Holographic polymer-dispersed liquid crystals are materials used to make switchable holograms exhibiting electrically controllable diffraction efficiency. Such devices form the building blocks for several applications in displays and telecommunications. Several demanding requirements are placed on switchable holograms for these applications, such as high diffraction efficiency, wide on/off dynamic range, polarization sensitivity control, low switching voltage and power consumption, high speed, uniformity and reliability, low cost, and manufacturability. One phenomenon affecting overall performance is polymer shrinkage. We focus on the effects of this on polarization dependence, index modulation, and chirp. Experimental and theoretical results are presented.

A planar oriented cholesteric liquid crystal (CLC) may selectively reflect light. The reflection wavelength is related to the helix pitch and the wavelength bandwidth depends on the birefringence which is typically less than 0.3. In the visible spectrum, the bandwidth is often limited to 100 nm which is not suitable for specific purposes such as white-on-black polarizer-free reflective displays. Here we present two novel designs of CLC materials exhibiting light reflections on several hundreds of nanometers in the visible spectrum. The first type of broadband reflector is a glass non-polymeric CLC with a pitch gradient which developed during a thermal annealing. The second kind of approach leads to a Polymer-Stabilized CLC which is switchable when an electric field is applied. Here the material has been UV-cured under out-of-equilibrium conditions, i.e., when the CLC helicoidal pitch was varying during a thermal process. The elaboration processes are investigated in relationship with the optical properties and the microstructure as observed by transmission electron microscopy.

A random oriented azo-polymer stabilized nematic liquid crystal is synthesized and used to record anisotropic holograms. An azo-dye is chemically attached to a diacrylate monomer at a base of the polymer network that can be optically reoriented with a polarized laser beam with (lambda) equals 514.5 nm. Raman-Nath scalar and vectorial gratings are recorded in 5-micrometers thick electro-optic cell without alignment layer. The electro-optical switch of the diffraction is realized using a moderate dc electric field of 6 V/micrometers .

Searches for new materials for electro-optic devices has led to the creation of heterogeneous materials in which liquid crystals (LCs) are dispersed in rigid matrices (confined/dispersed LCs) or filled with Aerosil particles forming flexible network (Filled LCs). We present the results of investigations of the influence of the confinement, interface and heterogeneity on relaxations of collective and molecular origin on LCs by photon correlation spectroscopy (PCS) and broadband dielectric spectroscopy (DS). The experiments show significant changes in the physical properties of LC confined in random porous medium and filled with Aerosil particles. The spatial confinement and a highly developed solid material-LC interface in these materials have a strong influence on physical properties of LC that is resulted in: appearance of at least two new dielectrically active modes, absent in the bulk LC and existence of slow glass-like relaxation process detected in both PCS and DS experiments. The collective relaxation processes due to fluctuations of director reorientations in these materials shows glass-like behavior. It is of non-Debye type and the temperature dependencies of the relaxation times obey Vogel-Fulcher law. Confinement modifies even bulk-like relaxation process due to the rotation of molecules around short axes, investigated in BDS experiment.

Eight novel low molecular weight liquid crystalline (LC) vitrifying materials with photopolymerizable acrylate groups have been synthesized. The molecules have a star-shaped topology with three and four arms. The mesogenic units were varied by the addition of lateral groups in different positions. In the case of the three-armed star-molecules crystallization from the melt was strongly suppressed. One of the three-armed stars (Triple-BTB) exhibits a nematic phase at room temperature and does not recrystallize for at least nine months. The photopolymerization behavior was studied over the whole liquid crystalline temperature range, and the novel nematic star-molecules were used as components for colored cholesteric polymer networks. Additionally, we describe the formation of holographic gratings in photopolymerizable cholesteric mixtures by using polarization holography, where we used two writing beams with perpendicular polarization instead of the usual setup with two writing beams with parallel polarization.

We report on recent experiments performed on Polymer Dispersed Liquid Crystals (PDLCs) and dye doped nematic liquid crystals (DDLCs), showing that these materials are potentially applicable in the development of novel optical devices free from the limitations typical of the currently used liquid crystal spatial light modulators. Nano-sized PDLCs, standard PDLCs with well patterned liquid crystal droplets distribution and dye doped liquid crystals with huge nonlinear response will be taken into account and shown to be among the most promising class of materials for developing high quality and low cost spatial light modulators.

We present a first investigation of liquid crystals in the frequency range around 1 THz using THz time-domain spectroscopy. As a model system we use 4-(trans-4'- pentylcyclohexyl)-benzonitril (PCH-5). The THz refractive indices for the nematic state are comparable to that for visible frequencies. Yet, the THz birefringence of PCH-5 is smaller than its optical birefringence. Furthermore we find that PCH-5 has a rather small absorption.

