Photoalignment Of Liquid Crystals Research Articles

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.

Morphological anisotropy of nano-dimensional arsenic trisulfide glass films and liquid crystal photoalignment

ABSTRACTThe results of high-resolution x-ray reflectivity study of nano-dimensionally thin arsenic trisulfide (As2S3) glass films reveal surface roughness anisotropy upon prolonged irradiation with polarized 436 nm blue light. The anisotropy correlates with the photoinduced anisotropy and LC photoalignment observed on these thin films. However, the As2S3 film's thickness reduces drastically during the reflectivity experiments, and ambient oxygen and moisture penetrate the film and create a second altered thin layer on top of a thicker film of As2S3. The results confirm the viability of As2S3 films for non-contact LC photoalignment material but suggests the need for further process optimization to reduce the films' degeneration to increase their effectiveness.

Command splay-bend switching in an optically-compensated bend cell with polymerized liquid crystal photoalignment layers

We demonstrate a fast-switching optically compensated bend (OCB) mode with polymerized liquid crystal photoalignment (PAL). The polymerized liquid crystal PAL is achieved by spin coating a mixture of reactive mesogens and a photoinitiator and polymerizing it with ultraviolet (UV) light on top of the PAL material, which is illuminated with linear-polarized ultraviolet light to introduce anisotropy and a pretilt angle for liquid crystal alignment. The experimental results show the dependency of the electro-optical properties of OCB cells on the morphology-modulated surface anchoring of polymerized liquid crystal PAL. Furthermore, the polymerized liquid crystal PAL exhibits superior stability against UV exposure and thermal stress, which makes PAL applicable in spatial light modulator application.

Optical field control via liquid crystal photoalignment

ABSTRACTOptical field control of laser beams is realized through photo-aligned liquid crystals. A dynamic micro-lithography system combined with a polarization-sensitive alignment agent is utilized to perform arbitrary fine liquid crystal alignment. Fork gratings are fabricated to generate high quality optical vortices. Forked polarization gratings with space-variant liquid crystal azimuthal orientations are proposed and corresponding optical vortices with arbitrary azimuthal and radial indices are demonstrated in high efficiency. Moreover, Airy masks for polarization selective Airy beam generation and polarization converters suitable for arbitrary vector beams are fabricated. This technology drastically enhances the capability of optical field control.

Liquid crystal photoalignment on As2S3 chalcogenide thin films

Recent studies of photoalignment of liquid crystals (LCs) on chalcogenide surfaces have a rich variety of mechanisms responsible for the photoalignment on these materials. Both chalcogenide surface-mediated and LC bulk-mediated photoalignment were observed. We report on investigation toward understanding the origin of the chalcogenide surface-mediated photoalignment. The contributions of light-induced optical and surface morphological anisotropy of the chalcogenide surface were studied. Light-induced optical anisotropy in the film was observed by polarization interferometry and the surface anisotropy was measured by high-resolution x-ray reflectivity. The data reveals the lack of a strong anisotropy in the surfaces’ morphology after irradiation with polarized blue light. At the same time, an evident correlation between the anchoring energy and the quality of the photoalignment was observed. This allows us to conclude that the photoalignment of LCs on chalcogenide surfaces is mainly determined by a light-induced anisotropic distribution of the glass structural elements in the bulk and on the chalcogenide surface.

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Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications.

Light-driven phenomena both in living systems and nonliving materials have enabled truly fascinating and incredible dynamic architectures with terrific forms and functions. Recently, liquid crystalline materials endowed with photoresponsive capability have emerged as enticing systems. In this Review, we focus on the developments of light-driven liquid crystalline materials containing photochromic components over the past decade. Design and synthesis of photochromic liquid crystals (LCs), photoinduced phase transitions in LC, and photoalignment and photoorientation of LCs have been covered. Photomodulation of pitch, polarization, lattice constant and handedness inversion of chiral LCs is discussed. Light-driven phenomena and properties of liquid crystalline polymers, elastomers, and networks have also been analyzed. The applications of photoinduced phase transitions, photoalignment, photomodulation of chiral LCs, and photomobile polymers have been highlighted wherever appropriate. The combination of photochromism, liquid crystallinity, and fabrication techniques has enabled some fascinating functional materials which can be driven by ultraviolet, visible, and infrared light irradiation. Nanoscale particles have been incorporated to widen and diversify the scope of the light-driven liquid crystalline materials. The developed materials possess huge potential for applications in optics, photonics, adaptive materials, nanotechnology, etc. The challenges and opportunities in this area are discussed at the end of the Review.

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.

