Nanoparticle-polymer composite gratings fabricated by holographic 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]. The spatial distribution of nanoparticles can be controlled artificially by holographic exposure. This method, the so-called holographic assembly of nanoparticles in polymer [2], enables us to construct the single step formation of large scale and multi-dimensional photonic crystal structures in thick films, which can be used for versatile applications in photonics and other fields of science and engineering. For example, the tailored design of NPCs provides nanocomposite volume gratings for holographic data storage, distributed feedback lasers, and electromagnetic/quantum beam control. In this summary we briefly describe our recent works on NPC volume gratings for novel applications in photonics and neutron optics. 2. Holographic assembly of nanoparticles Suppose that nanoparticles are uniformly dispersed in host monomer capable of radical photopolymerization as shown in Fig.1(a). Spatially inhomogeneous light illumination such as a two-beam light-intensity interference pattern produces free radicals by dissociation of doped initiators and the subsequent reaction of free radicals with monomer molecules leads to free-radical mediated polymerization in the bright regions. This photopolymerization process lowers the chemical potential of monomer in the bright regions, leading to the migration (diffusion) of the monomer from the dark to the bright regions. On the other hand, photoinsensitive nanoparticles experience the counterdiffusion from the bright to the dark regions since they are not consumed and their chemical potential increases in the bright regions due to the formation of polymer. Such a polymerization-driven mutual diffusion process essentially continues until the photopolymerization reaction completes. In this way the mutual diffusion of monomer and nanoparticles results in 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) during holographic exposure. (c) Density distribution of nanoparticles. (d) Density distribution of the formed polymer. 23rd ICO conference, Santiago de Compostela 26 29 August 2014 1
bright and the dark regions. It was confirmed experimentally that the distribution of dispersed nanoparticles followed the light-intensity interference pattern in an NPC film [2 4]. In this case the 180 phase shift of the formed polymer and nanoparticles distributions takes place as a result of the mutual diffusion process. Therefore, when a combination of monomer and nanoparticles having a large difference in their refractive indices is chosen appropriately, the saturated refractive index change ( n sat ) of a volume gratings recorded in an NPC film can be increased. Furthermore, the mechanical and thermal stability of a recorded volume grating against polymerization shrinkage and thermal changes in film thickness and refractive index can be improved by high dispersion of inorganic 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 n sat as large as 1 10 2, high recording sensitivity in the green and the blue, reduced polymerization shrinkage and high thermal stability, they can be used for optical recording media in holographic data storage systems [7]. It is shown that NPCs with thiol-ene monomers capable of free-radical mediated step-growth photopolymerizations gives substantive shrinkage suppression as low as 0.3% [6,8]. For this purpose the stoichiometric thiol-ene formulation of commercial secondary dithiol monomer, 1,4-bis(3-mercaptobutyryloxy)butane (Showa Denko K.K.), and triene monomer, triallyl-1,3,5-triazine-2,4,6(1h,3h,5h)-trione (Aldrich) were used together with the dispersion of 25 vol.% SiO 2 nanoparticles (the average size of 13 nm) and a photoinitiator/green-sensitizer system consisting of Irgacure784 (Ciba) and BzO 2 (Aldrich) [6]. Details of the sample preparation as well as the photopolymerization and (plane-wave) holographic recording properties were described in [6]. Figure 2 shows reconstructed digital data page images of 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 patterns with 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) lower than 1 10 2 and signal-to-noise ratios (SNRs) of larger than 2, implying that error-free retrieval of data pages is possible with error correction code. It was also shown that higher modulation coding 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 page holograms in recording order. 3-2. Nonlinear optics Since NPCs can be tailored by selecting a type of nanoparticles for a particular application, inorganic oxide and hyperbranched polymer (HBP) nanoparticles have been usually used for recording high contrast volume holograms [1, 6, 11]. In order to induce optical nonlinearities in a large area 23rd ICO conference, Santiago de Compostela 26 29 August 2014 2