We have developed a ferroelectric liquid crystal device with a novel structure containing a polymer fiber network for flexible lightweight displays using thin plastic substrates. The aligned polymer fibers of sub-micrometers -diameter were formed under ultraviolet light irradiation in a heated nematic- phase solution consisting of liquid crystal and monofunctional acrylate monomer. The rigid polymer network was found to adhere to the two plastic substrates, and the uniform liquid crystal alignment provided a contrast ratio of 100:1 for a monomer concentration of 20 wt%. This device achieves a continuous grayscale capability as a result of change in the spatial distribution of small liquid crystal domains, and also exhibits a fast response time of 80 microsecond(s) due to high-purity separation of polymer and liquid crystal materials. It therefore has attractive features for flexible moving-image display applications.

Relatively large electro-optic effects of nematic liquid crystal (LC) in the millimeter wavelength region have the possibility of an application to the electrically controlled millimeter-wave devices. The millimeter-wave transmission properties of the conventional nematic LC cells with a grating-patterned electrode structure are described. Novel LC cells with a stack-layered structure using metal substrates, which have a large effective path length, are prepared and large millimeter-wave transmission modulation of 50% can be attained by applying an external electric field to the LC cell. The stack-layered structure is useful for an application to the millimeter-wave devices. Then, the LC prism and LC lens as the quasi-optical millimeter-wave devices using the stack-layered structure are proposed. Deflection and focusing effects of the millimeter-wave are observed by applying an external electric field to the LC cells.

We have studied the optical nonlinearities of aligned nematic liquid crystalline films in the near IR communication spectral region (1.55 micrometers ). The measured refractive index coefficients are on the order of 10^-3 cm²/W. The origins of the refractive index changes are thermal indexing effect and director axis reorientation. Phase modulation of several (pi) s can be generated with mW-power near IR lasers in micron thick films.

The thermal and physical properties of several new classes of fluorinated liquid crystals of fused ring systems are discussed. The fluorinated phenyl-decahydronaphthalenes have low birefringence and relatively high clearing points. The naphthalenes with four fluoro-groups, including a fluoro- substituent at C-1 position for the naphthalene ring, have large dielectric anisotropy, large birefringence and low visco-elastic ratio which leads to quick response time in a twisted nematic liquid crystal display (TN-LCD). The pentafluorinated tetrahydronaphthalene has large dielectric anisotropy with relatively low birefringence and exhibits good stability of nematic phase at low temperature. We have designed some liquid crystal mixtures for an active matrix LCD with advanced performance. The liquid crystal mixtures with low birefringence, sufficiently high voltage holding ratio and wide operating temperature range, which are suitable for a reflective liquid crystal display, have been achieved. The liquid crystal mixtures, which exhibit about 10 ms of quick response time for a TN-LCD and are suitable for TV, also have been achieved.

A general equivalence theorem for all unitary optical system is presented. In particular the case of the twisted nematic liquid crystal (LC) cell is shown to have very interesting equivalence properties. Specific formulas for the equivalence are derived. Applications of this optical equivalence to LCD measurements and modeling are discussed.

New optical birefringent film for liquid crystal display (LCD) is described using composite materials consisting of photo-cross-linkable polymer liquid crystal (PLC) and photoreactive bifunctional monomers. The alignment of mesogenic groups in both PLC and monomers was achieved by irradiation with linearly polarized ultraviolet (LPUV) light and subsequent annealing. Axis-selective photo-cross-linking reaction of the mesogenic group in the PLC occurred by exposure with LPUV light, although the induced birefringence was very small. The annealing process generates a thermal reorientation of monomers in a direction parallel to the photo-cross-linked mesogenic groups, resulting in a reversion of the orientation direction and thermal enhancement of the alignment of the film. Since the orientation direction is parallel to the polarization direction of LPUV light, three-dimensional orientation can be feasible by oblique irradiation with LPUV light, and the angle of the molecular orientation is controlled by changing the exposure angle. By the use of this optical birefringent film, the improvement of the viewing characteristics of twisted-nematic (TN) LCD is demonstrated.

The photo-aligning materials based on azodye layers are proposed. The azodye aligning layers enable (i) high order parameter; (ii) excellent alignment quality of LCD with a high contrast ratio; (iii) temperature stability, suitable for LCD manufacturing; (iv) perfect adhesion and high voltage holding ratio due to the specific molecular groups (v) pretilt angle generation. The azodye layers can be used to fabricate thin internal patterned (pixelated) polarizers with different local orientations of the absorption axis and/or absorption colors. Our new methods allow to produce defect-free highly uniform alignment of lyotropic LC or iodine-doped azodye layers themselves with a fine resolution of the polarization pattern. The photo-aligned internal polarizers are cost-effective and enable new LCDs with excellent electro-optical response, including good viewing angles and high brightness. We prepared an internal phase retarder using UV-cured photo-polymerized material. 4-(6- acryloyloxyhexyloxy) benzoic acid had been synthesized and the synthesis procedure was modified for a better yield. We had shown that by applying an electric or magnetic field, the director deformation of the liquid crystalline monomer could be in-situ UV-cured for the optimal phase compensation generation.