Structure-performance relation of liquid crystal photoalignment with in-situ formation of protection layers

We demonstrate a stabilized liquid crystal photoalignment using a surface-localized polymer method. A protection layer is developed on a photoalignment layer (PAL) through phase separation of a mixture composed of reactive monomers (RMs) and liquid crystals (LCs). The RM is polymerized on the PAL which enhances its stability against heat and light with short wavelength. The effects of the concentration and molecular structure of RMs on the electro-optical response and surface anchoring of photoaligned LC device are studied. The concentration of RMs affects the effective cell gap of the LC device. The rigid core length of the RM structure modulates the surface anchoring strength of the alignment layer. Both effective cell gap and surface anchoring strength are key elements for the enhanced dynamic response of LC devices.

Meta-q-plate for complex beam shaping

Optical beam shaping plays a key role in optics and photonics. In this work, meta-q-plate featured by arbitrarily space-variant optical axes is proposed and demonstrated via liquid crystal photoalignment based on a polarization-sensitive alignment agent and a dynamic micro-lithography system. Meta-q-plates with multiple-, azimuthally/radially variant topological charges and initial azimuthal angles are fabricated. Accordingly, complex beams with elliptical, asymmetrical, multi-ringed and hurricane transverse profiles are generated, making the manipulation of optical vortex up to an unprecedented flexibility. The evolution, handedness and Michelson interferogram of the hurricane one are theoretically analysed and experimentally verified. The design facilitates the manipulation of polarization and spatial degrees of freedom of light in a point-to-point manner. The realization of meta-q-plate drastically enhances the capability of beam shaping and may pave a bright way towards optical manipulations, OAM based informatics, quantum optics and other fields.

Optically switchable and axially symmetric half-wave plate based on photoaligned liquid crystal films

We demonstrate an optically switchable half-wave plate (HWP) composed of a photoaligned and axially symmetric liquid crystal (ASLC) film containing two azobenzene derivatives, methyl red (MR) and 4-butyl-4′-methoxyazobenzene (BMAB). MR is responsible for photoalignment, and BMAB is used for optical tuning and switching the state of polarization (SOP) of probe beam (633 nm He–Ne laser) passing through the MR/BMAB doped ASLC film. The photoaligned ASLC film is first fabricated using a line-shaped laser beam (532 nm) exposure applied on a rotating LC sample. The fabricated ASLC film can passively change the linearly polarized light. Under UV light exposure, the formation of cis-BMAB (bend-like shape) within the film disrupts the LC molecules, switches the LC orientation, and further changes the SOP of the probe beam. Under laser irradiation (532 nm), the formation of trans-BMAB (rod-like shape) reverts the LC orientation back and simultaneously generates cis-MR, helping anchor the LC in the previously photoaligned orientation. The photoaligned MR/BMAB-doped LC HWP can change the linear SOP under alternating UV and visible light exposure.

Effect of surface condition on liquid crystal photoalignment by light-induced azo dye adsorption phenomena

ABSTRACTThe effect of surface condition on the photoalignment of light-induced methyl red (MR) adsorption phenomena on various substrates based on azo dye-doped liquid crystals is investigated. The results show that the polar surface energy determines the initiation of MR adsorption phenomena. The surface polarity energy of different substrates, which are modulated by oxygen plasma treatment (OPT), is further examined using the sessile drop method. The results also indicate that the uniformity and efficiency of MR adsorption can be enhanced by OPT. Light intensity influences the MR adsorption rate. The uniformity of MR adsorption onto various substrates is analysed via polarised optical microscopy.

Strengthening of liquid crystal photoalignment on azo dye films: passivation by reactive mesogens

Passivation of photoalignment films by continuous layers of reactive mesogen provides stable liquid crystal alignment and electro-optic performance equivalent to that of rubbed polyimide films.

Photoinduced anisotropy of the photo-alignment of liquid crystals on coumarin-containing copolymer film

In this paper, a copolymer containing photosensitive group was used as photo-alignment film through linearly polarised ultraviolet irradiation to a varying heating rate for an investigation of liquid crystal orientation. When the polymer film is exposed to linearly polarised ultraviolet, it can generate a polarisation-axis selective photoreaction. Then the thin film underwent thermally enhanced photoinduced molecular reorientation followed by subsequent annealing. Fixed exposure energy was 324 mJ cm−2 and annealing temperature was 98°C, heating rates varied from 5 to 30 S °C−1, a large anisotropy was generated. According to the method mentioned in the paper to calculate the enhanced ratio, the best heating rate is 20 S °C−1.

Anisotropic Shear Viscosity of Photoaligned Liquid Crystal Confined in Submicrometer-to-Nanometer-Scale Gap Widths Revealed with Simultaneously Measured Molecular Orientation.

