Photoalignment Of Nematic Liquid Crystals Research Articles

Electro-Optical properties of photoaligned Liquid Crystal cells prepared with obliquely irradiated Chalcogenide glasses

We report the effect of oblique irradiation of thin As2S3 chalcogenide glass films on the photoalignment of Nematic liquid crystals and their electro optical properties. After depositing 60 nm As2S3 films on ITO coated substrates, the substrates were irradiated with polarized blue light (436 nm) at angles 0°, 5°, 20° and 48° of the light propagation direction with respect to the substrate plane. Then, the substrates were assembled and filled with the liquid crystal. The device LC-20° exhibits good extinction, and lower threshold voltage when compared to all other devices. The contrast ratio of LC-20° almost double that of the LC-5° and LC-48° devices. The response time increases with the angle, so for LC-20° slightly longer than for LC-5° but shorter than that of LC-48°. It is evident from the observations that the obliquely irradiated cells show good photoalignment and enhanced electro optical properties, although no pretilt is generated due to the oblique irradiation. Therefore, contrary to the photoalignment using photosensitive polymers, chalcogenide glasses have the advantage of providing photoalignment with little sensitivity to the irradiation angle. This fact led us to propose a model for the photoinduced alignment on chalcogenide glasses based on directional structural changes, not related to oriented dipoles generation as in the case of photoalignment with polymers.

Plasmonic coupling induced by growing processes of metal nanoparticles in wrinkled structures and driven by mechanical strain applied to a polidimethisiloxisilane template

Plasmonic coupling induced by growing processes of metal nanoparticles in wrinkled structures and driven by mechanical strain applied to a polidimethisiloxisilane template

Homogenous photoalignment of nematic liquid crystals doped with photoalignable materials

We demonstrate the homogeneous photoalignment of low-molecular-weight liquid crystals (LC) using nematic LCs (NLCs) doped with photoalignable low-molecular-weight LC materials, achieved in the absence of substrate surface treatment; in-plane switching (IPS) is also shown using an IPS-mode LC cell. The exposure of the LC cells to linearly polarized (LP) light in the isotropic temperature range of the NLC composites generates homogenous NLC orientation, but exposure at room temperature results in the random orientation of the NLCs. Small amounts of photoalignable LC materials on the substrates, which have undergone axis-selective photoreaction, are found to control the NLC alignment.

Optical control of plasmonic heating effects using reversible photo-alignment of nematic liquid crystals

We demonstrate and characterize an optical control of the plasmonic heat delivered by a monolayer substrate of gold nanoparticles, obtained by modulating the effective refractive index of the neighboring dielectric medium. The effect, which exploits the dependence of the nematic liquid crystal (NLC) refractive index on the molecular director orientation, is realized by using a polarization dependent, light-induced molecular reorientation of a thin film of photo-alignment layer that the NLC is in contact with. For a suitable alignment, plasmonic pumping intensity values ranging from 0.25 W/cm2 to 6.30 W/cm2 can induce up to 17.4 °C temperature variations in time intervals of the order of seconds. The reversibility of the optically induced NLC molecular director orientation enables an active control of the plasmonic photo-induced heat.

Electrically controlled spatial-polarization switch based on patterned photoalignment of nematic liquid crystals.

A switching scheme for two orthogonal modes of laser radiation that is based on the total internal reflection effect realized at the interface of two liquid crystal regions with orthogonal director orientations is proposed. To create the photorefractive interface within the bulk of a liquid crystal, an original technique based on self-alignment of azo dye photoalignment and absorbing electrode patterns has been developed. Spatial separation of the orthogonally polarized light beams and their switching (when the positions of reflected and transmitted light beams are switched) due to the voltage applied has been experimentally realized.

Photoalignment Layers for Liquid Crystals from the Di-π-methane Rearrangement

Photoalignment of nematic liquid crystals is demonstrated using a di-π-methane rearrangement of a designed polymer. The alignment mechanism makes use of the strong coupling of the liquid crystal directors to dibenzobarrelene groups. The large structural changes that accompany photoisomerization effectively passivate segments of the polymer, allowing the remaining dibenzobarrelene groups to dominate the director alignment. Photoisomerization requires triplet sensitization, and the polymer was designed to have a uniaxially fixed rigid structure and rapid triplet energy transfer from the proximate benzophenone units to the dibenzobarrelene groups. The isomerization was observed to be regiospecific, and thin films showed alignment.

Azo containing thiophene based prop-2-enoates for photoalignment of a nematic liquid crystal

The ability of suitably derived prop-2-enoates from 3-(thiophen-2-yl)- and 3-(thiophen-3-yl)-prop-2-enoic acid and appropriately fluorinated and non-fluorinated 4-[(4-alkoxyphenyl)diazenyl]phenols to promote excellent photoalignment of a bulk commercial nematic liquid crystal is reported. The performed study showed that the photoalignment of the liquid crystal is influenced by the number and disposition of fluoro-substituents and the position of attachment of the terminal thiophene moiety, either 2- or 3-substituted, in the molecular structure of the alignment material. Inclusion of a lateral fluoro-substituent(s) induces an excellent photoalignment whereas the non-fluorinated counterpart tends to give an unsatisfactory alignment. 3-Substituted thiophenes give a better photoalignment than their 2-substituted isomeric counterparts. A prototype display, which contains an alignment layer made from one of the proposed materials, possessing good alignment ability, is presented in order to demonstrate the application potential of these materials in LCDs.

