We demonstrate and characterize volume holographic recording in ZrO2 nanoparticle-dispersed acrylate photopolymer films that have very low scattering loss. More than thirty-fold reduction in the scattering coefficient, as compared with those of previously reported TiO2 nanoparticle-dispersed photopolymers, is achieved. It is shown that the refractive index modulation as high as 5.3×10-3, together with substantive photopolymerization-shrinkage suppression, is obtained at the nanoparticle concentration of 15 vol.%. Dependences of nanoparticle concentration and grating spacing on the refractive index modulation are also investigated.
©2006 Optical Society of America
Various dry photopolymers [1–3] have been investigated so far for holographic/photonic applications such as holographic/micro-optic elements, optical waveguides, narrowband wavelength filters, information displays, and holographic data storage [4–6]. Recently, we have proposed a new type of holographic photopolymer in which inorganic [TiO2 (titania) or SiO2 (silica)] or organic (hyperbranched polymer) nanoparticles are incorporated for permanent volume holographic storage with high diffraction efficiency [7–11]. It was shown that the inclusion of nanoparticles also contributes to rapid grating buildup and noticeable suppression of polymerization shrinkage, yielding high recording sensitivity and dimensional stability. We also showed that the grating formation in nanoparticle-dispersed photopolymers could be explained in terms of the mutual diffusion of monomer molecules and nanoparticles during holographic exposure [12–14]: because reactive monomer is consumed more in the bright region than in the dark one under holographic illumination, monomer molecules diffuse from the dark to the bright regions due to the chemical potential difference between the bright and the dark regions. At the same time nanoparticles counter-diffuse from the bright to the dark regions. This is because the chemical potential of nanoparticles becomes higher in the bright region owing to their photo-insensitivity. As a result, compositional and density modulations of the formed polymer and nanoparticles having different refractive indices are spatially created, leading to the formation of the refractive index modulation (Δn) as large as 10-2.
Transparent inorganic oxides such as TiO2 and ZrO2 (zirconia) possess refractive indices higher than 2 in the visible (e.g., bulk refractive indices of 2.55 and 2.1 at 589 nm for TiO2 and ZrO2, respectively), which are much higher than those of available monomer and polymeric binder materials. Therefore such high-refractive-index inorganic nanoparticles may be used to increase Δn further. Indeed, we previously reported the first demonstration of volume holographic recording in TiO2 nanoparticle (the average diameter of 15 nm)-dispersed methacrylate photopolymers . We showed that this photopolymer system gave Δn as large as 5.1×10-3 and the polymerization-shrinkage suppression of approximately 69% was achieved at the nanoparticle concentration of 15 vol.%. However, prepared samples of 40-µm thickness suffered from noticeable scattering loss as high as 20% (without Fresnel correction) at a probe wavelength of 633 nm. Very recently, Sánchez et al.  reported a similar TiO2 nanoparticle-dispersed photopolymer system. They employed TiO2 nanoparticles having the average diameter of 4 nm, which were dispersed in the mixture of two kinds of acrylate monomers. They performed holographic recording at a wavelength of 351 nm and obtained Δn as large as 15.5×10-3 at 633nm. However, the scattering loss of their 15-µm-thick film was 12 % at 633 nm, which was higher than ours: if the film thickness were 40 µm, it would be 29 %. They suggested that the scattering loss could be attributed to, as similar to ours, partial aggregation of the nanoparticles.
A major cause of such relatively high scattering loss in high-refractive-index nanoparticle dispersed photopolymer is Rayleigh scattering  from interface discontinuities between nanoparticles/nanoparticle aggregates and the formed polymer. This scattering loss can be suppressed by reducing the nanoparticle size and at the same time by uniformly dispersing nanoparticles in monomer since the scattering coefficient the turbidity ) τ of Rayleigh scattering has a cubic dependence on the diameter for a spherical nanoparticle at a given nanoparticle volume fraction . Indeed, a substantive reduction in the scattering loss is very important for holographic data storage applications where a photosensitive layer thicker than 500 µm is required to record a large number of holograms in the same area of a recording medium .
In this paper we describe volume holographic recording in high-refractive-index ZrO2 nanoparticle-dispersed acrylate photopolymers. High contrast holograms with very low scattering loss are fabricated. Effects of ZrO2 nanoparticle dispersion on Δn and shrinkage suppression are investigated. Dependences of Δn and recording sensitivities on the concentration of a radical initiator are also evaluated. We note that previously Oliveira et al.  and very recently del Monte et al.  reported organic-inorganic composite photopolymers in which inorganic species is formed by the in situ condensation of methacrylic acid (MA) complexed zirconium isopropoxide [Zr(OiPr)4] in a matrix material of trimethoxy-[3-(oxiran-2-ylmethoxy)propyl]silane during the sol-gel process. Such an MA:Zr complex is considered to diffuse during holographic exposure to increase Δn. On the other hand, our proposed photopolymer system contains independently doped ZrO2 nanoparticles that play an important role in the holographic recording process.
