Koichiro Shirota

Modified Crystal Rotation Method for Measuring High Pretilt Angle in Liquid Crystal Cells

We propose a modified crystal rotation method which enables us to measure high pretilt angles in liquid crystal cells. This method is based on the use of two prisms made of the same material as the liquid crystal cell substrates. We demonstrate the ability for measuring a pretilt angle of over 21°.

Molecular Orientation in Mixed Monolayers of Hemicyanine and Fatty Acid at an Air/Water Interface Studied by Second Harmonic Generation

The molecular orientation of mixed monolayers consisting of hemicyanine dye and arachidic acid at an air/water interface is studied by optical second harmonic generation. It was found that the average molecular tilt angle with respect to the surface normal is almost unchanged throughout the compression process, and is smaller in a monolayer containing a higher fraction of hemicyanine. The effective local field factor was also evaluated as a function of surface number density and was found to agree qualitatively with the theoretical prediction.

Two-photon lasing of dye-doped photonic crystal lasers

We report two-photon pump lasing action of a photonic crystal laser, which comprises fluorescent dye-doped chiral nematic liquid crystals. The dye absorbs light peaking at 466 nm, while the pumping was conducted at 1000 nm wavelength. Due to redistribution of density of states by a stopping band, a strong single-mode stimulated emission was observed at the band edge wavelength of 608 nm with 1 nm detection-limited linewidth. A comparative study shows that under an identical pumping geometry, the two-photon lasing threshold is only 120 times larger than that for the single-photon excitation, a ratio lower than those generally reported for up-conversion lasing.

Surface Orientation of Polyimide Alignment Layer Studied by Optical Second-Harmonic Generation

We have determined the orientational distribution function (ODF) of the side chains of polyimide films for liquid crystal alignment. To determine ODF, optical second-harmonic generation was measured as a function of sample rotation angle. Using nonlinear susceptibility components obtained by a fitting procedure, ODF was obtained by means of information entropy. The symmetry of the polymer side chains changes from C∞v to Cs by rubbing treatment. Asymmetry of the polar angle along the rubbing direction is induced, though the change is very small.

Orientational relationship among polyimide alignment layer, liquid crystal monolayer, and bulk pretilt angle

We have determined the orientational distribution function (ODF) of side chains of rubbed polyimide (PI) films and liquid crystal (LC) monolayers on those films (PI & LC) by optical second‐harmonic generation (SHG). To separate the SHG signal of the LC monolayer from that of the (PI & LC) film, the phase difference between the SHG signals from PI and (PI & LC) was measured by an interference method. We found that the ODFs of PI and LC films are quite different. However, serious changes in these ODFs occur at the same rubbing strength, where the pretilt angle also shows a remarkable change.

Optical Switching of Antiferroelectric Liquid Crystal with Azo-Dye Using Photochemically Induced SmC*A-SmC* Phase Transition.

We have studied the photochemically induced phase transition from SmC A* to SmC* by Ar+ laser light irradiation (wavelength 514 nm) in an antiferroelectric liquid crystal doped with photochromic azo-dye molecules. Using this phase transition, we have demonstrated the optical switching under a bias voltage of 6.2 V. A response time of 1.5 s was achieved by 55 mW/cm2 irradiation at 30°C.

Orientation of Liquid Crystal Molecules on Side-Chain Polyimide Alignment Layer Studied by Second-Harmonic Generation Interferometry

Second-harmonic generation (SHG) interferometry has been applied to determine the phase of SHG from an SHG-active side-chain polyimide (PI) alignment layer with SHG-active liquid crystal molecules evaporated onto it. The phase difference between SH waves from the PI layer and from the PI layer with evaporated LC molecules monotonically increases with increasing evaporation time, while the SHG intensity increases after an inital decrease. These two facts indicate that LC molecules are adsorbed with an antiparallel polar orientation with respect to the polar side-chain orientation. The applicability of this method in studying the orientational relationship between an alignment layer and a LC monolayer is pointed out.

Influence of Compression Processes on the Formation of H-Aggregates in Hemicyanine Monolayer on Water Surface

The influence of the compression process on the formation of H-aggregates has been studied by means of the absorption spectra of the hemicyanine monolayer on the water surface. It was found that the conventional compression by moving the barrier does not bring about the formation of the H-aggregates but that the compression by spreading the dye solution does cause the formation. Absorption spectra in several geometries reveal that most of the aggregated molecules deposited on a substrate (LB film) are aligned along the surface normal and that the molecules on water (L film) tilt from the water-surface normal.

Optical control of the pitch in cholesteric liquid crystals

It has been shown that optical control of a spiral pitch in cholesteric liquid crystals (CLCs) by laser light irradiation is possible. Because of the spiral periodic structure, the CLCs shows unique optical characteristics with various polarization dependence. Then, the CLCs have a potential for application to polarization selective device whose properties can be changed by light irradiation. The CLC used was a mixture of cholesteryl chloride and cholesteryl pelagonate, and a mixing ratio selected for the wavelength of a probe laser was CC:CP equals 33.6:66.4. To control the pitch, CLCs was doped with azobenzene at a concentration of 5 mol%. Optical isomerization of the dye molecules induced a change of the pitch, and hence the selective reflection band is shifted. Irradiation by a He-Cd and an Ar+ laser shifted the selective reflection band of sample to shorter and longer wavelength, respectively. Hereby the spiral pitch in CLC was reversibly changed by 10 nm.

Symmetry and orientational structure in hemicyanine monolayer by surface second harmonic generation and absorption spectroscopy

Abstract We have investigated the influence of the deposition process on hemicyanine monolayer structure by surface second harmonic generation (SHG) and adsorption spectroscopy. The H-aggregates tend to dissociate by deposition. The orientation of the H-aggregates in the pure hemicyanine monolayer changes through the deposition process; the molecular long axes of the H-aggregate are parallel to the surface normal in the LB film made by vertical deposition, while their average direction makes a certain angle with respect to the surface normal in the LB film made by horizontal deposition.