Influence of dichroism on the optical properties of a twisted nematic liquid crystal cell in the terahertz region.

Polarization control is essential in terahertz (THz) imaging. Liquid crystals (LCs) have the potential to functionalize tunable polarization-control devices. Here, a twisted nematic (TN) cell using a hydrogen-bonded LC is fabricated, and the influence of dichroism in the THz region is discussed. Our results indicate that the polarization state in the Gooch-Tarry minimum condition is affected by the LC dichroism; a nondichroic LC is required for complete linearly polarized output. The output intensity of the dichroic LC-TN cell changed when electrically switched or when the incident THz wave polarization direction was rotated 90°. These intensity variations disappeared when using the nondichroic hydrogen-bonded LC.

Stokes imaging polarimetry using a twisted hybrid aligned nematic liquid crystal cell.

We propose a Stokes polarimetry system containing a twisted hybrid aligned nematic (HAN) liquid crystal (LC) cell as a simple and compact polarimetry setup that can function without any mechanical movement of the optical components. We measured the Mueller matrix of the twisted HAN-LC cell and compared it with the Mueller matrix of an ordinary nontwisted HAN-LC cell with respect to practicability for the proposed LC polarimetry system. The basic measurement principle of LC polarimetry is presented after determining the best set of four driving voltages for maximizing measurement performance. Finally, we conducted some demonstrative experiments using two different birefringent samples to ensure the applicability of the proposed LC polarimetry system.

Simple Stokes polarimeter using a liquid crystal grating with ternary orientation domains.

We propose a liquid crystal (LC) grating with a ternary orientation domain for application to optical polarization measurements. The ternary orientation domain, which generates two-dimensional diffraction properties, is the key to simultaneous acquisition of multiple of polarization parameters. The LC molecular orientation state and the polarization dependence of the diffraction efficiency were investigated experimentally, focusing on the applicability to a practical Stokes polarimeter. An experiment was conducted using the proposed LC grating as a Stokes polarimeter, and the four Stokes parameters (S0, S1, S2, and S3) were determined for linearly and circularly polarized incident light. As a result, the feasibility of the proposed LC grating Stokes polarimeter has been demonstrated experimentally. Finally, the operational performance of the proposed LC grating Stokes polarimeter is discussed using a figure of merit that is numerically derived from the measured polarization dependence of the diffracted light intensity.

Optical properties of a dichroic dye-doped liquid-crystal grating and its application to optical rotation measurement.

The diffraction properties of a twisted nematic liquid-crystal (TN-LC) grating, in which a dichroic dye was doped, were investigated experimentally in terms of the applicability to an optical system for determining the polarization angle of incident linearly polarized light. The main reason to dope a nematic LC material with a dichroic dye was to enhance the polarization dependence of the diffracted light intensities for the positive and negative first orders. We found the best values of the applied voltage for application in polarization angle measurement. The TN-LC grating with a high polarization sensibility was applied to the optical system for measuring the concentration of an optically active component in a liquid specimen. As a result, the applicability of the proposed optical system was revealed experimentally. Furthermore, the cause of the random measurement error is discussed briefly.

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.

Liquid-crystal-grating-based optical displacement sensors.

The optical properties of liquid crystal (LC) gratings with a mobile glass substrate, which can be used for optical displacement applications, were investigated. LC gratings are composed of homogeneous and twisted nematic domains. The displacement measurement principle is based on the change in diffraction efficiency induced by varying the mixing ratio of two orientation domains, in response to the lateral shift of the mobile substrate. Improved conditions for the polarization direction of incident light, as well as the application of voltage for achieving higher sensitivities, were explored. A simple demonstration of an LC-grating-based displacement sensor was also conducted.

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.

Twisted nematic liquid crystal polarization grating with the handedness conservation of a circularly polarized state.

