Eiji Hata

Holographic nanoparticle-polymer composites based on step-growth thiol-ene photopolymerization

We report on the photopolymerization kinetics and volume holographic recording characteristics of silica nanoparticle-polymer composites using thiol-ene monomers capable of step-growth polymerization. Real-time Fourier transform spectroscopy and photocalorimetry are used to characterize the visible light curing kinetics of a thiol-ene monomer system consisting of secondary dithiol with high self-life stability and low odor and triene with rigid structure and high electron density. It is shown that while the nanoparticle-(thiol-ene)polymer composites exhibit high transparency, their saturated refractive index modulation (Δnsat ) and material sensitivity (S) are as large as 1×10−2 and 1615 cm/J, respectively. The polymerization shrinkage is reduced as low as 0.4% as a result of the late gelation in conversion. These values meet the acceptable values for holographic data storage media (i.e., 5×10−3, 500 cm/J and 0.5% for Δnsat, S and shrinkage, respectively). It is also shown that because of the dispersion of inorganic silica nanoparticles and the use of the triene monomer having the rigid structure of the triazine functional group, the thermal stability of recorded holograms is much improved over our previously reported nanoparticle-polymer composites using organic nanoparticles and primary mercaptopropionate trithiol/allyl ether triene monomers [Opt. Lett. 35, 396 (2010)].

Order-of-magnitude polymerization-shrinkage suppression of volume gratings recorded in nanoparticle-polymer composites.

We demonstrate substantial shrinkage suppression of nanoparticle-polymer composite transmission gratings by use of the step-growth polymerization mechanism. It is shown that the polymerization shrinkage can be reduced as low as 0.3% at the nanoparticle concentration of 35 vol. % by which the refractive index modulation and the material sensitivity are maximized to be 8x10(-3) and 1014 cm/J, respectively, in the green. Our results offer a noticeable advance in the development of holographic data storage materials.

Shift-multiplexed holographic digital data page storage in a nanoparticle-(thiol–ene) polymer composite film

We demonstrate shift-multiplexed holographic storage of 180 digital data pages with low symbol-error rates in a thick (250 μm) SiO2 nanoparticle-polymer composite film using step-growth thiol-ene photopolymerization. A two-dimensional 2:4 modulation code was employed for formatting digital data pages in order to reduce the average intensity of code block without decreasing the coding efficiency. This study clearly shows the feasibility of the thiol-ene based nanoparticle-polymer composite system as a holographic data storage medium.

Stoichiometric thiol-to-ene ratio dependences of refractive index modulation and shrinkage of volume gratings recorded in photopolymerizable nanoparticle-polymer composites based onstep-growth polymerization

Spectroscopic, photocalorimetric and holographic measurements are conducted to investigate effects of stoichiometric thiol-to-ene ratio on the polymerization dynamics, refractive index modulation, recording sensitivity and polymerization shrinkage of volume gratings recorded in silica nanoparticle-polymer composite films based on step-growth radical addition polymerization. It is found that the polymerization rate of the composite system is maximized at the stoichiometric thiol-ene composition. It is also found that while the refractive index modulation and the recording sensitivity are maximized at the stoichiometric thiol-ene composition, polymerization shrinkage decreases with increasing the thiol monomer fraction. A negative correlation between gel point conversion and shrinkage is observed.

Photopolymerizable nanocomposite photonic materials and their holographic applications in light and neutron optics

We present an overview of recent investigations of photopolymerizable nanocomposite photonic materials in which, thanks to their high degree of material selectivity, recorded volume gratings possess high refractive index modulation amplitude and high mechanical/thermal stability at the same time, providing versatile applications in light and neutron optics. We discuss the mechanism of grating formation in holographically exposed nanocomposite materials, based on a model of the photopolymerization-driven mutual diffusion of monomer and nanoparticles. Experimental inspection of the recorded grating’s morphology by various physicochemical and optical methods is described. We then outline the holographic recording properties of volume gratings recorded in photopolymerizable nanocomposite materials consisting of inorganic/organic nanoparticles and monomers having various photopolymerization mechanisms. Finally, we show two examples of our holographic applications, holographic digital data storage and slow-neutron beam control.