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 nanocomposites were measured near the surface plasmon resonance (SPR) [12] that took place at the interface between metallic nanoparticles and a dielectric matrix host. The observed optical nonlinearity was attributed to the coherent oscillation of free electrons occupying states near the Fermi level in the conduction band, which gave rise to a surface plasmon absorption band whose peak and width were dependent on the size of metallic nanoparticles and the permittivity of a dielectric matrix host. This SPR resulted in an increase in the electric field inside the metal, the so-called local field enhancement [13], yielding to the enhancement of the optical nonlinearities of metal-dielectric nanocomposites. Recently, we studied the nonlinear optical properties of NPCs dispersed with HBP-metallic (Au or Pt) nanoparticle complex. It was shown that they exhibited the dielectric confinement effect near SPR and that the magnitude of their effective third-order nonlinear optical susceptibility 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 nonlinear optical study. QDs have interesting characteristics such as fluorescence tunability, enhanced photosensitivity and large optical nonlinearities. The electronic states of QDs are strongly influenced by the quantum confinement effect when the radius of QDs is smaller than approximately three times of the exciton Bohr radius. The band gap (Eg) of QDs increases with decreasing their size and the quantum confinement effect can strongly enhance the third-order optical nonlinearity [15]. The II-VI bulk semiconductor CdSe has the direct band gap Eg = 1.74 ev at 300 K and has the exciton Bohr radius of 5.6 nm. Therefore, the strong quantum confinement effect of CdSe QDs plays an important role in a large enhancement of the optical nonlinearity as compared with that of the bulk CdSe. We successfully fabricated a highly efficient Bragg grating in an NPC film dispersed with CdSe QDs [16]. It was found that a CdSe QD-dispersed NPC film gave n sat of 1 10 3 at a low CdSe QD concentration of 0.35 vol.%. We also observed the thirdand fifth-order optical nonlinearities in a uniformly cured CdSe QD-dispersed NPC film [17]. The effective third-order nonlinear optical susceptibility was found to be of the order of 10 11 esu at a wavelength of 532 nm. Figure 3 shows transmitted and self-diffracted beam patterns through a 50 µm-thick CdSe QD-dispersed NPC film in a degenerate multi-wave mixing setup [17], where a pair of self-diffracted beams due to the fifth-order nonlinear optical nonlinearity is seen. This result 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 QDdispersed NPC film. 3-3. Neutron optics Neutron optics and spectroscopy have been extensively studied from viewpoints of fundamental science and of medical/industrial applications [18]. Neutron optical elements such as mirror and beam splitters are essential for the construction of a neutron interferometer [19]. So far, a perfect silicon 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) provide better interferometric sensitivities as compared with thermal neutrons, they require other neutron optical elements because of the inability to diffract slow-neutron beams from a silicon. Rupp et al. demonstrated the diffraction of a cold neutron beam (λ= 1.5 nm) by a holographic volume grating optically recorded in a deuterium-substituted (poly)methylmethacrylate (PMMA) based 23rd ICO conference, Santiago de Compostela 26 29 August 2014 3
photopolymer [20]. Despite their success of slow-neutron diffraction the diffraction efficiency was limited by the Pendellösung oscillation averaging due to the very thick film ( 2 mm) and the limited collimation of a neutron beam. In order to circumvent this problem, we develop neutron optical elements by use of a volume grating optically recorded in a thick ( 100 µm) NPC film with (meth)arylate monomer capable of free radial mediated chain-growth polymerizations and with the dispersion of SiO 2 nanoparticles having a relatively large coherent scattering length for slow-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 and a 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 slowneutron mirror. Figure 4: A photograph of a free-standing NPC volume grating recorded at a wavelength of 532 nm 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 holography, nonlinear optics and neutron optics. Since NPCs can be tailored by selecting appropriate monomers and nanoparticles, a wide variety of material design and applications can be expected. In holographic data storage applications NPCs provide the realization of holographic recording media that possess large n sat, high recording sensitivity, low shrinkage and high thermal stability simultaneously. Such simultaneous improvement cannot be realized by conventional all-organic photopolymers. In nonlinear optics applications it is possible to construct multi-dimensional nonlinear photonic lattice structures in NPCs by holographic assembly of nanoparticles, which can be used for photonic applications such as optical switching/limiting and nonlinear photonic crystals. In neutron optics applications slow-neutron beams can be manipulated holographically by NPC volume gratings with high efficiency. It would lead to a new possibility of slow-neutron beam control. Acknowledgments The author is grateful for the support from the Ministry of Education, Culture, Sports, Science and Technology of Japan through grants no. 20360028, 23360030 and 23656045. The author would also 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. 23rd ICO conference, Santiago de Compostela 26 29 August 2014 4
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