Kohei Noda

Effects of plasticizer on the transient diffraction properties of polarization holographic gratings made from polarization-sensitive polymeric films

We recorded anisotropic gratings by polarization holographic recording in azobenzene-containing polymeric films doped with a 9-ethylcarbazole (ECZ) plasticizer. Using a polarimeter, we measured in real time the Stokes parameters of the beams diffracted from the gratings. We analyzed these data on the basis of a theoretical model that accounted for the distribution of birefringence caused by molecular reorientation as well as for surface relief deformation caused by mass transportation. Our results indicated that increasing ECZ doping level increased the formation speed of the anisotropic grating, but did not greatly affect the amplitude of the photoinduced birefringence.

Universal polarization terahertz phase controllers using randomly aligned liquid crystal cells with graphene electrodes

We present a universal polarization terahertz (THz) phase controller using a randomly aligned liquid crystal (LC) cell with graphene electrodes. The LC cell was fabricated using a nematic LC and two quartz substrates that were coated with a monolayer of graphene as the transparent electrode. The LC in the cell was prepared without any alignment treatments and was randomly aligned. The size of the random domains and the width of the disclination lines in the LC layer were several tens of microns. These textures disappeared when an alternating voltage was applied to the LC through the graphene layers. Using a THz time domain spectroscopic technique, we investigated the complex transmittance of the LC cell. The LC cell was highly transparent in the THz frequency range, and there was little change in the transmittance with the applied voltage. This indicated that the scattering loss originating in the randomly aligned LC molecules was small for the THz waves. We also demonstrated that the THz phase shift could be controlled by the applied voltage. The amplitude of the phase shift was explained by the ordinary and extraordinary refractive indices of the LC. These LC cells with graphene electrodes can be used to realize universal polarization THz phase controllers because of the random alignment.

Simple fabrication of liquid crystalline grating cells with homogeneous and twisted nematic structures and effects of orientational relaxation on diffraction properties

We have substantiated a simple yet efficient, competitive, and practical method to automatically fabricate liquid crystalline grating cells with homogeneous or twisted nematic (TN) structures by one-step ultraviolet holographic exposure on an empty glass cell coated with a photocrosslinkable polymer liquid crystal with 4-(4-methoxycinnamoyloxy)biphenyl side group (P6CB) films. The polarization diffraction properties in the resultant liquid crystalline grating cells have also been investigated extensively by varying the grating pitch. The theoretical considerations on the basis of elastic continuum theory revealed that accumulation of elastic free-energy density due to the TN structure affected the diffraction properties, although the strong anchoring of our photoalignment material contributed effectively to form the expected spatial distribution of liquid crystalline directors in the grating cells.

Liquid Crystal Gratings with Twisted Alignment Produced by One-Step Polarizer-Rotation Exposure on Photocrosslinkable Polymer Liquid Crystal Films

Liquid crystal (LC) gratings with twisted alignment were fabricated by simple one-step polarizer-rotation exposure. An empty glass cell coated with photocrosslinkable polymer liquid crystal (PCLC) films was exposed by UV light with modulation of intensity or polarization. LC grating cells were obtained by filling the resultant cells with a nematic LC and the LC alignment structures were well controlled by both the intensity and polarization of the UV light. The diffraction properties of the LC gratings were explained by considering the periodic director distribution in the cells, estimated based on the elastic continuum theory of nematic LCs and the photoalignment effects of the PCLC films.

Multilevel anisotropic diffractive optical elements fabricated by means of stepping photo-alignment technique using photo-cross-linkable polymer liquid crystals

Multilevel anisotropic diffractive optical elements (DOEs), in which digitized spatial patterns of optical birefringence are fabricated by means of stepping photoalignment technique, has been demonstrated using photo-cross-linkable polymer liquid crystals (PCLCs). The polarization state of incident light is converted into a different polarization state by diffracting light in the practical, i.e., transparent in visible region and thermally stable, multilevel anisotropic DOEs, and both polarization azimuth and ellipticity can be widely controlled by their birefringence patterns. Theoretical considerations for such polarization conversion were also performed using the Jones calculus and diffraction theory and well-explained experimental observations.

Three-dimensionally modulated anisotropic structure for diffractive optical elements created by one-step three-beam polarization holographic photoalignment

A diffractive optical element with a three-dimensional liquid crystal (LC) alignment structure for advanced control of polarized beams was fabricated by a highly efficient one-step photoalignment method. This study is of great significance because different two-dimensional continuous and complex alignment patterns can be produced on two alignment films by simultaneously irradiating an empty glass cell composed of two unaligned photocrosslinkable polymer LC films with three-beam polarized interference beam. The polarization azimuth, ellipticity, and rotation direction of the diffracted beams from the resultant LC grating widely varied depending on the two-dimensional diffracted position and the polarization states of the incident beams. These polarization diffraction properties are well explained by theoretical analysis based on Jones calculus.

Holographic binary grating liquid crystal cells fabricated by one-step exposure of photocrosslinkable polymer liquid crystalline alignment substrates to a polarization interference ultraviolet beam

Holographic binary grating liquid crystal (LC) cells, in which the optical anisotropy was rectangularly modulated even as the grating was fabricated using holographic exposure, were fabricated by one-step polarization holographic exposure of an empty glass cell, the interior of which was coated with a photocrosslinkable polymer LC (PCLC). The present study is of great significance in that three types of holographic binary grating LC cells containing twisted alignments can be fabricated by simultaneous exposure of two PCLC substrates to the UV interference beams, which are sinusoidally modulated. The polarization conversion properties of the diffracted beams are explained well by theoretical analysis based on Jones calculus.

Diffraction properties of a vector grating liquid crystal cell fabricated using a one-step exposure of a nonorthogonal elliptically polarized interference beam

Vector grating liquid crystal (LC) cells, with periodically intermixed 0° planar and 90° twisted nematic alignments and director distributions rotated in the grating vector, were fabricated using a one-step polarization holographic exposure. A nonorthogonal elliptically polarized interference UV beam was irradiated to the empty glass cell, which had the inner walls coated with a photocrosslinkable polymer liquid crystal film. The dependence of the diffraction properties on the birefringence of the nematic LC (E7) in the resultant vector grating LC cell was determined through experimental and theoretical analysis. The polarization azimuth and ellipticity of the diffracted beams were varied using the temperature of the nematic LC.

Three-dimensional vector recording in polarization sensitive liquid crystal composites by using axisymmetrically polarized beam.

Three-dimensional anisotropic structures were fabricated by a recording axisymmetrically polarized beam in azobenzene (azo)-dye doped liquid crystal polymer composites. Polarization and wavefront modulation properties of fabricated anisotropic structures are investigated by experimentally and theoretically analyzing the diffraction properties. Photo-induced anisotropic structures would be utilized to generate singular light waves, such as optical and polarization vortices.

Merged vector gratings recorded in a photocrosslinkable polymer liquid crystal film for polarimetry

A merged vector grating, which is holographically fabricated in an anisotropic medium by irradiation with interference light with intensity modulation and polarization modulation, was designed to detect the polarization of light. The merged vector grating is recorded by the interference of two elliptically polarized beams with equal intensities, parallel azimuths, equal ellipticities, and different directions of rotation. We clarified theoretically that the Stokes parameters of the incident light beam are described by the diffraction efficiency of the merged vector grating. Also, to apply this property to formation of a polarimeter without any moving parts or mechanisms, two merged vector gratings with different grating vectors were recorded in a photocrosslinkable polymer liquid crystal film by angle-multiplexed holography. By investigation of the diffraction properties of the gratings obtained, we demonstrated the applicability of the merged vector gratings for use in polarimetry.