Photocrosslinkable Polymer Liquid Crystal Films Research Articles

Polarized beam steering using multiply-cascaded rotating polarization gratings.

A polarized laser beam steering system using multiply cascaded rotating polarization gratings (PGs) is presented. The rotating PGs steer incident circularly polarized beams with high optical throughput because the theoretical limitation of the PG diffraction efficiency is 100%. The system also offers more rapid rotation when compared with wedge prism pairs because of the weight of the PGs. The beam steering performance was analyzed theoretically for schemes using two and four rotating PGs. The system's feasibility was demonstrated experimentally by projecting Lissajous and raster patterns using PGs fabricated from photocrosslinkable polymer liquid crystal films via a photo-alignment technique. The steered beam's diffraction efficiency and ellipticity were maintained at 83∼89% and 96∼99%, respectively, during PG rotation. This beam steering system will be applicable to light detection and ranging, optical imaging, and laser displays.

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.

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.

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.

Fabrication of twisted nematic structure and vector grating cells by one-step exposure on photocrosslinkable polymer liquid crystals

We present a simple yet efficient method to automatically fabricate the twisted nematic structure by one-step exposure on an empty glass cell coated with photocrosslinkable polymer liquid crystal (PCLC) films. The resultant photoalignment directions of two substrates can be orthogonal to each other by controlling the difference between the exposure energy for upper and lower PCLC films and the twisted nematic (TN) structure can be automatically fabricated. The vector grating liquid crystalline cell with TN structure was also fabricated by means of a developed method, and the diffraction properties were well explained by the theoretical calculation on the basis of Jones calculus.

Elastic continuum analysis and diffraction properties of two-dimensional liquid crystalline grating cells

A detailed analysis of two-dimensional liquid crystalline grating cells is presented. Crossed liquid crystalline gratings were prepared using two kinds of photo-alignment polymers, i.e., poly(vinyl cinnamate) (PVCi) and photocrosslinkable polymer liquid crystal (PCLC). The effects of the anchoring strength of photo-alignment polymer films and crossing angles of grating vectors on one pair of photo-alignment substrates were extensively investigated using elastic continuum analysis, Jones calculus, and diffraction theory. The anchoring strength of our PCLC film was sufficiently strong, and experimental observation was in good agreement with the theoretical values obtained assuming the strong anchoring (fixed boundary conditions), although PCLC still has limits with the capability of photo-alignment of the low-molar-mass liquid crystals.

Effects of anchoring strength on diffraction properties of liquid crystal phase gratings formed on photoalignment polymer films

The liquid crystal phase gratings were prepared using a polarization holographic exposure on poly(vinyl cinnamate) or photocross-linkable polymer liquid crystal (PCLC) films. Anchoring strength between the photoalignment layer and low-molar-mass liquid crystal is a very important role to realize the liquid crystal grating as a practical optical device. Anchoring strength of our PCLC film is sufficiently strong and optical properties of liquid crystal gratings were theoretically well explained assuming the strong anchoring.

Twisted phase gratings induced by photoregulated mesogenic molecules on the surface of photoreactive polymer liquid crystal film

Three-dimensional phase gratings with twisted structures were self-organized by photoregulated mesogenic molecules on the surface of photocrosslinkable polymer liquid crystal film. The molecules are selectively photoreacted only at the surface of the film under the polarization holographic illumination, and the resultant photocrosslinked mesogenic molecules induce the three-dimensional twisted gratings in the film during annealing process. The diffraction properties were well explained by means of Jones calculus and diffraction theory.

New fabrication method for anisotropic gratings formed in photocrosslinkable polymer liquid crystals

We present a new fabrication method, called ‘two-step ultraviolet exposure’, to prepare anisotropic gratings in photocrosslinkable polymer liquid crystal films. Using the controllability of the reorientation direction of mesogenic molecules, anisotropic phase gratings, in which the mesogenic molecules were periodically modulated in orthogonal directions, were prepared by the use of one grating photomask. The resultant gratings diffract the light and convert the polarization states at the same time, and have applications as diffraction optical devices.

Influence of substituents on the reorientational behavior in photocrosslinkable polymer liquid crystal films with p-substituted cinnamoyloxybiphenyl side groups by irradiating with linearly polarized ultraviolet light and annealing

This paper describes the synthesis and influence of the substituent on the reorientation behavior of photocrosslinkable methacrylate polymer liquid crystals (PPLCs), which contain p-substituted cinnamoyloxybiphenyl (CB) side groups. Irradiating with linearly polarized ultraviolet (LPUV) light induced optical anisotropy of all PPLC films due to an axis-selective photoreaction of the mesogenic groups. The thermally enhanced reorientation direction depended on the substituent groups and liquid crystalline nature of the PPLC. A high in-plane reorientational order ( S>0.6) parallel to the polarization of LPUV light was obtained for PPLCs with p-methoxy-CB and p-methyl-CB side groups, while out-of-plane reorientation was predominant for p-fluoro-CB and p-trifluoromethyl-CB side groups.

Highly stable polarization gratings in photocrosslinkable polymer liquid crystals

Highly stable polarization gratings have been prepared in photocrosslinkable polymer liquid-crystal films by the use of two orthogonally polarized He–Cd laser beams and subsequent annealing. The resulting pure polarization gratings exhibiting thermal stability up to 150 °C diffract the beam and convert the polarization state at the same time according to the theoretical expectation.

Photo-alignment of low-molecular liquid crystals on photo-crosslinkable polymer liquid crystalline film by linearly polarised UV light

Effects are described of the intensity and irradiation temperature of a linearly polarized (LP) UV light on a photoreaction of a photo-crosslinkable polymer liquid crystal (PLC) film comprising a cinnamoylethoxybiphenyl side group, and the photo-alignment behavior of nematic low-molecular (LM) liquid crystals (LCs) on a PLC film. Both homeotropic and homogeneous alignments of the LM-LC were possible on the photoreacted PLC film when the film was irradiated at room temperature. However, a homeotropic alignment was not observed when the film was exposed in the liquid crystalline temperature range of the PLC. When the light intensity was strong, the homogeneous alignment direction can be controlled both parallel and perpendicular to the electric vector (t E) of the incident LP-UV light, while it needed a large amount of irradiation energy to obtain a perpendicular alignment for the weak LP-UV irradiation. The LM-LC alignment behavior can be explained by a change of the interaction among the LM-LC, the photo-crosslinked mesogenic group of the PLC film and the photoinduced alignment of the photoreacted PLC film. Copyright © 1999 John Wiley & Sons, Ltd.