ZrO2 nanoparticles prepared by liquid-phase synthesis  were dissolved in toluene solution. Some chemical treatment to the surface of the nanoparticle was made to avoid unwanted aggregation in a host material. Figure 1 shows a transmission electron microscope (TEM) image of ZrO2 nanoparticles deposited on carbon-coated grids after evaporating a toluene suspension of the ZrO2 sol. It can be observed that the nanoparticles have the average size of approxi-mately 3 nm and that they partially aggregate due, probably, to their lateral diffusion during the evaporation process . The ZrO2 sol was dispersed to acrylate monomer [2-propenoic acid, (octahydro-4,7-methano-1H-indene-2,5-diyl)bis(methylene) ester] whose refractive indices were 1.50 in the liquid phase and 1.53 in the solid phase, respectively. We used titanocene (Irgacure784, Ciba) as a radical initiator providing the photosensitivity at wavelengths shorter than 550 nm. The concentration of the initiator was 1 wt.% unless it is otherwise stated later in this paper. The earlier ZrO2 nanoparticle-dispersed monomer that was mixed with the initiator was cast on a spacer-loaded glass plate. It was dried at 80°C for approximately 60 min in an oven and was finally covered with another glass plate tomake samples for holographic measurements. We prepared these samples with several ZrO2 nanoparticle-to-monomer concentration ratios (0:100, 5:95, 10:90, 15:85, and 20:80 in volume).
We used a conventional two-beam interference setup to write an unslanted transmission grating 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 buildup dynamics of the grating since the initiator was insensitive in the red. All the beams were s-polarized. 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 of each sample was estimated from a least-squares curve fit to the Bragg-angle detuning data with Kogelnik’s formula for an unslanted transmission grating . Then Δn was extracted from η with a help of Kogelnik’s formula and the estimated effective thickness.
To elucidate high transparency of a recorded hologram, we qualitatively examined the scattering properties of ZrO2 nanoparticle-dispersed photopolymer. Figure 2 shows photographs of a hologram recorded in a sample with the ZrO2 nanoparticle concentration of 15 vol.%. Good uniformity and high transparency of the hologram are seen. We measured the scattering loss of a recorded hologram, which we defined as unity minus the ratio of the sum of the transmitted and the diffracted powers to the incident power with Fresnel correction. It was found that a sample with the ZrO2 nanoparticle concentration of 15 vol.% had the scattering loss lower than 1% at the film thickness of 40 µm. This result is more than twenty-fold scattering noise reduction as compared with those of previously reported TiO2 nanoparticle-dispersed photopolymers [7, 16] and is nearly comparable to those of SiO2 nanoparticle-dispersed photopolymers  and sol-gel photopolymers containing Zr-based inorganic species . We used the measured and reported scattering losses (at 633 nm) of these photopolymers to estimate their corresponding τs. It was found that while τ was 1.6 cm-1 for our ZrO2 nanoparticle-dispersed photopolymer, they were 56 and 85 cm-1 for our  and Sánchez et al.’s  TiO2 nanoparticle-dispersed photopolymers, respectively. Again, our ZrO2 nanoparticle-dispersed photopolymer gains more than thirty-fold reduction in τ as compared with those of the above TiO2 nanoparticle-dispersed photopolymers. It is also lower than that (=12 cm-1) of the sol-gel photopolymers containing Zr-based inorganic species . Such a significant scattering noise reduction can be attributed to the uniform dispersion of nm-size ZrO2 nanoparticles. When these values are converted to transmittances of 500-µm film thickness that may be the minimum thickness required for holographic data storage media , they are approximately 92, 6, 1 and 55 % for our ZrO2, our TiO2, Sánchez et al.’s TiO2 nanoparticle-dispersed photopolymers and the sol-gel photopolymers containing Zr-based inorganic species, respectively. High optical quality of our ZrO2 nanoparticle-dispersed photopolymer is evident. Figure 3 shows a spectral dependence of τ for cured samples without and with ZrO2 nanoparticle dispersion. Curing was performed under uniform illumination of incoherent light from a thermal UV lamp (the center wavelength of 365 nm) for 15 min in order to avoid unwanted coherent scattering that often took place when a coherent laser beam was used . We extract τs for samples without and with ZrO2 nanoparticle dispersion from the measurement of their spectral transmittance data. The dash-dotted curve corresponds to a least-squares fit to the data with the Rayleigh scattering formula [17, 18, 24] for the ZrO2 nanoparticle-dispersed sample. It can be seen that the spectral curve for the ZrO2 nanoparticle-dispersed sample well follows the Rayleigh’s λ -4 law. The extracted value for the average diameter of the nanoparticle with n=2.1 was 2 nm, in good agreement with that estimated from the TEM image shown in Fig. 1. This result also indicates that substantive aggregation of ZrO2 nanoparticles in the cured film is negligible. Therefore, the scattering loss of our ZrO2 nanoparticle-dispersed photopolymer is predominantly determined by Rayleigh scattering that depends on the nanoparticle size, the nanoparticle concentration and a refractive-index difference between the nanoparticle and the formed polymer.
