In Kyu Moon

The effect of the molecular structure of the chromophore on the photorefractive properties of the polymer systems with low glass transition temperatures

Seven chromophores with different dipole moments and polarizability anisotropies were synthesized by varying conjugation bridges, such as polyene, thiophene, indole, and benzene combined with strong electron-donor and electron-acceptor groups. The birefringence of photorefractive materials containing 30 wt% of chromophore was observed to be in the range of 0.45 × 10−3 to 5.0 × 10−3 at an electric field of 70 V µm−1, depending on the chromophore structure. The photorefractive material containing a polyene-based chromophore exhibited the highest photorefractive property, followed by those containing thiophene-based, benzene-based and indole-based chromophores. It was observed that, within material systems used in this study, photorefractive properties of guest–host systems were linearly proportional to the orientational birefringence.

Determination of the space-charge field in polymeric photorefractive material

We proposed a method for measuring the magnitude of the space-charge field of the polymeric photorefractive materials. In the case of polymeric photorefractive material with low glass transition temperature, optically anisotropic chromophores are known to be reoriented under space-charge field. Simply by adding a pair of crossed polarizer units to a conventional degenerated four wave mixing setup, we could measure the birefringence of the photorefractive materials induced by a newly formed space-charge field. Since the birefringence of a given material is governed by the applied electric field, the space-charge field can be determined from the variation of birefringence using the oriented gas model.

Temperature Dependence on the Grating Formation in a Low-Tg Polymeric Photorefractive Composite

We investigated a dependence of the grating formation on the temperature in polymeric photorefractive (PR) composite, in terms of magnitude and buildup speed of the PR grating. For polymeric PR materials, the temperature is one of the most important factors together with the external electric field because it is closely related on photocharge generation efficiency, mobility of generated carrier, electro-optic coefficient tensor, and so on. Above the glass transition temperature, the diffraction efficiency of degenerate four-wave mixing decreased with increasing the temperature; it can be explained with the magnitude of space-charge field and the electro-optic behavior at different temperatures. The space-charge field decreased linearly with increasing temperature due to a decrease in the photocharge generation efficiency and an increase in the hole detrapping by the high dark conductivity. Also as we expected, the PR grating buildup speed, which is strongly dependent on the photoconductivity, steeply decreased with increasing the temperature, and its tendency was similar to the temperature dependence of the phase shift.

Ultra-high pseudocapacitance of mesoporous ZnCo2O4 nanosheets on reduced graphene oxide utilizing a neutral aqueous electrolyte

A novel strategy to prepare hierarchical mesoporous ZnCo2O4 nanosheets, decorated on a reduced graphene oxide/nickel foam template, was constructed. This unique nanostructure electrode, consisting of mesopores/micropores for pseudocapacitors, showed high capacitance, excellent cycling performance and good rate capability.

High-performance photorefractive organic glass based on diphenylhydrazone

A multifunctional molecule that possesses photoconductive and electro-optic properties in one molecule was synthesized as a photorefractive organic glass. Diphenylhydrazone as a photoconductive moiety was covalently bound to the enamine derivative as an electro-optic chromophore via a flexible ester link. The sample prepared from a mixture of the multifunctional molecule (99 wt %) and fullerene C60 (1 wt %) showed good photorefractive properties. The 100 μm thick film showed a maximum diffraction efficiency of 69% at a low applied field of 25 V/μm, corresponding to a refractive index modulation (Δn) of 1.50×10−3 and a photoconductivity of 0.46 pS/cm.

Highly water-soluble multi-walled carbon nanotubes amine-functionalized by supercritical water oxidation

Multi-walled carbon nanotubes (MWNTs) have been amine-functionalized by eco-friendly supercritical water oxidation. The facilely functionalized MWNTs have high solubility (~84 mg L(-1)) in water and 78% transmittance at 30-fold dilution. The Tyndall effect is also shown for several liquids.

New Photorefractive Polymer Composites Doped with Liquid Nonlinear Optical Chromophores

Photorefractive polymer composites were synthesized based on poly (N-vinylcazbazole) (PVK) doped with liquid nonlinear optical chromophores and a sensitizer C60. PVK/liquid NLO chromophores/C60 devices showed no signs of phase separation and did not require a plasticizer, such as ethylcarbazole. The composites showed 69% diffraction efficiency (C3) and a rapid response time of 46 ms (C1) in four-wave mixing experiments at a light intensity of 60 mW/cm2 and a wavelength of 633 nm.

Dependency of Ionization Potentials of Photoconductive Polymers on Photorefractive Response Time

Photorefractive response time has been an important issue to be improved. We investigate the dependency of photorefractive speed against ionization potentials of the photoconductive polymers. The composite with the lowest ionization potential gives the highest photoconductivity. Fast component of the response time is found to be strongly dependent of photoconductivity of the composite.

TEMPERATURE DEPENDENCE ON THE GRATING FORMATION IN PHOTOREFRACTIVE POLYMERIC COMPOSITE

We investigated the dependence of the grating formation on temperature in photorefractive polymeric composite, especially the index contrast of the grating. For photorefractive polymeric materials, temperature is one of the most important factors together with the external electric field, because it is closely related with photogeneration efficiency, carrier mobility, electro-optic coefficient tensor, and so on. The diffraction efficiency of the photorefractive polymeric composite decreased with increasing the temperature, and it could be explained with the magnitude of space-charge field and the electro-optic behavior at various temperatures.

Dependence of the Bragg condition on an external electric field for a polymeric photorefractive material

We investigated the effect of an applied electric field on the Bragg condition of degenerate four-wave mixing a polymeric photorefractive material with a low glass-transition temperatue. For a polymeric photorefractive material the application of an external electric field is necessary for photorefractivity leads to birefringence of the material by poling of the nonlinear optical chromophore. Because the propagation vectors of the pumping and reading beams inside the material are influenced by the refractive index of the material, the Bragg condition depends on the magnitude of the external field. Using an oriented gas model and the-coupled-mode theory, we numerically analyzed the Bragg-mismatch effect that causes a reduction in diffraction efficiency as a function of an external field. We present the boundary conditions for sample thickness and grating spacing for which the Bragg-mismatch effect must be taken into account.