Photoalignment of Liquid Crystals on Chalcogenide Glass As20Se80 Surface

Photoalignment of nematic liquid crystals on the chalcogenide glass As20Se80 surface is studied by the digital processing of the optical textures of light-induced twist deformations of a liquid crystal (LC) in a cell irradiated with Gaussian polarized light. The original experimental method allowed obtaining the dependence of the light-induced twist angle for the As20Se80 surface on the exposure dose by the analysis of the only irradiated spot. The dependences of the light-induced twist angle on the exposure dose are found to be qualitatively different for LC~5CB and LC E7, which points on a possibility of different mechanisms of photoalignment on chalcogenide glass.

Permanent photoalignment of liquid crystals on nanostructured chalcogenide glassy thin films

Photoalignment of nematic liquid crystals is obtained on the chalcogenide glassy thin film of As2S3 using irradiation with polarized blue light. A uniform homogeneously aligned device is obtained with high contrast and strong anchoring. The device alignment quality is permanent as checked by following its functionality over a period of few months. The origin of the observed photoalignment is attributed to the photoinduced anisotropy in chalcogenide glasses. No differences between the different As2S3 film thicknesses observed, thus supporting the proposition that some orientational order is photoinduced on the surface of the glass and responsible for the photoalignment.

Photoalignment of nematic liquid crystal on polyamic‐acid‐based soluble polyimide with no side fragments

Abstract— The photoalignment of nematic LC on a layer of newly synthesized UV‐sensitive polyimide without side fragments is reported. The photoaligning polymer advantageously differs from other photoaligning materials because of its extremely low sticking effect and residual DC voltage. These parameters, as well as the UV and thermal stability of the alignment, are not worse than those of rubbed polyimides. At the same time, the new material possesses all the advantages of photoaligning polymers.

Photoinduced anisotropy and photoalignment of nematic liquid crystals by a novel polymer liquid crystal with a coumarin-containing side group

A new type of photo-crosslinkable methacrylate polymer liquid crystal (PLC) with a coumarincontaining mesogenic side group was synthesized and applied as the photoalignment layer for low molecular mass nematic liquid crystals. Linearly polarized ultraviolet light was directed onto a thin film of PLC under various exposure conditions. When a film was irradiated at room temperature, a small negative optical anisotropy was generated due to angular-selective photo-crosslinking. In contrast, when the film was exposed near the clearing temperature of the PLC, the induced anisotropy was positive due to thermally enhanced photoinduced reorientation of the side groups. The aggregation of the mesogenic groups was also observed when the irradiation was carried out in the liquid crystalline temperature range of the PLC. The LC alignment on the photoreacted film was greatly dependent on these irradiation conditions. It was made clear that the LC alignment was regulated by the interaction among the LC, the photo-crosslinked side groups and the remaining mesogenic side groups, and that the aggregated mesogenic groups inhibited the LC alignment.

Key Molecular Structure Determination of Photoalignment Materials from the Effects of Linearly Polarized Deep UV Light on Several Polymers

We studied the photoalignment of nematic liquid crystals by examining polyimide, polyester, polystyrene, polyvinylalcohol (PVA), and polymethylmethacrylate (PMMA) exposed to linearly polarized deep UV (LPDUV) light. Polystyrene aligned the liquid crystal parallel to its LPDUV polarization axis, while polyimide and polyester aligned the liquid crystal perpendicular to the polarization axis. Polyimide and polyester exhibited order parameter peaks for small dosages of LPDUV. PVA and PMMA did not align the liquid crystal. The presence of a phenyl ring is a key to the decomposition-type photoalignment, whose alignment direction depends on the position of the phenyl ring.

Photogeneration of pretilt angles of nematic liquid crystals by azobenzene-containing monolayers on poly(acrylic acid) films

A convenient procedure to fabricate azobenzene-containing monolayers capable of performing the photoalignment of nematic liquid crystals (LCs) with pretilt angles has been elaborated. The photoactive monolayers were readily prepared by the surface-selective adsorption of p-butylazobenzene substituted with an ω-aminoalkyl residue to the outermost surface of a poly(acrylic acid) thin film. After the fabrication of an LC cell, the monolayers generated homeotropic LC orientation, which was transformed into homogeneous alignment with tilting angles by oblique irradiation with non-polarized UV light. The azimuthal direction of the photoinduced LC alignment was consistent with the propagation direction of the UV light, suggesting that the LC photoalignment results from direction-selective E to Z photoisomerization leading to re-orientation of the azobenzene chromophores. The photogenerated pretilt angles of LC alignment were significantly dependent on the dielectric anisotropy of LC and the surface density of the azobenzene in the monolayers.

Thermal Stability of Photoalignment of Nematic Liquid Crystals by Azobenzene Polymer Thin Films.

Thermal Stability of Photoalignment of Nematic Liquid Crystals by Azobenzene Polymer Thin Films.

Liquid crystal alignment on photopolymer surfaces exposed by linearly polarized uv light.

A photo-alignment of nematic liquid crystals (NLC) on a photopolymer film of polyvinyl cinnamate (PVCi) has been achieved by using linearly-polarized UV light. The mechanism of the LC alignment is clarified and good unidirectional LC alignment with pretilt angle is obtained by appropriate combination of normal and oblique exposure of linearly-polarized UV light to the PVCi films. This photo-alignment method is applied to fabricate super-multidomain (SMD) TN-LCDs making a feature of quartered pixels, and we demonstrate that reverse tilt disclination (RTD)-free SMD TN-LCDs is successfully fabricated by using the photo-alignment method.