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Holographic manipulation of nanoparticle distribution morphology in nanoparticle-dispersed photopolymers Yasuo Tomita and Naoaki Suzuki Department of Electronics Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
Katsumi Chikama Chemical Research Laboratories, Nissan Chemical Industries, Ltd., 722-1 Tsuboi, Funabashi, Chiba 274-8507, Japan Received November 12, 2004 We report on experimental verification of mass transfer of nanoparticles during holographic recording in nanoparticle-dispersed photopolymers. Through direct observations of the microscopic structure of recorded holograms as well as optical measurements of the phase shift between the light interference pattern and a recorded hologram we find that holographic exposure causes nanoparticles to be redistributed from bright to dark regions, leading to periodic assembly of nanoparticles and thereby to formation of high-contrast holograms. © 2005 Optical Society of America OCIS codes: 090.2900, 090.7330, 160.4890, 160.5470.
Recently, Tomita and co-workers proposed a new type of organic–inorganic nanocomposite photopolymer system in which inorganic nanoparticles with a larger refractive-index difference from photopolymerized monomers are dispersed in unreacted monomers, as shown in Fig. 1(a).1–4 Inorganic materials possess a wide variety of refractive indices that give us the opportunity to obtain much higher refractiveindex changes ⌬n than conventional photopolymers.5 Inclusion of nanoparticles also contributes to rapid buildup of fixed holograms and noticeable suppression of polymerization shrinkage, giving high recording sensitivity and dimensional stability. Despite such high performance, the mechanism of holographic grating formation has not been clearly understood. In this Letter we show experimental evidence of holographic control of nanoparticledistribution morphology in nanoparticle-dispersed photopolymers. Analogous with two-organic-component photopolymers6 and holographic polymer-dispersed liquid crystals,7 one may understand holographic grating formation in our photopolymer system by considering the chemical potential. If one ignores interactions between components of a mixture (monomers and nanoparticles), the chemical potential of the ith component of the mixture is approximately written as i = 0i + kT ln共Ni / ⌺jNj兲, where 0i is the chemical potential of the pure ith component, Ni is the number density of the ith component, and kT is the thermal energy.8,9 Under thermodynamic equilibrium i is constant everywhere in the mixture. For monomers with radical photopolymerization spatially nonuniform light illumination produces free radicals by dissociation of initiators, and subsequent reaction of free radicals with monomers leads to chain polymerization of individual monomers in the bright regions. This polymerization process lowers the chemical potential of monomers in the bright regions, leading to 0146-9592/05/080839-3/$15.00
migration (diffusion) of monomers from the dark to the bright regions. On the other hand, photoinsensitive inorganic nanoparticles experience counterdiffusion from the bright to the dark regions, as illustrated in Fig. 1(b), since the particles are not consumed and their chemical potential increases in the bright regions as a result of consumption of monomers. Such a mutual diffusion process essentially continues until the monomers are consumed completely by the monomolecular and bimolecular termination processes and (or) until the high viscosity of a surrounding medium consisting of polymerized monomers makes monomers and nanoparticles immobile. As a result the spatial distribution of nanoparticles is also fixed and a refractive-index grating (a hologram) is created as a result of compositional and density differences between the bright and the dark regions. Diffusion of nanoparticles of the order of 10 nm in diameter in monomer syrup would be possible based on the following numerical estimation: assuming a spherical nanoparticle with diameter d in a liquid with viscosity , the diffusion constant D of
Fig. 1. Schematic of distributions of constituents (monomers and nanoparticles) (a) before and (b) during holographic exposure to describe the holographic recording process in a nanoparticle-dispersed photopolymer. © 2005 Optical Society of America
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Fig. 2. TEM images of cross sections of recorded holograms in SiO2 nanoparticle-dispersed photopolymer samples after (a) uniform and (b) holographic exposure. Note that the black portions in these images correspond to nanoparticles. Total exposure intensity was 100 mW/ cm2.
