Junho Mun

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.

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).

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.

Large two-beam coupling effect in poly(methylmethacrylate) doped with hemicyanine dye

Unexpected asymmetric energy exchange (two-beam coupling) between two writing beams in two-wave mixing experiments was observed in poly(methylmethacrylate) doped with hemicyanine dye. It was found that photobleaching of hemicyanine dyes was responsible for the formation of refractive index gratings. Instability of the laser-based optical system could be the cause of the phase shift between the interference pattern of the writing beams and the refractive index gratings, such a phase shift being a necessary condition for the two-beam coupling.

Photo-aligned and photo-conductive polymer layers for photorefractive liquid crystal cells of high transmittance

The optical performance of photorefractive cells of liquid crystals strongly depends on the alignment of liquid crystals stabilized by surface states. Two photo-aligned polymer layers with relatively weak anchoring energy were investigated by FT-IR and UV/VIS spectroscopies. The cells without alignment layers showed significant scatterings due to random switching under an applied field. The cells with photo-alignment layers showed the high transmittance because of the high orientation order, but a little decrease in gain coefficient and diffraction efficiency because of the decrease in trap sites. Therefore, it is concluded that the reorientation of liquid crystals onto photo-conduction layer would govern the photorefractivity in two-beam coupling experiments.

Second Harmonic Generation and Photorefractive Effect in Dye-Doped Liquid Crystals

Second harmonic generation (SHG) and photorefractive (PR) effects were observed in a PR cell of liquid crystals doped with nonlinear optical (NLO) chromophore. In general, photorefractive materials of liquid crystals have no second harmonic (SH) signals due to the absence of nonlinearity. However, we observed SHG using a Maker Fringes experimental setup and, simultaneously, the asymmetric energy exchange of high PR gain coefficient, 815 cm-1, using the PR cell.

Photorefractive effect in nematic liquid crystals doped with nonlinear optical chromophores

Photorefractive effect in nematic liquid crystals is attributed to reorientation of liquid crystal molecules by light induced space-charge field. In the absence of external field, nematic liquid crystals form domain structure, which is responsible for high threshold voltage. By doping nonlinear optical chromophores into liquid crystals, the threshold voltage could be reduced significantly and the mechanism involved was investigated.