Tadashi Akahane

Determination of Director Profile of Twisted Nematic Liquid Crystal Cell with Tilted Surface Alignment by Renormalized Transmission Ellipsometry

Applying renormalized transmission ellipsometry (RTE), the evaluation of a director profile in the case of a twisted nematic cell with a relatively high pretilt angle was performed. To determine the director profile, we proposed a novel technique for determining four cell parameters, namely, the cell gap d, the pretilt angle θp, the twist angle of the director alignment t, and the angle between the axis of the polarizer and the entrance liquid crystal director i. To date, several methods of measuring the average tilt angle of a twisted nematic (TN) cell have been proposed, but our method of simultaneously determining the four cell parameters is considered to be quite effective in the evaluation of liquid crystal displays.

Renormalized Ellipsometry for Determining the Anisotropic Refractive Indices of Nematic Liquid Crystals

Renormalized transmission ellipsometry was applied to the precise measurement of anistropic refractive indices of nematic liquid crystals. It is quite beneficial for electrical industry that the conventional sandwich-type liquid crystal cell becomes an applicable sample and experimental measurement can be carried out using a conventional instrument for transmission ellipsometry. A simplified measurement procedure can provide continuous values of refractive indices within the visible wavelength range. At the same time, the cell gap and twist angle of the liquid crystal cell can be estimated by this measurement precisely.

Measurement of Genuine Azimuthal Anchoring Energy in Consideration of Liquid Crystal Molecular Adsorption on Alignment Film

The photopolymer material of poly(vinyl cinnamate) (PVCi) is known as a liquid-crystal (LC) alignment film whose surface azimuthal anchoring energy is relatively small. However, the results of an experiment on LC reorientation under an electric field suggest that the genuine azimuthal anchoring energy is one order of magnitude higher than that measured by the conventional torque balance method and/or the N?el wall method. Here we clarified that this inconsistency is due to the LC molecular adsorption on the alignment film occurring during the isotropic-nematic phase transition process. In this paper, we propose an improved torque balance method by which the genuine azimuthal anchoring energy can be estimated.

Surface alignment of liquid crystal multilayers evaporated on photoaligned polyimide film observed by surface profiler

Abstract The investigation of the surface alignment of liquid crystal (LC) multilayers evaporated on photoaligned polyimide vertical alignment (PI-VA) film was carried out by means of the novel three-dimensional (3D) surface profiler. We report the first use of the surface profiler to visualize a microscopic image of the monolayer arrangement of LC molecules in contact with the surface of photo-treated PI-VA film. The photoinduced anisotropy of partially UV-exposed PI-VA film can be visualized as a topological image of LC multilayers. It seems that the topology of LC multilayers is indicating the orientational distribution of LC molecules on the treated film. It was found that the periodically photoaligned PI-VA film surface can align an adsorbed LC monolayer and the LC molecular alignment can be extended to the bulk via the epitaxylike LC–LC interaction, i.e. a short-range molecular interaction. With regard to the unexposed PI-VA film surface, noticeable anisotropy in the monolayer alignment was not observed, indicating that the long-range elastic interaction may be responsible for the bulk alignment. The appearance of small droplets in the masked region may be presumably related to the dewetting phenomena.

Quantitative analysis of nematic director reorientation dynamics studied by time-resolved spectroscopic ellipsometry

The director reorientation dynamics of the nematic liquid crystal (NLC) filled in a conventional sandwich cell was demonstrated by means of time-resolved spectroscopic ellipsometry (TRSE). By using a dye-doped NLC, the transient behavior of the NLC director near the alignment layer as well as that in the bulk can be observed clearly without using any special cavity such as a waveguide. It is shown that the observed transient change of the ellipsometric angles, which corresponds to the director reorientation dynamics, can be completely reproduced by the numerical simulation based on the Franks elastic continuum theory. From these numerical and experimental results, it is emphasized that TRSE is a potential instrument to analyze the director reorientation dynamics of NLC.

Improved Method for the Dispersion of Refractive Indices Based on Transmission Spectroscopic Ellipsometry

In our previous study, we have proposed a novel analysis method to determine the dispersion of refractive indices based on the plural incidence renormalized transmission spectroscopic ellipsometry. This method was, however, applicable only to the homogeneous cell. In this paper, we introduce the improved method to determine the wavelength dispersion of refractive indices by using the twisted nematic cell as well as by the homogeneous cell. Numerically calculated refractive indices of ordinary and extraordinary rays were in good agreement with the refractive indices experimentally measured by the Abbe Refractometer.

Surface Azimuthal Anchoring Energy between the Grating Surface and Nematic Liquid Crystal Layer Studied by Finite Element Method

It has been pointed out that Berremans topological alignment model is applicable to a fairly shallow grating surface, but not for gratings with deep groove profiles. The reason for Berremans model breakdown was presumed to be the absence of a second derivative in the algebra of elastic theory. In this paper, the saturation tendency of surface azimuthal anchoring energy as a function of the grating height as decribed by numerical simulation on the basis of finite element method (FEM). A good agreement between FEM and experimental results was also demonstrated.

Analysis Technique for Wavelength Dispersion of Refractive Indices by Renormalized Transmission Spectroscopic Ellipsometry

An improved analysis technique to determine the wavelength dispersion of refractive indices based on renormalized transmission spectroscopic ellipsometry was demonstrated. Twelve device parameters, including cell gap, twist angle, and rotation angle, nine Cauchy coefficients for birefringence, and refractive indices for ordinary and extraordinary rays are determined in a three-step manner. The refractive index for an ordinary ray at the wavelength λ=589.0 nm, which is measured by using an Abbe refractometer, is substituted as an original refractive index to guarantee the accuracy of the analysis for the absolute value of refractive indices. Compared with our previous technique, this analysis procedure is more simplified; as a result, the reproducibility and applicability were remarkably enhanced.

Characterization of Bistable Hybrid-Twisted Nematic Liquid Crystal Mode by Means of Renormalized Transmission Spectroscopic Ellipsometry

We report on how to determine asymmetrical director distribution throughout a liquid crystal layer such as a bistable hybrid-twisted nematic (BHTN) liquid crystal device by means of renormalized transmission spectroscopic ellipsometry. The advantage of the plural oblique incidence spectroscopic ellipsometry (POISE) is that the azimuthal angle as well as the tilt angle with a relatively high zenithal angle can be determined simultaneously without using a special cell with a coupling prizm. It was demonstrated that five device parameters, namely, cell gap, pretilt angles of upper and lower surfaces, twist angle of the director alignment, and azimuthal angle of the entrance liquid crystal director, can be determined uniquely. It is experimentally confirmed that each bistable state has an opposite twist sense and the twist angle difference between two stable states is approximately 180°.

Evaluation of Alignment of Nematic Liquid Crystal Thin Film by Spectroscopic Ellipsometry

Generally, macroscopic molecular orientation state in liquid crystal layer can be described by Ossen and Franks continuum theory. However, the validity of applying the continuum theory to nematic liquid crystal thin film (NLCF) has not been confirmed yet. In this study, we experimentally examined NLCF by employing spectroscopic ellipsometer. As a result, theoretical and experimental result represent a good agreement when NLCF thickness is from 60 to 250 nm. Furthermore, the influence of surface roughness on alignment film surface and glass substrate was also examined.