction kinetics except for the case of the dispersion of 30 vol.% SiO2 nanoparticles with 20 wt.% NVP doping where the thiol conversion tends to take over the yne conversion. Such an exceptional trend is unexpected since thiol monomers cannot homopolymerize: when a thiyl radical abstracts a hydrogen atom from an unreacted thiol, the original thiyl radical reforms a thiol so that there is no net consumption of thiol groups [40]. Namely, thiol functional groups usually undergo repeated chain-transfer reactions that do not have an effect on the overall thiol conversion. We speculate that the reaction of thiyl radicals with highly doped NVP having a single C=C bond causes the inhibition of the yne reaction events and forms the step-growth polymerization. It can be seen in Fig. 5(b) that the relative conversion ratio of thiol to yne functional groups close to unity. The final conversions of thiol and yne monomers saturate at approximately 60% for sample II, which may be explained by an increase in viscosity with increasing HBP dispersion.
Fig. 5 Parametric dependences of thiol and yne functional group conversions for (a) sample I and (b) sample II at various concentrations of nanoparticles. The solid lines correspond to stoichiometric functional group conversion.
In order to investigate the influence of NVP on the thiol-yne photopolymerization process, we measured thiol and yne functional group conversions for sample I with 25 vol.% SiO2 nanoparticle dispersion. The result is shown in Fig. 6. It can be seen that while the final thiol functional group conversion is more or less unchanged above 90%, the yne functional group conversion decreases with an increase in NVP. This trend clearly shows the inhibition of the yne reaction events by the addition of NVP. Note that the final yne functional group conversion with 12.5 wt.% doping of NVP is lower than that with 15 wt.% doping of NVP. This result may be explained by the fact that while 12.5 wt.% doping of NVP in sample I with 25 vol.% SiO2 nanoparticle dispersion is highly viscous, the inhibition of the yne reaction events is prominent. Figure 7 shows temporal traces of the thiol and yne functional group conversions in early curing time duration for various formulations of thiol-yne monomers without SiO2 nanoparticles and NVP, thiol-yne monomers without SiO2 nanoparticles and with NVP (15 wt.%), and thiol-yne monomers with SiO2 nanoparticles (25 vol.%) and NVP (15 wt.%). It can be seen that the conversion rates for the two formulations with NVP are much higher than that without NVP (i.e., the neat thiol-yne formulation). Figure 8 shows thiol and yne functional group conversions versus the polymerization rate for various formulations of thiol-yne monomers, thiol-yne monomers with NVP (15 wt.%) and thiol-yne monomers with silica nanoparticles (25 vol.%). It can be seen that the polymerization rate increases with the addition of NVP, independently of SiO2 nanoparticles. It can also be seen that the gel-point conversion with NVP occurs earlier as compared with the neat thiol-yne monomer formulation. These trends are in consistent with the result shown in Fig. 7, where the conversion rates with NVP are much higher than that without NVP. We consider that the addition of NVP in the neat thiol-yne formulation accelerates the reaction event since doped NVP decreases the viscosity of the thiol-yne NPC system and significantly increases the thiol conversion. Further investigation is necessary to clarify the reaction kinetic process of NVP in thiol-yne photopolymerizations.
Fig. 6 Parametric dependences of thiol and yne functional group conversions for sample I with 25 vol.% SiO2 nanoparticle dispersion at NVP concentrations of 15, 20, 25 and 30 wt.%. The solid line corresponds to stoichiometric functional group conversion.
Fig. 7 Temporal traces of early thiol and yne functional group conversions for the stoichiometric thiol-yne monomer formulation (black), the stoichiometric thiol-yne formulation without SiO2 nanoparticles and with 15 wt.% NVP (blue) and the stoichiometric thiol-yne formulation with 25 vol.% SiO2 nanoparticles and 15 wt.% NVP (red). The solid and dotted curves correspond to thiol and yne functional group conversions, respectively.
Fig. 8 Parametric dependences of thiol and yne functional group conversions and their polymerization rates (Rps) for the stoichiometric thiol-yne monomer formulation (black), the stoichiometric thiol-yne formulation without SiO2 nanoparticles and with 15 wt.% NVP (blue) and the stoichiometric thiol-yne formulation with 25 vol.% SiO2 nanoparticles and 15 wt.% NVP (red). The solid and dotted curves correspond to thiol and yne functional group conversions, respectively.
