Ryouta Ito

Rotational Behavior of Stripe Domains Appearing in Hybrid Aligned Chiral Nematic Liquid Crystal Cells

The appearance of stripe domains is strongly affected by various cell conditions such as thickness, applied voltage, and surface alignment. In this work, we adopt hybrid surface alignment to investigate the appearance of stripe domains under asymmetric surface anchoring. It is observed that the stripe direction rotates continuously according to the cell thickness and applied voltage, which has potential as a unique diffraction grating. It is also found that the rotational response time is always several seconds for any size of cell, because a large area rotation is achieved by the sum of small poly domain rotations related to defect creation–extinction processes on their boundaries.

Tunable Terahertz Filter Using an Etalon with a Nematic Liquid Crystal Layer and its Response Speed

We demonstrate a tunable terahertz (THz) filter using an etalon with a nematic liquid crystal (NLC) layer. The etalon is composed of water-free fused silica glass plates and air layers, and contains a 75 μm-thick defect layer filled with homeotropically-aligned NLCs. Frequency tuning of the transmission peak is achieved by applying an in-plane electric field across the NLC layer: the transmission peak shifts from 1.025 THz to 1.013 THz. The obtained rise and decay times are 5.3 s and 12.2 s respectively and found to be 10 times faster than previous THz LC devices because of the thin NLC layer.

Determination of birefringence and slow axis distribution using an interferometric measurement system with liquid crystal phase shifter

It is known that liquid crystal (LC) cells are useful as compact and easy-to-handle phase shifters that are readily coupled into the optics of standard microscope systems. Here, a uniformly aligned molecular LC phase shifter is introduced into a polarization microscope to attain a birefringence imaging system, using the phase-shift interferometric technique. Since the birefringence can be determined accurately only when the optical axis of the sample is parallel or perpendicular to the slow axis (variable axis) of the LC phase shifter, an improved data analysis method is proposed for determining the birefringence independently of the direction; a simple method of determining the slow axis distribution is also demonstrated. Measurements of the birefringence and slow axis distribution properties of a potato starch particle are demonstrated to confirm the novel determination method.

High frequency performance extending to millimeter-waves in inverted-microstrip-line-type LC phase shifter

Various liquid crystal (LC) phase shifters that operate in the super-high-frequency electromagnetic-wave regions have been investigated using planar-type excellent waveguides such as the microstrip line (MSL) and coplanar waveguide (CPW). First planar-type LC phase shifters were constructed using MSL, which was developed as an excellent planar waveguide for super-high-frequency electromagnetic waves. CPW-type LC phase shifters have attracted continued attention, because when they are used, all the signal and ground electrodes are at the same surface, which leads to ease in integration for constructing various functional devices. However, they suffer from an essential drawback of degradation in the phase shift magnitude, which is because the propagating electromagnetic waves encounter the permittivity of both the substrate and the LC materials, which reduces the modulation effect of the LC materials to less than half. In this work, a novel MSL-type LC phase shifter is investigated to achieve excellent phase shifting performance while maintaining ease in integration, as offered by the CPW-type phase shifter. Several device structural parameters are investigated to improve the transmission and phase shifting properties. Some LC materials are also tested for further improvement in the high-frequency operation extended to the millimeter-wave region.

THz Nematic Liquid Crystal Devices Using Stacked Membrane Film Layers

Twisted nematic type THz polarization control devices using liquid crystal immersed membrane films were fabricated by using simple stacking process. First of all, we verify the liquid crystal alignment effect in various kinds of membrane films. From THz Time-Domain Spectroscopy measurements, apparent refractive index anisotropy was observed in liquid crystal immersed Polytetrafluoroethylene and Polyolefin membrane films. Furthermore, transmission properties of twisted nematic type THz device are in rough agreement with 4 × 4 matrix calculation data. We believe that primary characteristic of THz TN device can be obtained by optimizing the fabrication processes and measurement method.

Basic Performance of Refractive Index Measurement Method for LC Materials in Super High Frequency Region by Using Coplanar Wave Guide

Novel measurement method of complex refractive indices of liquid crystal (LC) materials in the ultrahigh frequency region such as microwave and millimeter-wave (MMW) is investigated to reduce the quantity of test sample by using a coplanar waveguide (CPW) substrate, which is known as an excellent planar type waveguide. Although it needs some calibration method for precise measurement, the amplitude and phase properties of the propagating electromagnetic waves are sufficiently sensitive to the extremely small amount of test sample and the basic performance for the loss and refractive index measurements is investigated by using the well known K15 LC material.

THz Wave Transmission Properties of LC Composite Membrane Films

In this study we investigate anisotropic transmission properties of the membrane films impregnated with nematic LC materials in the THz frequency region. Typical experimental result shows that transmittance of membrane film without LC material is almost 100% in THz region. Phase change of the THz wave is observed but the transmittance does not decrease so much after impregnating LC material. Refractive index anisotropy increases after impregnating LC material which is related to the molecular orientation in the membrane film. These results show that the membrane film has a potential application to the LC device components in the THz region.

Differential interference contrast imaging using a pair of twisted nematic cells.

Nematic liquid crystal behaves like an optically uniaxial crystal whose optical axis coincides with the direction of molecular orientation. When an electric field is applied, a lateral shear of incident light is induced, depending on the angle of molecular inclination. While this may degrade the image quality for display applications, the precise electrical tunability of the lateral shear distance is desirable for differential interference contrast (DIC) imaging. In this Letter, a pair of twisted nematic (TN) cells is used for DIC imaging instead of the normal DIC prisms, and the unique optical properties of the TN cell are investigated for DIC imaging applications.

Improved High-Frequency Performance of Microstrip-Line-Type Liquid Crystal Phase Shifter

A novel microstrip-line (MSL)-type liquid crystal (LC) phase shifter is designed to retain both the ease of fabrication with a coplanar waveguide (CPW)-type LC phase shifter and the excellent phase-shifting properties of the MSL-type one. The converting circuit, which is a key component of the device, is optimized so as to improve the operating frequency. Then, the phase-shifting properties and loss phenomena are investigated by using various LC materials. The operational frequency is extremely improved by employing a novel large birefringence LC material and the optimized design.

Fundamental Properties of Novel Design Microstrip Line Type of Liquid Crystal Phase Shifter in Microwave Region

A planar-type electrically tunable phase shifter is demonstrated in the microwave region by using liquid crystal (LC) materials. A novel planar waveguide structure based on the microstrip line, which can convert the normal microstrip line to the inverted one, is designed. The new conversion circuit enables us to introduce LC materials on the microstrip line as easily as in the case of using the coplanar waveguide type of liquid crystal phase shifters. Larger phase shifting properties are confirmed empirically up to approximately 10 GHz with the application of a driving voltage of less than 10 V.