Low polymerization-shrinkage nanoparticle-polymer composite films based on thiol-ene photopolymerization for holographic data storage

We demonstrate volume holographic recording in nanoparticle-polymer composites using thiol-ene monomers capable of step-growth polymerization by which shrinkage can be reduced as low as 0.2%. The reduced shrinkage is 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. It is shown that the thiol-ene based organic nanoparticle-polymer composites possess the refractive index modulation and the material sensitivity of 8×10-3 and 1014 cm/J, respectively, in the green, larger than the minimum acceptable values of 0.005 and 500 cm/J for holographic data storage.

Volume holographic recording in nanoparticle-polymer composites with reduced polymerization shrinkage

Holographic dry photopolymers have been studied for various applications such as holographic optical elements, volume holographic data storage, narrowband optical filters, optical interconnects, waveguide couplers, electrically switchable Bragg gratings, photonic crystals, head-up/head-on displays and three-dimensional displays. High-contrast refractive index changes Δn with high recording sensitivity and dimensional/environmental stability are usually required to make these applications practical. Recently, we have developed a new class of holographic dry photopolymers, nanoparticle-polymer composites [1], in which inorganic or organic nanoparticles are dispersed in (meth)acrylate monomers. In this photopolymer system the phase separation of monomer molecules and photo-insensitive nanoparticles takes place during holographic exposure, providing high contrast volume holograms [2]. The inclusion of nanoparticles also yields to the suppression of polymerization shrinkage and to high thermal stability of fixed holograms, giving high dimensional and environmental stability [3]. In this paper we demonstrate an order-of-magnitude suppression of polymerization shrinkage of volume holograms recorded in nanoparticle-polymer composites by green light initiated thiol-ene photopolymerization, as compared to our previously reported nanoparticle-polymer composites with multifunctional (meth)acrylate monomers.

Characterization of holographic digital data page recording in nanoparticle-polymer composite films based on thiol-ene photopolymerization

We report on holographic storage of digital data pages in a thick silica nanoparticle-polymer composite film that uses step-growth thiol-ene photopolymerization at a wavelength of 532 nm. Shift-multiplexed holographic storage of 180 digital data pages with a two-dimensional 2:4 modulation code was successfully demonstrated with low symbol-error rates. This result clearly shows the feasibility of the thiol-ene based nanoparticle-polymer composite system as holographic data storage media.

Photopolymerizable thiol-ene nanocomposite materials for holographic applications

We describe an experimental investigation of the photopolymerization kinetics and volume holographic recording characteristics of silica nanoparticle-polymer nanocomposites using thiol-ene monomers capable of step-growth polymerization. We characterize the visible light curing kinetics of a thiol-ene monomer system consisting of secondary dithiol with high self-life stability and low odor and triene with rigid structure and high electron density by using real-time Fourier transform spectroscopy and photocalorimetry. In plane-wave volume holographic recording at a wavelength of 532 nm it is shown that while volume holograms recorded in the nanocomposites exhibit high transparency, their saturated refractive index modulation (Δnsat) and material sensitivity (S) are as large as 1x10-2 and 1615 cm/J, respectively. The polymerization shrinkage is reduced as low as 0.4% as a result of the late gelation in conversion. These values meet the acceptable values for holographic data storage media (i.e., 5x10-3, 500 cm/J and 0.5% for Δnsat, S and shrinkage, respectively). The improved thermal stability of volume holograms recorded in the nanocomposites is also confirmed experimentally.

Readout fidelity of coaxial holographic digital data page recording in nanoparticle–(thiol–ene) polymer composites

We report on an experimental investigation of nanoparticle-concentration and thiol-to-ene stoichiometric ratio dependences of symbol error rates (SERs) and signal-to-noise ratios (SNRs) of digital data pages recorded at a wavelength of 532 nm in thiol–ene based nanoparticle–polymer composite (NPC) films by using a coaxial holographic digital data storage method. We show that SERs and SNRs at the optimized material condition can be lower than 1 × 10−4 and higher than 10, respectively, without error correction coding. These results show the usefulness of thiol–ene based NPCs as coaxial holographic data storage media.