In the context of the use of liquid crystals (LCs) as lubricants and lubricant additives, this study investigates the anisotropic shear viscosity of LCs confined in nanometer-sized gap widths subject to both shearing and photoalignment. The photoalignment is achieved using anisotropically dimerized polyvinyl cinnamate (PVCi) films coated on substrates. We simultaneously measure the viscosity and order parameter of a liquid crystal (4-cyano-4'-pentylbiphenyl) confined and sheared in the gap range of 500 nm down to a few nm. We achieve this simultaneous measurement using an original method that combines a highly sensitive viscosity measurement and a sensitive birefringence measurement. When the LC is sheared in the same direction as the photoalignment (parallel shearing), the order parameter, which is around 0.3 in the bulk state, increases up to around 0.4 at a gap width of less than 50 nm and the viscosity is smaller than half the bulk viscosity. We consider that this increase in the order parameter is due to the highly ordered photoaligned LC layer near the PVCi film, and the viscosity decrease is due to shear thinning of this layer enhanced by both confinement and molecular ordering. In addition, we observe a gradual decrease in viscosity starting at a gap of less than around 300 nm in the parallel shearing. Based on the apparent slip model, we show that the LC layer near the PVCi film can also cause this gradual viscosity decrease. In contrast, when the LC is sheared in the direction perpendicular to the photoalignment direction (perpendicular shearing), the viscosity increases as the gap decreases. We speculate that this is due to the rotational motion of the LC molecules caused by the competing effect of shear alignment and photoalignment. We believe our findings can significantly contribute to a better understanding of the confined LCs utilized for lubrication.

Effect of photoaligning azo dye structure on liquid crystal anchoring energy

A method of controlling anchoring energy of surface interaction of liquid crystal photoaligning substances is considered to enhance the liquid crystal display performances. An important parameter of the dye's molecular structure that determines the ratio of polar and azimuthal anchoring energy is the ability to form dimers. The values of dimerization thermodynamic potentials have been found. The probability of the formation of dye molecules dimers is evaluated. The bonds conjugation transfer via intermolecular hydrogen bond is revealed, and anisotropy of polarizability of the hydrogen bond is evaluated. The effect of dimerization on polar and azimuthal anchoring energy of liquid crystal — azo dye system — is shown.

Paper No S6.3: Photoaligned Diffractive Thin Film Polymer Polarizer to Double the Efficiency of Conventional Polarizers

AbstractIn this paper, we disclose a novel nano‐scale photoaligned liquid crystals polymer thin film polarizer that converts ordinary nonpolarized light to polarized light with efficiency close to 100%, which aims to double the polarization efficiency of conventional polarizers. Different from the mechanisms of conventional film polarizers, which lie in either absorption or reflection and the loss is normally more than 50%, the proposed thin film polarizer is a diffractive polarizer that makes use majority of the incident light and convert it into linear or circular polarized light with minimum loss.

Paper No S6.2: An Advanced Optical Thin Film Structure for Fabricating Multidomain Display

AbstractIn this paper, an advanced optical thin film structure, which can be used for fabricating multidomain display is introduced. It is composed of multilayer patterned birefringent thin films, of which each thin film is formed by a liquid crystal (LC) polymer layer with spatially varying optical axes controlled by a patterned photoalignment layer so that it can spatially modulate light propagation directions and polarizations. The overall thickness of the multilayer films, excluding the substrate, can be less than 1 µm, which offers the flexibility to generate high resolution light wavefront structure and the light diffraction within the film is also suppressed. As a result, the optical thin film can be ideally working as a photomask for exposure on a LC photoalignment layer to generate multidomain alignment structures in a single exposure, which can be applied for the manufacturing of LC multidomain displays. To verify the effectiveness of the proposed photomask, a periodic two‐domain thin film structure has been designed and fabricated and a liquid crystal cell with two‐domain alignment structure has been successfully fabricated in a single exposure with the photomask.

Multilayer photo-aligned thin-film structure for polarizing photonics.

In this Letter, an advanced multilayer photo-aligned liquid crystal polymer (LCP) thin-film structure with multiple optical functions is introduced. Within each LCP layer, a spatially distribution of local optical axes can be controlled by a patterned photo-alignment layer. As an embodiment of the proposed structure, a two-layer structure with pixelated controlled light-propagation directions and polarizations has been studied, which has shown the potential to be used as a photomask for generating multi-domain photo-alignment structures with a single exposure step. The combination of the multilayer structure with patterned photo-alignment technology provides a new perspective of designing optical structures for polarizing photonics applications.