Kotaro Kawai

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.

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.

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.

Polarization grating fabricated by recording a vector hologram between two orthogonally polarized vector vortex beams

Polarization gratings (PGs) were fabricated by the recording of vector holograms between two orthogonally polarized vector vortex beams (VVBs). The polarization and diffraction properties of the resulting PGs were analyzed theoretically. The feasibility of the process was demonstrated experimentally using a photo-crosslinkable liquid crystal polymer film as the polarization-sensitive material. The fabricated PGs can convert homogeneously polarized laser beams into VVBs, vector beams, optical vortices, and ring-shaped optical lattices by controlling the incident beam’s state of polarization. The PGs that are presented will be applicable to optical communications and optical manipulation processes as vortex generators and converters.

Tunable dichroic polarization beam splitter created by one-step holographic photoalignment using four-beam polarization interferometry

A tunable dichroic polarization beam splitter (tunable DPBS) simultaneously performs the follow functions: 1. Separation of a polarized incident beam into multiple pairs of orthogonally polarized beams; 2. Separation of the propagation direction of two wavelength incident beams after passing through the tunable DPBS; and 3. Control of both advanced polarization and wavelength separation capabilities by varying the temperature of the tunable DPBS. This novel complex optical property is realized by diffraction phenomena using a designed three-dimensional periodic structure of aligned liquid crystals in the tunable DPBS, which was fabricated quickly with precision in a one-step photoalignment using four-beam polarization interferometry. In experiments, we demonstrated that these diffraction properties are obtained by entering polarized beams of wavelengths 532 nm and 633 nm onto the tunable DPBS. These diffraction properties are described using the Jones calculus in a polarization propagation analysis. Of si...

Alignment structures and diffraction properties of chiral nematic liquid crystal cells with periodically patterned photoalignment films

Liquid crystal (LC) cells with periodic alignment distributions were fabricated using chiral nematic LCs (N*LCs), which were prepared using mixtures of a nematic LC and a chiral dopant, along with photoreactive liquid crystalline polymer (PLCP) films. Periodic structures were formed by polarization holographic recording in the PLCP films. The director distribution in each cell depended on the ratio of chiral dopant present, i.e., the inherent helical pitch of the N*LCs. These periodic alignment structures with line defects in the LC grating cells were well explained on the basis of the elastic continuum theory of the N*LCs and the photoalignment effect of the PLCP films. The diffraction properties of the grating LC cells were also investigated using a polarized visible laser. The observed intensity and polarization states of the diffracted beams were consistent with theoretical ones calculated using the director distribution models. Our results clarify that the diffraction properties of the grating LC cells can be controlled by the helical pitch of the N*LCs.

Fabrication of anisotropic diffractive optical element by using polarization drawing method based on galvanometer scanner

Polarization is one of the important parameters of the light wave. Diffractive elements, which can control the polarization, have been attracted as high-performance light control device. We have implemented various studies on the formation method and the diffraction characteristics of the anisotropic diffractive element using a photoreactive material. Photocross linkable polymer liquid crystal (PCLC) is an attractive material that can induce anisotropy along the polarization direction of linearly polarized ultraviolet light (LPUV). Also, owing to its relatively large anchoring strength, PCLC have been used as an alignment film of low-molar-mass liquid crystal (LC). Galvanometer scanners (GS) can freely control the exposure position of the laser beam by adjusting the two mirrors, it is possible to form a highly functionalized optical element by drawing the arbitrary exposure lines to the photo-reactive material with temporally changing the polarization state of the laser beam. In this study, we report the polarization drawing method based on GS for the fabrication of anisotropic diffractive optical elements. First, the two types anisotropic diffractive optical elements were fabricated on the PCLC films. To investigate the diffraction properties of fabricated anisotropic diffractive optical elements, we used a polarized He-Ne laser beam as probe and observed diffracted lights. Diffracted beam was twodimensionally emitted depending on the formed anisotropic optical distribution. Then we fabricated LC cell, which works as polarization dependent anisotropic Fresnel lens. The experimental investigations show that it has functions of light condensing and polarization control. From these results, high-performance light control device can be fabricated by the polarization drawing method.