Nanoparticle-polymer composite gratings fabricated byholographic assembly of nanoparticles and their applications

Yasuo Tomita

Department of Engineering Science, University of Electro-Communications,1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan

∗e-mail: ytomita@uec.ac.jp

1. Introduction

Photopolymerizable nanoparticle-polymer composites (NPCs) consist of photopolymerizable monomer(photopolymer) dispersed with inorganic or organic nanoparticles at high concentrations [1]. Thespatial distribution of nanoparticles can be controlled artificially by holographic exposure. Thismethod, the so-called holographic assembly of nanoparticles in polymer [2], enables us to constructthe single step formation of large scale and multi-dimensional photonic crystal structures in thickfilms, which can be used for versatile applications in photonics and other fields of science andengineering. For example, the tailored design of NPCs provides nanocomposite volume gratingsfor holographic data storage, distributed feedback lasers, and electromagnetic/quantum beam con-trol. In this summary we briefly describe our recent works on NPC volume gratings for novelapplications in photonics and neutron optics.

2. Holographic assembly of nanoparticles

Suppose that nanoparticles are uniformly dispersed in host monomer capable of radical photopoly-merization as shown in Fig.1(a). Spatially inhomogeneous light illumination such as a two-beamlight-intensity interference pattern produces free radicals by dissociation of doped initiators andthe subsequent reaction of free radicals with monomer molecules leads to free-radical mediatedpolymerization in the bright regions. This photopolymerization process lowers the chemical po-tential of monomer in the bright regions, leading to the migration (diffusion) of the monomerfrom the dark to the bright regions. On the other hand, photoinsensitive nanoparticles experi-ence the counterdiffusion from the bright to the dark regions since they are not consumed andtheir chemical potential increases in the bright regions due to the formation of polymer. Sucha polymerization-driven mutual diffusion process essentially continues until the photopolymeriza-tion reaction completes. In this way the mutual diffusion of monomer and nanoparticles resultsin macroscopic assembly of nanoparticles during holographic exposure [see Fig.1(b)]. As a result,a refractive index grating is induced owing to compositional and density differences between the

(a) (b) (c) (d)

Figure 1: Schematic of distributions of monomer and nanoparticles (a) before and (b) duringholographic exposure. (c) Density distribution of nanoparticles. (d) Density distribution of theformed polymer.

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bright and the dark regions. It was confirmed experimentally that the distribution of dispersednanoparticles followed the light-intensity interference pattern in an NPC film [2–4]. In this case the180◦ phase shift of the formed polymer and nanoparticles distributions takes place as a result ofthe mutual diffusion process. Therefore, when a combination of monomer and nanoparticles havinga large difference in their refractive indices is chosen appropriately, the saturated refractive indexchange (∆nsat) of a volume gratings recorded in an NPC film can be increased. Furthermore, themechanical and thermal stability of a recorded volume grating against polymerization shrinkageand thermal changes in film thickness and refractive index can be improved by high dispersion ofinorganic nanoparticles and by use of free-radical mediated step-growth polymerizations [1, 5, 6].

3. Applications

3-1. Holographic data storage

Because NPCs provide volume gratings with ∆nsat as large as 1×10−2, high recording sensitivityin the green and the blue, reduced polymerization shrinkage and high thermal stability, they can beused for optical recording media in holographic data storage systems [7]. It is shown that NPCs withthiol-ene monomers capable of free-radical mediated step-growth photopolymerizations gives sub-stantive shrinkage suppression as low as 0.3% [6,8]. For this purpose the stoichiometric thiol-ene for-mulation of commercial secondary dithiol monomer, 1,4-bis(3-mercaptobutyryloxy)butane (ShowaDenko K.K.), and triene monomer, triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (Aldrich) wereused together with the dispersion of 25 vol.% SiO2 nanoparticles (the average size of 13 nm) anda photoinitiator/green-sensitizer system consisting of Irgacure784 (Ciba) and BzO2 (Aldrich) [6].Details of the sample preparation as well as the photopolymerization and (plane-wave) holographicrecording properties were described in [6]. Figure 2 shows reconstructed digital data page imagesof the 21st [Fig. 2(a)] and 241st holograms recorded in an 250 µm-thick NPC film, respectively.In this example, 265 volume holograms were stored for different 2D digital data page patternswith the 2:4 modulation coding in a shift-multiplexed two-beam holographic recording setup [9].It was found that most of reconstructed data page images had symbol error rates (SERs) lowerthan 1× 10−2 and signal-to-noise ratios (SNRs) of larger than 2, implying that error-free retrievalof data pages is possible with error correction code. It was also shown that higher modulationcoding could give lower SERs and higher SNRs [10].

(a) (b)

Figure 2: Reconstructed 2D digital data page images of (a) the 21st and (b) the 241 data pageholograms in recording order.

3-2. Nonlinear optics

Since NPCs can be tailored by selecting a type of nanoparticles for a particular application, inor-ganic oxide and hyperbranched polymer (HBP) nanoparticles have been usually used for recordinghigh contrast volume holograms [1, 6, 11]. In order to induce optical nonlinearities in a large area

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NPC film, one may employ metallic nanoparticles (e.g., Au) and semiconductor quantum dots(QDs) as nanoparticles. In the past nonlinear optical responses of metal-dielectric nanocompositeswere measured near the surface plasmon resonance (SPR) [12] that took place at the interfacebetween metallic nanoparticles and a dielectric matrix host. The observed optical nonlinearity wasattributed to the coherent oscillation of free electrons occupying states near the Fermi level in theconduction band, which gave rise to a surface plasmon absorption band whose peak and widthwere dependent on the size of metallic nanoparticles and the permittivity of a dielectric matrixhost. This SPR resulted in an increase in the electric field inside the metal, the so-called localfield enhancement [13], yielding to the enhancement of the optical nonlinearities of metal-dielectricnanocomposites. Recently, we studied the nonlinear optical properties of NPCs dispersed withHBP-metallic (Au or Pt) nanoparticle complex. It was shown that they exhibited the dielectricconfinement effect near SPR and that the magnitude of their effective third-order nonlinear opticalsusceptibility was of the order of 10−10 esu at a wavelength of 532 nm [14].

