Photoalignment Of Liquid Crystals Research Articles

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.

Photoalignment of liquid crystals: basics and current trends

The review describes the status of the studies and the recent achievements in the field of photoalignment of liquid crystals. An update classification of photoaligning materials and exposure schemes, and analyzes of the relationship between the molecular structure of the materials and characteristics of LC alignment are provided. In addition, bulk mediated photoalignment and combination of photoalignment with other alignment methods are discussed. Along with traditional, recently proposed applications of the photoalignment technique are considered.

ChemInform Abstract: Photoalignment and Photoinduced Molecular Reorientation of Photosensitive Materials

AbstractReview: 110 refs.

Photoinduced phenomena in nano-dimensional glassy As_2S_3 films

Photoinduced anisotropy in nano-dimensional (ND) thin films (<50nm thick) of chalcogenide glasses is observed for the first time. Results of photoinduced dichroism and photodarkening effect in 10, 20, 50 and 100 nm glassy As2S3 films are presented and compared with photoinduced phenomena in thick films (>1µm). ND As2S3 films are shown to function also as efficient photoresists. Preliminary model based on two exponential rate processes involving defects generation and stabilization is proposed explaining the main features of the photoinduced phenomena. These observations widen the range of applications of chalcogenide glassy films such as for higher capacity of optical data storage and for the photoalignment of liquid crystals.

A novel liquid crystals photoalignment layer-by-layer self-assembled film fabricated with long side-chain cinnamate polyelectrolyte

A novel photoalignment film for liquid crystals (LC) was prepared based on layer-by-layer self-assembly of photosensitive long side-chain cinnamate polyelectrolyte. A series of self-assembled films with different methylene spacer groups was prepared and used as alignment film. The film became anisotropic, and could induce uniform alignment of LC after irradiation by linearly polarised ultraviolet light (LPUVL). The effects of spacer chain lengths of the cinnamoyl polycations on the structure and photoalignment properties of the self-assembled film were studied. The polycation films with longer spacer chain obtained a larger dichroic ratio after LPUVL irradiation. The contrast ratio (T max/T min) of the LC cell increased with spacer chain length increase. However, it was found that the thermal stability of PSS/PACPYn films decreased with increasing chain length of polycation.

Photoalignment and Photoinduced Molecular Reorientation of Photosensitive Materials

Abstract Photosensitive materials for photoalignment of liquid crystals (LCs) and photoinduced molecular reorientation are described, especially focusing on azobenzene-containing materials and photocrosslinkable polymers. All the materials generate photoinduced optical anisotropy by means of linearly polarized (LP) light exposure. The large optical anisotropy is induced when the axis-selective photoreaction is accompanied by molecular reorientation.

Photochemical Transformations in Bis-Methacrylic Polymers for Liquid Crystal Photoalignment: IR Spectroscopy Studies

Fourier transform infrared spectroscopy (FTIR) is applied to study photochemical transformations in poly(p-methacryloylaminophenyl methacrylate), a material with pronounced liquid crystal photoalignment capability. The characteristic bands corresponding to photosensitive groups of this polymer are identified by comparing its FTIR spectra with the spectra of model compounds having a reduced number of photochemical transformations. The changes in FTIR spectra of this material under ultraviolet illumination give evidence of two Fries rearrangements (in arylamide and arylester groups) and a conversion of C˭CH2 methacrylic bonds. This allows one to refine the microscopic model of photoinduced ordering in this class of polymers.

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.