Figure 9 shows thiol and yne functional group conversions versus the polymerization rate for sample II at various concentrations of HBP nanoparticles. It can be seen that the polymerization rates of thiol and yne functional groups significantly decrease with the addition of HBP nanoparticles. This is so because the viscosity of the formulations increases with the dispersion of HBP nanoparticles. It can also be seen that the gel-point conversion is lowered significantly with the dispersion of HBP nanoparticles due to the increase in viscosity.
Fig. 9 Parametric dependences of (a) thiol and (b) yne functional group conversions and their polymerization rates for sample II at various concentrations of HBP nanoparticles.
2.3. Holographic recording properties
We used a two-beam interference setup to record an unslanted transmission grating of 1-μm spacing with two mutually coherent beams of equal intensities from an Nd:YVO4 laser operating at 532 nm. A low intensity He-Ne laser beam operating at 633 nm was employed as a Bragg matched readout beam to monitor the grating buildup dynamics since the initiator system was insensitive in the red. All the beams were s-polarized. A schematic of the optical setup was described in [41]. We measured the diffraction efficiency η that was defined as the ratio of the 1st-order diffracted signal to the sum of the 0th- and 1st-order signals. The effective thickness ℓ of each sample was estimated from a least-squares curve fit of Kogelnik’s formula for an unslanted transmission grating [42] to the Bragg-angle detuning data of the saturated η (ηsat), so that Δnsat was extracted from ηsat with a help of Kogelnik’s formula and ℓ. Note that Δnsat measured at 633 nm was converted to that at 532 nm by multiplying the former by a factor being the ratio of Δnsat at 532 nm to that at 633 nm. Applying the factor to the buildup dynamics of Δn, we evaluated S at 532 nm. Since a weak recording intensity dependence of Δnsat was observed at various concentrations of SiO2 nanoparticles, I0 was determined to be 5 mW/cm2 maximizing Δnsat. This recording intensity was used in all the data shown below. We also evaluated the fractional thickness change σ arising from polymerization shrinkage by means of Dhar et al.’s holographic method [43].
Figure 10(a) shows a dependence of Δnsat on NVP concentration for sample I with the dispersion of 25 vol.% SiO2 nanoparticles. It can be seen that there exists the optimum NVP concentration (15 wt.%) maximizing Δnsat. This NVP-concentration dependence can be explained qualitatively by that of the final yne functional group conversion shown in Fig. 6, where the final yne conversion at the NVP concentration of approximately 15 wt.% is the highest among other NVP concentrations. Figure 10(b) shows a dependence of S on NVP concentration for sample I with the dispersion of 25 vol.% SiO2 nanoparticles. It can be seen that S is maximized at 15 wt.% NVP. This dependence is similar to the trend of Δnsat, both of which are attributed to the thiol-yne photopolymerization dependence on NVP concentration as shown in Figs. 7 and 8 that show the acceleration of the photopolymerization reaction events by the addition of NVP to the thiol-yne monomer formulation. Using the same procedure, we determined the optimum NVP concentrations maximizing both Δnsat and S at a given concentration of SiO2 nanoparticles for sample I as described earlier. These combinations were used in Figs. 11 and 16.
Fig. 10 Dependences of (a) Δnsat and (b) S on NVP concentration for sample I with the dispersion of 25 vol.% SiO2 nanoparticles.
Fig. 11 Dependences of (a) Δnsat and (b) S on nanoparticle concentration for sample I (red) and sample II (blue).
Fig. 12 Nanoparticle concentration vs. fractional thickness changes (shrinkage) σ measured in % for sample I (red), sample II (blue) and a methacrylate-based NPC sample (black).
Fig. 13 Thermo-optic coefficients dn/dT at 25 °C and at a wavelength of 546 nm as a function of nanoparticle concentration for uniformly cured film sample I (red), sample II (blue) and a methacrylate-based NPC sample (black).
Fig. 14 Linear coefficients of thermal expansion αL as a function of nanoparticle concentration for uniformly cured film sample I (red), sample II (blue) and a methacrylate-based NPC sample (black).
Fig. 15 Holographic recording of a two dimensional digital data page pattern using sample I with the dispersion of 25 vol.% SiO2 nanoparticles: (a) a single input image and (b) a reconstructed image.
Fig. 16 Shift-multiplexed holographic recording of 80 digital data page patterns using sample I with the dispersion of 25 vol.% SiO2 nanoparticles: (a) SERs and (b) SNRs as a function of stored digital data pages.