We also fabricated semiconductor QD-dispersed NPCs with ionic liquid monomer for their non-linear optical study. QDs have interesting characteristics such as fluorescence tunability, enhancedphotosensitivity and large optical nonlinearities. The electronic states of QDs are strongly influ-enced by the quantum confinement effect when the radius of QDs is smaller than approximatelythree times of the exciton Bohr radius. The band gap (Eg) of QDs increases with decreasing theirsize and the quantum confinement effect can strongly enhance the third-order optical nonlinear-ity [15]. The II-VI bulk semiconductor CdSe has the direct band gap Eg = 1.74 eV at 300 Kand has the exciton Bohr radius of 5.6 nm. Therefore, the strong quantum confinement effect ofCdSe QDs plays an important role in a large enhancement of the optical nonlinearity as comparedwith that of the bulk CdSe. We successfully fabricated a highly efficient Bragg grating in anNPC film dispersed with CdSe QDs [16]. It was found that a CdSe QD-dispersed NPC film gave∆nsat of 1 × 10−3 at a low CdSe QD concentration of 0.35 vol.%. We also observed the third-and fifth-order optical nonlinearities in a uniformly cured CdSe QD-dispersed NPC film [17]. Theeffective third-order nonlinear optical susceptibility was found to be of the order of 10−11 esu ata wavelength of 532 nm. Figure 3 shows transmitted and self-diffracted beam patterns througha 50 µm-thick CdSe QD-dispersed NPC film in a degenerate multi-wave mixing setup [17], wherea pair of self-diffracted beams due to the fifth-order nonlinear optical nonlinearity is seen. Thisresult indicates a CdSe QD-dispersed NPC film possesses a large nonlinear optical effect.

Figure 3: Transmitted and self-diffracted beam patterns through a uniformly cured CdSe QD-dispersed NPC film.

3-3. Neutron optics

Neutron optics and spectroscopy have been extensively studied from viewpoints of fundamentalscience and of medical/industrial applications [18]. Neutron optical elements such as mirror andbeam splitters are essential for the construction of a neutron interferometer [19]. So far, a perfectsilicon wafer is used to control a thermal neutron beam for this purpose. While slow-neutrons(cold and very cold neutrons) possessing their longer wavelengths (0.4 nm < λ < 10 nm) providebetter interferometric sensitivities as compared with thermal neutrons, they require other neutronoptical elements because of the inability to diffract slow-neutron beams from a silicon. Rupp etal. demonstrated the diffraction of a cold neutron beam (λ= 1.5 nm) by a holographic volumegrating optically recorded in a deuterium-substituted (poly)methylmethacrylate (PMMA) based

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photopolymer [20]. Despite their success of slow-neutron diffraction the diffraction efficiency waslimited by the Pendellösung oscillation averaging due to the very thick film (∼ 2 mm) and thelimited collimation of a neutron beam. In order to circumvent this problem, we develop neutronoptical elements by use of a volume grating optically recorded in a thick (∼ 100 µm) NPC filmwith (meth)arylate monomer capable of free radial mediated chain-growth polymerizations andwith the dispersion of SiO2 nanoparticles having a relatively large coherent scattering length forslow-neutrons. So far, operations of a half-mirror for cold (λ=2 nm) and very cold (λ=4.1 nm)neutrons, a mirror with the diffraction efficiency of 90% for very cold (λ=4.1 nm) neutrons anda triple beam splitter for cold (λ=2 nm) neutrons have been demonstrated successfully [21–24].Figure 4 shows a photograph of a free-standing NPC volume grating that can be used for a slow-neutron mirror.

Figure 4: A photograph of a free-standing NPC volume grating recorded at a wavelength of 532nm for a slow-neurton mirror. The grating spacing is 0.5 µm and the film thickness is 115 µm.

4. Conclusions

We have described the properties and applications of holographic NPC volume gratings for holog-raphy, nonlinear optics and neutron optics. Since NPCs can be tailored by selecting appropriatemonomers and nanoparticles, a wide variety of material design and applications can be expected.In holographic data storage applications NPCs provide the realization of holographic recordingmedia that possess large ∆nsat, high recording sensitivity, low shrinkage and high thermal stabil-ity simultaneously. Such simultaneous improvement cannot be realized by conventional all-organicphotopolymers. In nonlinear optics applications it is possible to construct multi-dimensional non-linear photonic lattice structures in NPCs by holographic assembly of nanoparticles, which can beused for photonic applications such as optical switching/limiting and nonlinear photonic crystals.In neutron optics applications slow-neutron beams can be manipulated holographically by NPCvolume gratings with high efficiency. It would lead to a new possibility of slow-neutron beamcontrol.

Acknowledgments

The author is grateful for the support from the Ministry of Education, Culture, Sports, Scienceand Technology of Japan through grants no. 20360028, 23360030 and 23656045. The author wouldalso like to thank a number of key collaborators, including N. Suzuki, K. Furushima, Y. Endoh, T.Nakamura, K. Matsumura, E. Hata, X. Liu, K. Momose, K. Mitsube, S. Takayama, K. Chikama,M. Fally, J. Klepp, and C. Pruner, for their contribution to the NPC work.

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