We propose a liquid crystal (LC) polarization grating that conserves the polarization state of incident light, wherein the variation range of the twist angle is 2π. The design scheme for theoretically 100% diffraction efficiency of the first-diffraction order is derived, and a prototype LC grating is evaluated. Under zero voltage, the fabricated LC grating exhibits high efficiency of the first-order diffraction, validating the proposed design scheme. The high efficiency of the second-order diffraction can also be achieved under a high voltage so that the LC director in the midplane is vertical to the substrate plane. The circular polarization sense of the second-order diffraction is identical to that of the incident light as in the case of the first-order diffraction. This grating functions as a beam deflector, steering the input beam in three different directions (zeroth-, first-, and second-order diffractions) by adjusting the applied voltage.

Liquid-crystal variable-focus lenses with a spatially-distributed tilt angles.

A pretilt angle controlling method by the density of rubbings using a tiny stylus is proposed. The control of the surface pretilt angle is achieved by rubbing a side-chain type polyimide film for a homeotropic alignment. Smooth liquid crystal (LC) director distribution in the bulk layer is successfully obtained even though the rough surface orientation. This approach is applied to LC cylindrical and rectangular lenses with a variable-focusing function. The distribution profile of the rubbing pitch (the reciprocal of the rubbing density) for small aberration is determined to be quadratic. The variable focusing function is successfully achieved in the LC rectangular lens, and the voltage dependence of the focal length is tried to be explained by the LC molecular reorientation behavior.

Liquid crystal polarization-converting devices for edge and corner extractions of images using optical wavelet transforms.

Optical properties of radially polarized light generated by liquid crystal (LC) polarization-converting devices with two LC molecular orientations (+/-180 degrees and +/-90 degrees twist modes) are measured and the applicability to optical wavelet transforms (WTs) are discussed. It is found that the radially polarized lights in both the +/-180 degrees and +/-90 degrees twist modes can be regarded as wavelets. Based on the principle of WT, feature extractions of a simple binary image are carried out by applying the LC polarization-converting device to a shadow-casting system. Unique feature extraction properties of the shadow-casting system using the LC polarization-converting device and a LC microlens array are discussed. It is experimentally confirmed that edge and corner features are successfully extracted using the LC polarization-converting device with +/-180 degrees and +/-90 degrees twist modes, respectively.

Simple method for the determination of refractive indices and loss parameters for liquid-crystal materials in the millimeter-wave region.

A reflection-type liquid crystal (LC) test cell is prepared with a rectangular waveguide for investigation of a novel method to determine refractive indices and loss parameters of nematic liquid-crystal materials. As the bottom of the test cell is sealed with a glass window and the top of the cell is capped with a metal-tipped movable reflector after the LC materials are injected, both ends of the waveguide test cell have large reflectance. Thus the reflection properties of the LC test cell can be well described by a multiple-beam interference model. A simple method for the determination of refractive indices from the reflection measurement data is proposed based on results of some investigations with the theoretical model. Commercially available LC materials have been measured with this method at a millimeter-wave frequency (50 GHz) by use of a simple experimental setup with a Gunn oscillator and a diode detector.

Liquid-crystal blazed grating with azimuthally distributed liquid-crystal directors.

We propose a novel formation method of arbitrary phase profiles of circular light by controlling azimuthal angles of liquid-crystal directors; its principle is described theoretically. A new liquid-crystal blazed grating is demonstrated by use of the proposed method. It is revealed that the first-order diffraction efficiency reaches the maximum value (theoretically 100%, experimentally approximately 90%) at an optimum applied voltage when the phase difference between the extraordinary and ordinary rays agrees with one-half the wavelength. Furthermore, the polarization states of the diffracted light beams are analyzed by Stokes parameter measurements, and unique polarization-splitting properties are revealed.

Liquid-crystal depolarizer consisting of randomly aligned hybrid orientation domains.

A novel depolarization method for linearly polarized incident light that uses a liquid-crystal (LC) cell with randomly aligned hybrid orientation domains is theoretically described by use of Mueller matrix calculations. The depolarization effect of the incident linear polarization is confirmed with Stokes parameter measurements. The unique optical properties of the fabricated LC depolarizer are revealed; that is, the intensity of the transmitted light is independent of the rotation of the analyzer. The degree of polarization becomes zero when the retardation of the LC depolarizer coincides with a half-wavelength.