Figure 11(a) shows dependences of Δnsat on nanoparticle concentration for samples I and II. It can be seen that there exists the optimum nanoparticle concentration of 25 vol.% maximizing Δnsat for sample I, which is higher than 20 vol.% for sample II. The maximum values for Δnsat are comparable to those of NPCs using the thiol-ene based NPC system reported previously [32, 33], and they are larger than the required minimum value of 5×10−3 for holographic data storage [26]. Figure 11(b) shows dependences of S on nanoparticle concentration for samples I and II. It can be seen that values for S are maximized at nanoparticle concentrations of 20 and 15 vol.% for samples I and II, respectively. These values are slightly lower than those maximizing Δnsat shown in Fig. 11(a). While, thanks to NVP doping, all measured values for S of sample I are higher than the required minimum value of 500 cm/J for holographic data storage [26], those of sample II are much lower than that. Note that S is approximately given by πΔnsat/(λI0τ) [18], where λ is the recording wavelength in vacuum and τ is a grating buildup time constant. Since values of Δnsat for sample I and II comparable to each other, a large difference in S between samples I and II is attributed to the addition of NVP in sample I.
Figure 12 shows dependences of σ of recorded volume gratings on nanoparticle concentration for samples I and II, together with the result for a methacrylate-based NPC sample dispersed with 34 vol.% SiO2 nanoparticles [4] for comparison. It can clearly be seen that σ decreases with increasing nanoparticle concentration for all the samples. It can also be seen that σs for samples I and II are much lower than that for the methacrylate-based sample. This result indicates that the step-growth polymerization process is dominant in samples I and II as is also clearly seen in Fig. 5. We also observe that while σ for sample I is approximately 1%, that for sample II is smaller and comparable to that for a thiol-ene based NPC dispersed with SiO2 nanoparticles [32, 33]. Since the addition of NVP to the thiol-yne monomer formulation tends to move the gel-point conversion toward earlier polymerization stages (see Fig. 8), σ for sample I is larger than that for sample II. Such a correlation between σ and the gel-point conversion was also observed for non-stoicihometric thiol-ene system [34].
We also measured temperature-induced optical and mechanical distortions of plane-wave volume holograms recorded in samples I and II to examine environmental thermal stability of the thiol-yne based NPC system. Figure 13 shows nanoparticle-concentration dependences of the thermo-optic coefficient dn/dT for uniformly cured samples I and II measured at 25 °C and at a wavelength of 546 nm. For comparison, the data for a methacrylate-based NPC sample dispersed with 34 vol.% SiO2 nanoparticles [8] is also plotted. It can be seen that |dn/dT| is a decreasing function of nanoparticle concentration for sample I since the refractive indices of inorganic materials (i.e., SiO2 nanoparticles) are mainly determined by the temperature-dependent polarizability whose dn/dT is positive [44]. On the other hand, organic materials (i.e., the formed polymer) have negative dn/dT due to the main contribution of the temperature-dependent volume change to the refractive indices [44]. Such a difference in a sign of dn/dT between inorganic nanoparticles and the formed polymer results in the compensation of a temperature-dependent change in dn/dT for sample I. Therefore, |dn/dT| is an increasing function of HBP nanoparticle concentration for sample II with increasing temperature as seen in Fig. 13. We note that the methacrylate-based NPC sample has the smallest |dn/dT| owing to high crosslinking networks of the formed methacrylate polymer. Then, we estimated coefficients of linear thermal expansion αL from the measurement of temperature-dependent Bragg-angle detuning by means of Dhar et al.’s holographic method [43]. This additional information is necessary to evaluate the temperature stability of recorded volume gratings since thermal changes in refractive index and volume additively alter the Bragg-angle selectivity [45]. Namely, decreasing behavior of |dn/dT| and αL as a function of temperature leads to a reduction in temperature-induced optical distortions of recorded volume holograms. The detailed experimental procedure is described elsewhere [8].