the nanoparticle is given by the Stokes–Einstein relation D = kT / 3d. Substituting d = 10 nm and = 50 cP (a typical value of photopolymerizable monomers for holographic recording) into this formula, we obtain D ⬇ 1 ⫻ 10−8 cm2 / s at room temperature. This value is comparable with that of monomers 共10−9 – 10−7 cm2 / s兲,10 suggesting that mutual diffusion is possible. To examine the applicability of the mutual diffusion model to our photopolymer system, we first investigated the morphology of nanoparticle distributions embedded in a polymerized monomer material after holographic exposure. We prepared a sample by use of silica 共SiO2兲 nanoparticles with a bulk refractive index of 1.46, and an average diameter of 13 nm as an inorganic guest material. These nanoparticles were initially dispersed in a solution of methyl isobutyl ketone and were mixed in 34 vol. % to methacrylate monomers [2-methylacrylic acid 2-4-[2-(2-methyl-acryloyloxy)ethylsulfanylmethyl]-benzylsulfanyl-ethyl ester] (Ref. 11) whose refractive indices were 1.55 in the liquid phase and 1.59 in the solid phase, respectively, at a wavelength of 589 nm. Titanocene as an initiator was also mixed in 1 wt. % with respect to monomers. The chemical mixture described above was cast on a glass plate with a polyester-film spacer in approximately 50-m height. The mixture was dried in an oven and finally was covered with another glass plate. Such a sample was holographically exposed to two mutually coherent beams at a wavelength of 532 nm so that a transmission-type hologram was recorded at a grating spacing of 1 m. Figure 2 shows TEM images of cross sections of recorded holograms in the thickness direction when exposed uniformly [Fig. 2(a)] and holographically [Fig. 2(b)]. It can be clearly seen from Fig. 2(b) that the distribution of SiO2 nanoparticles indeed follows the
intensity interference fringe pattern at a grating spacing of 1 m. We also observed such a phenomenon of periodic assembly of nanoparticles in holograms at grating spacings of 1.5 and 2 m. Given ⌬n 共⬇0.01兲 and the average volume fraction 共=34% 兲 of the nanoparticles in this sample,4 we could estimate the relative volume fraction change between the bright and the dark regions to be 22% in the nanoparticle distribution after holographic exposure, which is consistent with the nanoparticledistribution morphology shown in Fig. 2(b). We also found that the volume change as a result of polymerization shrinkage after holographic exposure induced small surface corrugation of the sample, as shown in Fig. 3. However, the height of the corrugation was of the order of 1 nm. The fractional thickness change as a result of polymerization shrinkage was also a few percent.4 These observations indicate that periodic assembly of nanoparticles as shown in Fig. 2(b) can be attributed to mass transfer of nanoparticles during holographic exposure. To determine the mass-transfer direction of nanoparticles during holographic exposure, we measured the phase shift between the intensity interference fringe pattern and the corresponding recorded hologram in real time. We applied the running fringe method of Kondilenko et al.,12 by which could be extracted by analyzing temporal changes in transmitted intensities of two recording beams as a function of external phase shift that was intentionally introduced in one of the recording beams during holographic recording. The dynamics of diffraction efficiency as the ratio between the Bragg diffracted and the total incident light intensities of one of the two recording beams were also measured at the same time to monitor the grating-buildup process. If nanoparticles move from the bright to the dark regions while monomers move from the dark to the bright regions during holographic exposure, would be 0° (180°) when the refractive index of nanoparticles nn is lower (higher) than that of photopolymerized monomers np. To evaluate these two cases (i.e., nn ⬍ np and nn ⬎ np), we prepared a SiO2 nanoparticledispersed photopolymer sample with nn ⬍ np as described above and also another sample with nn ⬎ np by use of titania 共TiO2兲 nanoparticles with a bulk re-
Fig. 3. Atomic-force microscope image of the surface morphology of a SiO2 nanoparticle-dispersed photopolymer sample after holographic exposure at a grating spacing of 1 m.
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lographic data storage5 and linear photonic crystals13 but also for adding completely new functionality to holographic applications that use photopolymers. For example, nonlinear periodic structures14 and nonlinear photonic crystals15 can be holographically fabricated by use of highly nonlinear optical nanoparticles such as II–VI semiconductor CdSe nanocrystals.16 Other new possibilities include the use of magnetic nanocrystals for new magneto-optic applications and the use of inorganic nanocrystals for multifunctional light-emitting devices. Furthermore, the method may also be used to periodically align chemical and biological species attached to nanoparticles.
Fig. 4. Temporal evolution of diffraction efficiency (open circles) and phase shift (filled circles) for (a) SiO2 and (b) TiO2 nanoparticle-dispersed photopolymer samples in which a transmission-type hologram with 1-m spacing was recorded in each sample by two mutually coherent beams at a wavelength of 532 nm.
fractive index of 2.55 and an average diameter of 15 nm. The details of the TiO2 sample preparation are described in Ref. 1. The measured results are shown in Fig. 4(a) for the SiO2 nanoparticledispersed sample and in Fig. 4(b) for the TiO2 nanoparticle-dispersed sample. It is indeed seen that is approximately 0° (i.e., ⌬n is the highest in the bright region) for the SiO2 nanoparticle-dispersed sample while it is 180° (i.e., ⌬n is highest in the dark region) for the TiO2 nanoparticle-dispersed sample during and after hologram buildup. These results, together with the result shown in Fig. 2, indicate that nanoparticles indeed experience counterdiffusion from bright to dark regions under holographic illumination, confirming the applicability of the mutual diffusion model and the definitive role of nanoparticles as a mobile component that enhances ⌬n in holographic recording. Our finding of all-optical control of nanoparticledistribution morphology in photopolymers by holographic means can be used not only for realizing high-contrast holograms for applications such as ho-
This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan under grant 16651062, the 21st Century Centers-of-Excellence (COE) program “Coherent optical sciences,” and the Iketani Science and Technology Foundation. Y. Tomita’s e-mail address is
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