Mesomorphic and luminescent poly(propiolates) with different skeleton structures (-{(R)CequalsC[CO₂(CH₂)₆OCO-Biph- OC₇H₁₅]}_n-; R equals H (1), CH₃ (2), C₆H₅ (3), Biph equals biphenylyl} are synthesized. The backbone absorption in 2 and 3 are weak but upon photoexcitation, the polymers emit strong violet light of 369 nm, whose intensities are higher than that from poly(l-phenyl-l-octyne), a well-known highly-fluorescent polyacetylene. The main chain of 3 absorbs strongly at 380 nm, and the polymer is completely nonluminescent. All the polymers are thermally stable and form enantiotropic monolayer SmA phase, with 1 adopting well-ordered packing arrangements.

Reliable performance of liquid crystal (LC) materials in display applications is critically dependent on the behavior of surface treatments that induce molecular alignment parallel to the cell windows. While general understanding of the nature of this effect has been available for some time, the detailed characteristics of the LC-surface interaction are not sufficiently well understood to enable reliable interface engineering. This is particularly true for new materials that might otherwise possess favorable qualities but cannot be reproducibly oriented in a device. Our work is concentrated on understanding optical and mechanical modification of organic monolayers that have been self- assembled on dielectric surfaces. We use vibrationally resonant sum-frequency generation and ellipsometry as well as non-optical techniques, such as contact angle testing, to evaluate the influence of these perturbations. We have been able to follow subtle conformation changes as well as more dramatic photochemical effects. The results help reveal the anchoring mechanisms and provide insight into ways of tailoring interfaces for new materials.

We report optical and photoresponsive behavior of nonlinear liquid crystals in two-dimensional (2D) periodic structure. 2D structure made of photoresist and titania is constructed by interference photolithography using grating mask. Then azobenzene-doped nematic liquid crystal is infiltrated into these arrays, and photoresponsive behavior of the azobenzene-doped liquid crystal in the periodic structure is investigated. In particular, we show that the diffraction from these liquid crystal infiltrated grating structures can be optically modulated by an Ar⁺ laser at 488 nm.

Optical properties in the liquid crystal (LC) microlenses are studied on molecular orientations with large and axially symmetrical electric field. The LC microlenses with a thick LC layer are also investigated by experiments and simulations of 3 dimensional finite difference method (3D-FDM). The LC microlens has a converging property with low applied voltage as well as a diverging property with high applied voltage. The dependence of converging property on D/t is investigated in the LC microlenses, where D/t is the ratio between a hole-pattern diameter D and LC thickness t, and the simulation by the 3D-FDM in terms of the molecular orientation state is successfully carried out. It is found that the lateral distribution of the LC molecular orientation in the thickness direction is not uniform and changes depending on the D/t ratio, and good converging properties can be obtained when the D/t value is around 2.

In this paper we present analytical and numerical calculations of electric field profile for interdigital electrode configuration and corresponding threshold voltage. Our results for threshold voltage for nematic director reorientation take account of non-homogeneous profile of the electric field in the IPS mode. We found there exists optimal ratio between cell thickness, electrode width and electrode gap which corresponds to minimal value of the threshold voltage. Our results on operating voltage dependence on the cell thickness and electrode gap are in good agreement with the experimental data.

Photochemically induced nematic (N)-isotropic (I) phase transition behavior and domain formation of the azobenzene/liquid crystal (LC) mixtures in the presence of non-mesogenic dopants were evaluated. When a bent-shaped non-mesogenic molecule was added to the azobenzene LCs, formation of I domains was formed effectively on photoirradiation to cause the trans-cis isomerization of the azobenzene. The domain formation results from the aggregation of cis isomers, which could be enhanced to more extent with the aid of the bent-shaped dopant with a common structural feature to the cis-azobenzene. When the formation of the I domain proceeds quickly upon photoirradiation, the photochemical phase transition is induced more effectively.

The simulation of polymer-dispersed cholesteric liquid crystals is carried out using a model, which takes into account the domain structure of the layer. The model is based on the 4x4 matrix calculation method of Berreman. One considers different orientations of the helical axis in multi- and single-domain configurations. The distribution of the helical axis orientations in the multi-domain model allows the description of optical properties of multi-stable director configurations.

This paper investigates the effects of cure temperature and composition on the morphology of polymer dispersed liquid crystals (PDLCs) composed of the liquid crystal, K21 (4-heptyl-4'-cyanobiphenyl), in a thiol-ene based pre-polymer. PDLCs are composites composed of micron-size liquid crystal droplets dispersed in a solid polymer matrix. The PDLCs were fabricated by photo-initiated, polymerization induced phase separation using a differential photo-calorimeter (DPC). The cure temperatures were chosen based on the temperature-composition phase diagrams of the liquid crystal/pre-polymer mixtures. The characterization of the PDLCs included differential scanning calorimeter (DSC) to determine the nematic-to-isotropic transition temperature (T_NI) of the liquid crystal and the glass transition temperature (Tg) of the polymer matrix. Scanning electron microscopy (ESEM) was used to examine phase separated morphology.