Choon Sup Yoon

Transport and trapping of photocharges in liquid crystals placed between photoconductive polymer layers

The transport and trapping of photocharges in liquid crystals placed between photoconductive polymer layers was investigated systematically. The transport of the photocharges is explained in terms of current paths that are formed along the bright sites of an interference pattern. Our study shows clearly that charge trapping occurs predominantly in the photoconductive poly(N-vinylcarbazole) layers and not in the insulating poly(vinyl alcohol) layers, contrary to a previous report.

Domain switching characteristics and fabrication of periodically poled potassium niobate for second-harmonic generation

We report on the 180° domain switching characteristics and fabrication of periodically poled KNbO3 for second-harmonic generation. Using a modified triangular pulse, a periodically poled KNbO3 of very high quality was fabricated with dimensions of 3×4×0.925 (a×b×c) mm3 and a period of 32.5 μm to utilize the d31 component of the second-order nonlinear optical coefficient tensor. An efficient second-harmonic generation of a mode-locked Nd:YAG laser, operated at 1064 nm with a pulse width of 35 ps, was achieved with a maximum conversion efficiency of 34.8%. The effective nonlinear coefficient, measured in the limit of undepleted pump approximation, was 5.2 pm/V, which is 68% of the theoretical value.

Surface morphology study of corona-poled thin films derived from sol–gel processed organic–inorganic hybrid materials for photonics applications

Dye-doped and a dye-attached sol–gel films using the chromophore (E)-N-butyl-(4-{2-[4-bis(2-hydroxyethyl)amino]phenyl}-ethenyl)pyridinium tetraphenylborate (BPTP) and N,N-bis(2-hydroxyethyl)amino-N′-methylstibazoliumtoluene-p-sulfonate (BAST) were fabricated by sol–gel processing. Surface morphology of the films was studied using an atomic force microscope. In the dye-doped film, many surface defects were observed. The morphology of the surface defects varied as a function of chromophore concentration. The shapes of the defects became more complicated as the chromophore concentration increased. On the other hand, no evidence of such defects was present in the dye-attached film and the quality of the film was high. Also for the dye-attached film, the surface was optically flat and smooth, and the surface roughness was measured to be less than 4 nm. It was found that corona poling processes also affected the quality of the dye-attached film significantly. Corona poling with a needle electrode resulted in many irregular shaped defects in the film. However, poling with a tungsten wire stabilized the corona discharge and thus prevented the formation of defects, which led to films of excellent quality. The electro-optic coefficient, r33 , of the BPTP dye-attached film was measured to be 5.0 pm V–1 and this value was maintained even after 54 days at room temperature.

Local structure of LiB3O5 single crystal from 7Li nuclear magnetic resonance

The local structure of LiB3O5 single crystal was investigated with 7Li (I=3/2) nuclear magnetic resonance measurements. We observed four different spectra, which could be divided into two groups corresponding to two kinds of lithium atoms, LiA and LiB, lying at crystallographically equivalent sites and magnetically inequivalent sites. From these results, the quadrupole coupling constants and the asymmetry parameters were determined at room temperature and are e2qQ/h=143±1 kHz and η=0.6±0.1 for 7Li. The directions of the principal axes of the electric field gradient tensors were also determined. The spectra for the two groups have the same principal values of the electric field gradient tensor, but different orientations, and originate from magnetically inequivalent sites. Also, the 7Li spin-lattice relaxation rate was measured, and the measured relaxation rate was found to be proportional to the temperature. The temperature-dependent single phonon process is considered to be more effective than the Raman ...

DIELECTRIC PROPERTIES OF LITHIUM TRIBORATE SINGLE CRYSTALS

From the impedance analysis, we report the dielectric constants of lithium triborate single crystals for the three principal axes in the frequency range of 100 Hz–1 MHz and in the temperature range of 303–773 K. The observed highly anisotropic behavior of dielectric dispersions is mainly related to the anisotropy of the activation energy for Li+ ion hopping, which is constituted by the B3O7 channel framework lain along the c axis.

Synthesis and properties of oligomeric poly(glycidyl ether)s with a carbazole-based multifunctional photorefractive chromophore

Abstract Novel photorefractive poly(glycidyl ether) oligomers of a carbazole-based multifunctional chromophore were synthesized via ring-opening cationic polymerization. This new class of materials showed good solubility in various organic solvents and thermal stability up to 400°C. Optical quality films of excellent transparency at the light wavelength above 430 nm were obtained by spin-coating or casting. Photoconductive sensitivity of the film was as high as 10−13 S cm−1/mW cm−2 at 532 nm. Electrooptic coefficients (r33) of 5–7 pm V−1 were easily achieved by the contact poling of spin-coated films. As evidence of photorefractivity, an asymmetric two-beam coupling gain of 3.2 cm−1 was demonstrated at an electric field of 30 V μm−1 with a low-Tg polymer blend of multifunctional poly(glycidyl ether).

Multilevel charging and discharging mechanisms of vertically stacked Ni1-xFex self-assembled nanoparticle arrays embedded in polyimide layers

Capacitance-voltage curves for the Al/polyimide (PI)/multilayered Ni1−xFex nanoparticle array/PI/p-Si (100) devices at 300K showed that the flatband voltage shift of the metal-insulator-semiconductor capacitor was affected by the value of sweep voltage, indicative of the variations in the charged electron number in the multiple-stacked Ni1−xFex nanoparticle arrays in the floating gate. Current-voltage results showed that the electron charging and discharging in the Ni1−xFex nanoparticles were attributed to thermionic emission and Fowler-Nordheim tunneling, respectively. The multilevel charging and discharging mechanisms of vertically stacked Ni1−xFex self-assembled nanoparticle arrays embedded in PI layers are described on the basis of the experimental results.

Layer-Structured Photorefractive Composite Containing Nematic Liquid Crystal(E7)-C60-PVK (Poly(N-Vinylcarbazole))

Abstract We present layer-structured photorefractive composite which contains nematic liquid crystal(E7), C60 and PVK(poly(N-vinylcarbazole)). The evolution of the grating formation in composites was probed by a He-Ne laser.

Dielectric properties of RbTiOAsO4 single crystals

Using the impedance analysis, we obtain the dielectric constants of RbTiOAsO4 (RTA) single crystals for the three principle crystallographic axes in the frequency range of 1 kHz–6 MHz and in the temperature range of 303–1123 K. The conductivity of RTA along the polar c axis is five orders-of-magnitude smaller than that of KTiOPO4 (KTP) in the temperature range of 500–970 K. The conductivities of RTA along the three principle axes have the same order of magnitude, 10−5 Ω−1 m−1 at 720 K, which is contrasted with the high anisotropy shown in KTP.

Sign change in the organic magnetoresistance of tris(8-hydroxyquinolinato)aluminum upon annealing

We report the sign change in the magnetoresistance of a tris(8-hydroxyquinolinato)aluminum film with the morphological change from amorphous to crystalline state upon annealing. The negative component of the magnetoresistance followed power law behavior, whereas the positive one showed non-Lorentzian function behavior. The decreasing absolute values of the negative component with increased annealing temperature may be understood by both intersystem-crossing-based mechanism and quenching of triplet excitons. The increasing values of the positive component with increased annealing temperature may be explained by the increase in the hopping probability of charge carriers with increased crystallinity of the film.