Figure 4 shows the recording dynamics of Δn at a grating spacing of 1.0 µm for samples with different ZrO2 nanoparticle concentration ratios at a recording intensity of 10 mW/cm2. It can be seen that the saturation of Δn reaches at an exposure fluence of approximately 10 J/cm2 and there exists the optimum value for the ZrO2 nanoparticle concentration. Namely, the sample with the ZrO2 nanoparticle concentration of 15 vol.% gives the largest steady-state value for Δn (=3.9×10-3) at a recording intensity of 10 mW/cm2, which is nearly the same as that of our previously reported TiO2 nanoparticle-dispersed photopolymers . Figure 5 shows a dependence of the saturated Δn on the ZrO2 nanoparticle concentration at several recording intensities. It can be seen that the optimum concentration maximizing the saturated Δn weakly depends on the recording intensity and it is between 10 and 20 vol.%. This trend can be explained as follows: when no nanoparticle is dispersed, no steady-state refractive index modulation is created since unreacted monomer is eventually polymerized in the dark region of the intensity-interference fringe pattern. As the doped nanoparticle concentration increases, Δn increases due to the mutual diffusion of monomer molecules and nanoparticles during holographic exposure. However, too much dispersion of nanoparticles causes a decrease in the mutually diffusing amount of monomer molecules and nanoparticles, leading to a decrease in Δn. The evidence of holographically controlled periodic assembling of ZrO2 nanoparticles in photopolymer as a result of the mutual diffusion [12, 14] is shown in Fig. 6.
Figure 7 shows a grating-spacing dependence of the saturated Δn for the sample with the ZrO2 nanoparticle concentration of 15 vol.% at several recording intensities. It can be seen that, as similar to the cases of TiO2 and SiO2 nanoparticle dispersion [7, 10], the saturated Δn is peaked at grating spacings longer than 1 µm. We speculate that this peculiar trend is attributed to the nonlocal spatial response of photopolymer materials [15, 25]. The decrease in Δn at short grating spacings happens because of the spatial extension of radical monomer chains to the dark region. On the other hand, the decrease in Δn at long grating spacings is caused by the dominance of the polymerization rate over the mutual diffusion rate. It can also be observed that the influence of the nonlocal spatial response on Δn appears to be weaker with an increase in the recording intensity. This may be explained by the decreased effective polymer chain length owing to the increased termination rate of radical monomer at high recording intensities as observed in holographic polymer-dispersed liquid crystals .
We investigated dependences of the saturated Δn and recording sensitivities on the initiator concentration. Table 1 shows the saturated Δn and static/dynamic sensitivities S (≡ Δn 0/E) and S* (≡dΔn 0/E|t=tind) (where Δn 0, E and tind are the saturated Δn, the exposure fluence on the saturation of Δn and the induction time period, respectively) [8, 27] at a grating spacing of 1.0 µm and at several recording intensities for samples with several concentrations of the initiator. It can be seen that while the saturated Δn, S and S* increase significantly with an increase in the initiator concentration at a low recording intensity of 1 mW/cm2, they are less influenced at higher recording intensities of 10 and 100 mW/cm2. Such a trend happens because of an increase in the amount of radical monomer termination with an increase in the recording intensity. It is observed that the initiator concentration of approximately 3 wt.% at the recording intensity of 10 mW/cm2 gives the best performance. We note that the absorption coefficient α of the uncured sample with the initiator concentration of 5 wt.% was 8.1 cm-1 at 532 nm, giving the corresponding effective thickness α -1 is longer than 1 mm. Therefore, a volume hologram thicker than 500 µm can be recorded uniformly along the thickness direction at the initiator concentrations up to approximately 5 wt.%.
We also evaluated the fractional thickness change σ  arising from polymerization shrinkage. We first measured Bragg-angle detuning ΔθB as a function of tilt angle measured from the normal to the sample’s surface with respect to the bisector of the two recording beams. We estimated s from a least-squares fit to the measured data of ΔθB with the theoretical formula  as a function of tilt angle. It was found that estimated values for σ were 4.9, 4.0, 3.9, and 3.4 % for the samples with the ZrO2 nanoparticle concentrations of 5, 10, 15 and 20 vol.%, respectively. Because the monomer’s σ is approximately 9.1 % (obtained from the density measurement), the effectiveness of the ZrO2 nanoparticle dispersion on shrinkage suppression is evident.
We have demonstrated permanent volume holographic storage in ZrO2 nanoparticle-dispersed acrylate photopolymer. We have shown that the refractive index modulation as large as 5.3×10-3 is recorded at a grating spacing of 1.5 µm with markedly high suppression of scattering loss (i.e., more than thirty-fold reduction in τ as compared with those of previously reported high-refractive-index TiO2 nanoparticle-dispersed photopolymers). Such a highly transparent and high contrast hologram results from the use of nm-size ZrO2 nanoparticles capable of excellent uniform dispersion in monomer. We have also confirmed substantive shrinkage suppression by the ZrO2 nanoparticle dispersion. We expect that ZrO2 nanoparticle-dispersed photopolymers are useful for many holographic applications.
We thank K. Odoi for fruitful discussions. This work was supported by the 21st Century Center-of-Excellence (COE) program, the University of Electro-Communications, granted byMinistry of Education, Culture, Sports, Science and Technology, Japan.
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