Munehiro Kimura

Novel Method of Measuring Surface Torsional Anchoring Strength of Nematic Liquid Crystals

A simple and accurate method of measuring the surface torsional anchoring strength of nematic liquid crystals is proposed. This method is based on the simultaneous determination of the actual twist angle and the cell gap (optical retardation) of a twisted nematic cell in which the easy axes of the upper and lower substrates are at a particular angle ( e.g., 90°). The TN cell is placed between two polarizers. The TN cell and the analyzer are mounted on rotatable stages. The measurement procedure consists of the following steps. (1) Using monochromatic light, we minimize the amount of light transmitted by rotating both the TN cell and the analyzer (we denote these minimizing angles of the analyzer and the TN cell from the polarizer as Ψ p(1) and Ψ0(1), respectively. (2) We set the TN cell at a particular angle from Ψ0(1), and minimize the amount of light transmitted by rotating only the analyzer (we denote this minimizing angle of the analyzer as Ψ p(2)). From the values of Ψ p(1) and Ψ p(2), we can determine both the actual twist angle and the optical retardation using the analytical expression for optical transmissivity based on the Jones matrix method. We discuss the accuracy of this method. Experimental examples are also shown.

Observation of Nematic Liquid Crystal Director Reorientation at the Interface between Substrate and Liquid Crystal Layer by Total Reflection Ellipsometry.

The liquid crystal (LC) director reorientation near the interface between the substrate and the LC layer was studied using total reflection ellipsometry (TRE). The dynamic and static responses of the ellipsometric angles to the electric field were measured by TRE. It was experimentally confirmed that total reflection occurs in the LC cell. The reflected light is dominantly modified by the interfacial LC director reorientation. The polar anchoring strength of the cell was estimated by analyzing the electrical response of the phase difference response. We confirm that the TRE is useful for studying the LC director reorientation near the interface between the substrate and the LC layer.

Electro-optical characteristics and switching behavior of a twisted nematic liquid crystal device based upon in-plane switching

A driving mechanism for a twisted nematic liquid crystal device that could possibly improve the viewing-angle characteristics and the cell gap error margin is proposed. It is important that the surface azimuthal anchoring strength of the liquid crystal cell differs at the upper and lower substrates, unlike a conventional twisted nematic (TN) mode. Electro-optical characteristics were investigated with an electric field applied in the plane of the substrates. It is numerically demonstrated and predicted that the proposed driving mechanism can have both the desirable features of in-plane switching mode (such as an excellent viewing angle) and those of a conventional TN type device (such as a small color shift and wide cell gap margin).

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.

Offset of Multiple-Beam Interference inside Anisotropic Multilayered Structure: Determination of Polar Anchoring Strength at Nematic Liquid Crystal-Wall Interface

An optical technique for eliminating the inaccuracy in the measurement of retardation caused by multiple-beam interference in anisotropic multilayer-structured thin film is developed by the detailed analysis of obliquely incident transmission ellipsometry. The director distribution of a nematic liquid crystal inside a conventional sandwich-type cell which constitutes glass substrates, alignment films and transparent electrodes can be accurately determined without taking complicated multilayer analysis into account. As a typical application, a novel method of determining of the polar anchoring strength at the nematic liquid crystal-wall interface is proposed. The accurate measurement of the director distortion of a NLC without information on the refractive indices of ITO films, alignment films and glass substrates results in the realization of polar anchoring strength with a high accuracy. This technique can be used to achive a strong polar anchoring reaching the order of 10-3 J/m2.

Determination of Polar Anchoring Strength at Vertical Alignment Nematic Liquid Crystal-Wall Interface Using Thin Hybrid Alignment Nematic Cell

A method of determining the polar anchoring strength Wd of a vertical alignment nematic liquid crystal-wall interface is proposed. Wd is evaluated from the measurements of the optical retardation of a thin hybrid alignment nematic cell as a function of the wavelength of the incident light and cell rotating angles. As an example, the anchoring strength Wd = 2.5 ×10-4 [J/m2] is measured at the interface between the nematic mixture ZLI-2293 and the polyimide vertical alignment film PI-1.

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.

Analysis of Liquid Crystal Orientation at Interface between Liquid Crystal and Alignment Film by Total Reflection Ellipsometry.

The total reflection ellipsometry (TRE) method is proposed to investigate the behavior of the liquid crystal (LC) molecules at the interface between LC and an alignment film. Theoretical analysis indicated that the TRE method is useful for studying the behavior of interfacial LC molecules. From the numerical results of the TRE method, it was found that the phase difference versus applied voltage depends on the polar anchoring strength. The TRE method is sensitive enough to estimate the polar anchoring strength of LC cells.

Study of Liquid Crystal Alignment Formed Using Slit Coater

Liquid-crystal (LC)-coated substrates with a planar LC alignment can be fabricated by UV irradiation during LC coating using a slit coater. The alignment transition from a planar alignment to a vertical alignment was caused by UV irradiation after LC coating when a certain UV-curable additive was doped with the nematic LC. These results are applicable to the development of a novel fabrication technology for printable LC devices.

Study of Liquid Crystal Display Fabricated Using Slit Coater under Two Ultraviolet Irradiation Conditions

We have recently reported on a method that utilizes a slit coater as a novel liquid crystal alignment technique. We carried out this study under two ultraviolet (UV) irradiation conditions, viz., (i) UV irradiation from the side of a glass substrate (exposure type 1) and (ii) UV irradiation from the side of the air–liquid crystal interface (exposure type 2), and investigated the electro-optical characteristics and azimuthal anchoring energy of liquid crystal sample cells performed under these two UV irradiation conditions.