Highly sensitive photoalignment of calamitic and discotic liquid crystals assisted by axis-selective triplet energy transfer

Highly sensitive photoalignment of liquid crystals (LCs) can be realized by axis-selective triplet energy transfer. Addition of a triplet photosensitizer (phosphorescent donor) into a photocrosslinkable polymer tethering E-cinnamate side chains ensures dramatic enhancement of photosensitivity to generate the optical anisotropy of polymer film and surface-assisted LC photoalignment. Photoirradiation of triplet photosensitizer-doped polymer films with linearly polarized 365 nm light for the selective excitation of triplet sensitizer gives rise to optical anisotropy of cinnamates as a result of axis-selective triplet energy transfer. By analyzing phosphorescence spectra with theoretical Perrin's formula, we find that triplet energy transfer is efficient within a radius of ∼0.3 nm from the triplet photosensitizer. Such photoaligned polymer films can be used for the surface-assisted orientation photocontrol of not only calamitic LC, but also discotic LC, even for extremely low exposure energies. The present procedure would be greatly advantageous for high-throughput fabrication of optical devices by photoalignment techniques.

Liquid crystals photoalignment by films of side-chain azobenzene-containing polymers with different molecular structure

Liquid crystals photoalignment on the films of several side-chain azobenzene-containing polymers with various molecular structures was studied. Kinetics of the alignment and realignment of the liquid crystals director under irradiation by the polarized light was investigated and a comparison of the rates of polymers’ azobenzene moieties reorientation and director realignment were performed. The degree of the orientation, stability of the alignment, the possibility of multiple reorientation and other parameters of the photoinduced processes occurring in polymers films and liquid crystal cells were studied in detail. A rubbing technique was applied as alternative method for LC-alignment using these azobenzene-containing polymers. It was shown that the studied azobenzene-containing polymers can be considered as a photocontrollable command surfaces for the application in display technology and optoelectronics.

Liquid Crystal Photoalignment: A New Challenge for Liquid Crystal Photonics

Photoalignment possesses obvious advantages in comparison with the usually "rubbing" treatment of the substrates of liquid crystal display (LCD) cells. The liquid crystal photoalignment is nano-technology, as the thickness of the alignment layer is about 2-15 nm. The photoalignment materials can be very useful for a new generation of liquid crystals displays and Photonics Devices. The advantages of the photoalignment technology and its application to Photonics LC devices will be reviewed here based on research in HKUST. Full Text: PDF

Liquid-crystal micropolarimeter array for full Stokes polarization imaging in visible spectrum

In this paper, we describe the design, modeling, fabrication, and optical characterization of the first micropolarimeter array enabling full Stokes polarization imaging in visible spectrum. The proposed micropolarimeter is fabricated by patterning a liquid-crystal (LC) layer on top of a visible-regime metal-wire-grid polarizer (MWGP) using ultraviolet sensitive sulfonic-dye-1 as the LC photoalignment material. This arrangement enables the formation of either micrometer-scale LC polarization rotators, neutral density filters or quarter wavelength retarders. These elements are in turn exploited to acquire all components of the Stokes vector, which describes all possible polarization states of light. Reported major principal transmittance of 75% and extinction ratio of 1100 demonstrate that the MWGP's superior optical characteristics are retained. The proposed liquidcrystal micropolarimeter array can be integrated on top of a complementary metal-oxide-semiconductor (CMOS) image sensor for real-time full Stokes polarization imaging.

15.1: Optically Rewritable LiquidCrystal Technology: A New Green EPaper Approach

AbstractOptical rewritable Epaper is a valuable contribution to the green technology, as Epaper replaces usual paper with the same quality of image. The light printable rewritable paper is based on liquid crystal photoalignment and is suitable for labels and plastic card displays, price labels, Ealbums, Eadvertisements.

Effects of UV Exposure Conditions on Photoalignment of Liquid Crystals

It has been shown that the photoinduced liquid crystal (LC) alignment is inferior to the rubbing-induced alignment, which requires the optimal UV exposure conditions for improvement of photoalignment. For a cinnamate-branched polymer, we have investigated the effects of various LPUV exposure parameters on the photoinduced LC alignment over it. The uniform photoalignment with good thermal stability has been induced by LPUV irradiation with much more (about 10 times) exposure energy than that generating the maximum anisotropy of the photoalignment layer, employing both a polarizer with a higher extinction ratio and an optical filter to cut off deep UV.

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.

