Toshiyasu Tadokoro

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.

Optical Properties of BiFeO3-System Multiferroic Thin Films

The optical properties of the chemical solution deposition (CSD)-derived BiFeO3 (BFO) film and the pulsed laser deposition (PLD)-derived (Bi,Nd)(Fe,Mn)O3 (BNFM) film were evaluated by spectroscopic ellipsometry, and their optical constants (n, k) and band gaps were determined. At a wavelength of 600 nm, the refractive indexes of 3.22 and 2.87 were estimated for the BFO and BNFM films, respectively, although the existence of a refractive index gradient was suggested in the BFO film. In addition, at a wavelength of 1550 nm, which is generally used for optical communication, the refractive indexes of 2.91 and 2.59 were estimated for the BFO and BNFM films, respectively. The band gaps of the BFO and BNFM films were estimated to be 2.79 and 2.72 eV, respectively, and it was confirmed that the extinction coefficients of both films were almost zero at wavelengths larger than 600 nm. These results suggest that the BFO-system multiferroic films have a high potential as an optical material with a high refractive index.

Design and Implementation of Near-Field Scanning Optical Microscope for Observation of Interfacial Liquid Crystal Orientation

A near-field scanning optical microscope (NSOM), optimized for observation of liquid crystal (LC) orientation at the LC/alignment layer interface, has been developed. The NSOM operates in the illumination mode using a metal-coated optical fiber probe together with a probe-positioning system based on the optical feedback technique. This novel system enables the depth profiling and two-dimensional imaging of the LC orientation distribution under an applied local electric field with high spatial resolution. In this paper we describe the design and implementation of the developed NSOM system and report its first application to the microscopic analysis of LC orientation at the LC/alignment layer interface.

Analysis of Totally Reflected Light from Liquid Crystal Cell using Renormalized Ellipsometry

Standard ellipsometry is usually applied for the evaluation of an optical isotropic system, but it is insufficient to analyze light reflected from an anisotropic system such as a liquid crystal (LC) cell. In order to analyze the light reflected from the LC cell, the standard ellipsometry system combined with the photo elastic modulator was extended to renormalized ellipsometry by introducing the definition of the renormalized reflection coefficients without any modification of the conventional experimental procedure. The experimental results such as the ellipsometric angles versus the rotation angle of the LC cell and the dependence of the ellipsometric angles on the applied electric field were in good agreement with the theoretical results based on renormalized ellipsometry. From these results, it was confirmed that renormalized ellipsometry was quite a useful technique to analyze the totally reflected light from the LC cell.

Spectroscopic Ellipsometry Measurements for Liquid and Solid InSb around Its Melting Point

We have carried out spectroscopic ellipsometry measurements for liquid- and solid-phase InSb around its melting point for wavelengths of 300 to 1700 nm. The real and imaginary parts of the complex refractive index for liquid and solid InSb appear to be completely different, with the imaginary part for the liquid being higher than that of the solid. This result agrees with the proposed mechanism for the super-resolution readout effect in optical disks using an InSb film. From a dielectric function analysis, we find that InSb characteristics clearly change from semiconducting in the solid to metallic in the liquid.

Two-Dimensional Analysis of Liquid Crystal Orientation at In-Plane Switching Substrate Surface Using a Near-Field Scanning Optical Microscope

The microscopic liquid crystal (LC) orientation distribution in an in-plane switching (IPS) LC display device has been studied using a near-field scanning optical microscope (NSOM). The new NSOM system, which features an improved metal-coated optical fiber probe and is specifically designed for LC orientation analysis, has enabled successful visualization of the two-dimensional LC orientation distribution at the substrate surface. The observed NSOM images can be reproduced by theoretical simulations. These images together with the simulations reveal that the uneven-level structure of the interdigitated electrodes causes the asymmetric local LC orientation distribution near the electrode edges.

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.

Application of time-resolved spectroellipsometry to the study of liquid crystal reorientation dynamics

Abstract Polarization modulated spectroellipsometry (PMSE), with a 1-ms time resolution and 0.01° phase difference angular sensitivity, was used to study the dynamics of the electric field-induced homogeneous–homeotropic transition of nematic liquid crystals. Reflection PMSE carried out for a dye-doped liquid crystal cell can isolate the signals from molecules in the vicinity of substrate walls. Information about bulk molecules is obtained from transmission PMSE experiments. The transition of interface molecules is fast in both rise and decay processes (3 ms). The rise of bulk molecules is similarly fast, but the decay is slow (100 ms). The fast reorientation exhibited by interface molecules indicates the presence of a strong anchoring effect imposed by the substrate surface.

Dynamics of Surface-Stabilized Ferroelectric Liquid Crystals at the Alignment Layer Surface Studied by Total-Reflection Ellipsometry

The reorientation dynamics of surface-stabilized ferroelectric liquid crystals (SSFLCs) at the substrate surface have been studied by time-resolved spectroellipsometry (TRSE). The results of total-reflection TRSE indicate that the surface motion of SSFLC molecules is hindered by a strong anchoring system. The observed dependence of the ellipsometric parameter Ψ on the orienting layer material is discussed in terms of the competitive effects of the surface tilt angle and the strength of the surface anchoring effect. It is also demonstrated that TRSE can analyze the mode of motion of SSFLC molecules in three-dimensional space and can therefore characterize the hysteresis properties of the SSFLC cell in terms of its structural changes.

High-Resolution Examination of Recording Marks in Phase-Change Media Using a Scanning Near-Field Optical Microscope

Recording marks in digital versatile disc-random access memory (DVD-RAM) were examined using a scanning near-field optical microscope (SNOM) incorporating an improved probe tip design. The marks were recorded on an experimental DVD-RAM medium with a track-pitch of 0.6 µm. The tip size of the SNOM was made wider than the track width in order to avoid optical ghosting arising from the track edge. The recording power was varied in order to investigate the dependence of the mark shape on power and it was found that the mark shape correlated well with the reproduced signal. The SNOM is considered to be a good candidate for a practical characterization tool for optical recording media.