Figure 14 shows out-of-plane αLs as a function of nanoparticle concentration. Once again, the data for a methacrylate-based NPC sample dispersed with 34 vol.% SiO2 nanoparticles is also plotted. It can be seen that the inclusion of inorganic SiO2 nanoparticles in sample I reduces αL significantly. This happens because changes in chain conformation of a formed polymer are constrained at boundaries of SiO2 nanoparticles that have very large surface areas relative to their volumes and possess smaller αL than the formed polymer. It is also found that sample I has smaller αLs than those of a thiol-ene based NPC system [33] at the same concentration of SiO2 nanoparticles. This result indicates that the thiol-yne photopolymerization in sample I provides a higher crosslinking density than that of the thiol-ene based NPC system. We expect further reduction in αL for sample I when the dispersion of SiO2 nanoparticles is possible without doping NVP. On the other hand, sample II has an increasing trend in αL with increasing temperature. We speculate that volume of a dispersed HBP nanoparticle increases with increasing temperature. Among the three NPC systems shown in Fig. 14 the methacrylate-based NPC sample gives the best performance in thermal stability owing to the dispersion of SiO2 nanoparticles and the high crosslinking networks in the formed methacrylate polymer. However, it exhibits much higher shrinkage than thiol-ene- and thiol-yne-based NPC systems as shown in Fig. 12. Therefore, we expect that the thiol-yne based NPC system with SiO2 dispersion can be a candidate for a well-balanced holographic data storage material possessing large Δnsat, high S, low shrinkage and high thermal stability.
Furthermore, we performed holographic digital data page storage in sample I with 25 vol.% SiO2 nanoparticles. The data page pattern contains 3600 symbols and 7200 bits of data information with the 2:4 symbol modulation code [36]. The coding efficiency and white rate (i.e., the average rate of bright bits in one symbol modulation code pattern) for the coded pattern was 0.50 and 0.25, respectively. Such a relatively low white rate avoids the saturation of a recording medium in low spatial frequency areas on recording and thereby improving an SER and an SNR [46]. Fourier transform holography setup was employed to record the data page pattern as described in [36]. Figure 15 illustrates a single input image [Fig. 15(a)] and the corresponding reconstructed image [Fig. 15(b)]. The output image with good fidelity can be quantitatively understood by the calculated SER and SNR that are lower than 1.4 × 10−4 and equal to 5, respectively. These values imply that error-free retrieval of data pages is possible with error-correcting coding (ECC) [47]. We also performed shift-multiplexed recording of 80 digital data pages with the 2:4 symbol modulation code. The optical setup and multiplexing scheme were the same as our previous experiment using a thiol-ene based NPC [36].
Figure 16 shows SERs [Fig. 16(a)] and SNRs [Fig. 16(b)] of reconstructed 80 holograms as a function of data page number. It can be seen that all SERs and SNRs are lower than 1 × 10−2 and are approximately equal to 4, respectively, implying that error-free retrieval of data pages is possible with ECC.
3. Conclusion
We have described volume holographic recording in NPCs using the thiol-yne chemistry of photopolymerization. We have found that the stoichiometric mixture of primary trithiol and diyne monomers partially exhibits homopolymerization behavior of yne monomer during curing. However, the dispersion of nanoparticles inhibits the homopolymerization. This trend is significant when the concentration of SiO2 nanoparticles is high. We have found that the thiol-yne mixture with nanoparticle dispersion exhibits the gelation point at early conversion. We have also found that doping of NVP is necessary to uniformly disperse SiO2 nanoparticles in the thiol-yne monomer formulation. The addition of NVP at the appropriate concentration (15 wt.%) results in a high conversion rate, resulting in high S. Measured values for Δnsat and S for the thiol-yne mixture with the optimum dispersion of SiO2 nanoparticles and NVP meet the media requirement for holographic data storage. We have confirmed that polymerization shrinkage can be suppressed by nanoparticle dispersion in the thiol-yne based NPC system. We have also shown that thermal thickness changes are substantively suppressed by the use of the thiol-yne formulation together with the dispersion of SiO2 nanoparticles owing to decreases in both |dn/dT| and the out-of-plane αL. We have demonstrated holographic digital data page storage with high fidelity readout in a thiol-yne based NPC film dispersed with SiO2 nanoparticles. We have shown that 80 digital data pages with low average SERs (< 1 × 10−2) and high SNRs (≈ 4) can be stored. Finally, we would like to comment on our present thiol-yne based NPCs. Doping of NVP as a plasticizer is necessary when SiO2 nanoparticles are employed. Although the addition of NVP increases S, it may cause some negative effect on shrinkage reduction and the improvement of thermal stability. It is therefore desirable to find any combination of thiol-yne formulation and inorganic (e.g., SiO2) nanoparticles, which does not requires any plasticizer. Such an investigation is underway.
Acknowledgments
This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan under grant 20360028.
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