Liquid crystal applications in photonics

We developed new optical switches based on nematic and ferroelectric liquid crystal (LC) cells for photonics applications. Certain new LC switches based on the effect of total internal reflection in nematic LC and deformed helix ferroelectric effect ferroelectric LC with very fast response time were developed. Fast bistable optical switches of the light polarization based on ferroelectric liquid crystal cells were proposed. The switches are characterized by 100 is switching time and 26 dB crosstalk at the wavelength of 632.8 nm and bistable, i.e., required zero power consumption in the switch state. High frequency hysteretic free electrically controlled 0–2π phase modulation of light has been proposed using a very short helix pitch (less than 400 nm) deformed helix ferroelectric liquid crystal. The electrically controlled 0–2π hysteretic free phase modulation was achieved at the driving voltage frequency up to 4 kHz and the voltage amplitude of 32 V. The application of fast V-shaped deformed helix ferroelectric ferroelectric LC (DHF-FLC) for new active-matrix liquid crystal display (LCD) and optical data processing devices is envisaged. Photoalignment technology can be very useful for the new generation of liquid crystal devices as well as in new photovoltaic, optoelectronic and photonic devices based on highly ordered thin organic layers. We have investigated the LC photoalignment in superthin tubes, which are basic elements of switchable photonic crystal/liquid crystal structures and obtained the order parameter comparable with usual homogeneous nematic LC cells. We studied LC alignment on silicon surfaces with submicrometer-sized straight and curved waveguide profiles. The liquid crystal cladding refractive index was then varied according to the applied voltage, and subsequently the microresonator resonance wavelengths were tuned. Based on our initial measurements, the free spectral range (FSR) wavelength shift within the range of 20 nm was obtained, which is comparable with a thermooptic effect. The new voltage controllable Si-based add drop filters are envisaged based on this principle.

Liquid crystal photoalignment by a polymaleimide having a photoreactive 2-styrylpyridine derivative as aN-substituent

A high molecular weight polymaleimide with 2-styrylpyridine unit as a photosensitive group and dodecylene group as a flexible spacer in the side group bonded to the nitrogen atom was synthesized. The polymer was soluble in organic solvents such as chloroform, dichloromethane, DMF, and THF, and its thin film was formed by solution casting. The polymer showed high thermal stability, high photosensitivity, and excellent optical transparency. The polar diagram of the LC molecules in the polymer film cell demonstrated parallel alignment to the polarization direction of LPUV light irradiation. The alignment direction was explained with the aromatic π-π interaction between the side group of the polymer and the LC. The LC in the film cell gave rise to the maximum order parameter of 0.40 at a small exposure dose of 0.05 J/cm2. Therefore, this polymer will be applicable to birefringent materials for LC display devices.

High-resolution photoaligned liquid-crystal micropolarizer array for polarization imaging in visible spectrum

We report a superior high-resolution micropolarizer array fabrication technology that combines a linear polarizer with a micropatterned liquid-crystal (LC) cell. A 2 microm pitch is achieved by using UV light to define the orientation of the micropolarizer elements. Reported experimental results validate the concept of high-performance photoaligned LC micropolarizer arrays with major principal transmittance of approximately 80% and extinction ratio as high as approximately 3200(35 dB).

Stabilization of liquid crystal photoaligning layers by reactive mesogens

Via passivation of liquid crystal (LC) photoaligning layers by thin layers of reactive mesogens, strong enhancement in LC alignment stability is achieved. Simultaneously, the passivation layers improve alignment uniformity and chemical stability of photoaligning layers and widely change pretilt angle of liquid crystal.

P‐134: New Mechanism of Photoalignment Based on Reversible Intermolecular Bonds

AbstractWe present new mechanism of liquid crystal photoalignment based on reversible intermolecular bonds (hydrogen bonds or metal‐oxygen bonds) between dye molecules. This mechanism differs from known before and possesses temporal stability of photoinduced alignment against thermal disordering at room temperature. Predicted photoalignment dye was synthesized and verified in experiment.