Investigations on the polarization induced orientational order in three different azo-polymers are presented and discussed with respect to the formation of surface relief gratings induced by holographic exposure. For this purpose the birefringence induced for different polarization-states of the light, e.g. linear, elliptical or circular, has been studied. By placing the sample between crossed polarizers at various angles of incidence of the probe-laser, the symmetry and the induced amount of order is determined. The orientational order is characterized by the scalar Maier- Saupe order parameter S, known from nematic liquid crystals. By holographic exposure different kinds of polarization gratings were induced in these materials and investigated with polarization- and atomic force microscopy. The gratings are characterized by diffraction efficiencies and by depth and shape of the surface modulation. Large surface modulations (> 2.5 micrometers ) and diffraction efficiencies (> 90%) for a 5 micrometers thick film were obtained.

Photoconductive polymers are doped into liquid crystals to create a new mechanism for space-charge field formation in photorefractive liquid crystal composites. The composites contain poly(2,5-bis(2'-ethylhexyloxy)-1,4- phenylenevinylene) (BEH-PPV) and the electron acceptor N,N'- dioctyl-1,4:5,8-naphthalenediimide, NI. Using asymmetric energy transfer (beam coupling) measurements that are diagnostic for the photorefractive effect, the direction of beam coupling as a function of grating fringe spacing inverts at a spacing of 5.5 micrometers . We show that the inversion is due to a change in the dominant mechanism for space-charge field formation. At small fringe spacings, the space-charge field is formed by ion diffusion in which the photogenerated anion is the more mobile species. At larger fringe spacings, the polarity of the space charge field inverts due to dominance of a charge transport mechanism in which photogenerated holes are the most mobile spaces due to hole migration along the BEH-PPV chains coupled with interchain hole hopping. Control experiments are presented, which use composites that can access only one of the two charge transport mechanisms. The results show that charge migration over long distances leading to enhanced photorefractive effects can be obtained using conjugated polymers dissolved in liquid crystals.

Possible applications of the novel antiferroelectric/ferroelectric phases exhibited by achiral bent-core mesogens (termed banana phases, B phases, or bow- phases in the literature) are considered. Specifically, in a ferroelectric bow-phase, alignment such that the molecular bow plane of the bent mesogens is parallel to the substrates, anchoring the polarization parallel to the substrates, should afford an ideal geometry for analog `V- shaped switching' devices.

Preliminary attempts at aligning nematic liquid crystals using windows having sub-wavelength surface gratings show that good planar aligned cells can be fabricated. We also demonstrate all-optical nonlinear diffraction effect using methyl-red doped nematic liquid crystal that possess a large intensity refractive index change coefficient.

We investigate the effect of monomer blends on the performance of holographically-formed polymer dispersed liquid crystals (H-PDLCs). Difunctional, trifunctional, and hexafunctional urethane acrylate resins are mixed in different ratios to yield monomers characterized by an effective functionality. Reflection H-PDLCs are formed using the different blends. Reflection efficiency and voltage response are measured. Monomer blends prepared with trifunctional/hexafunctional constituents are formed to yield H-PDLCs with higher reflection efficiency than their difunctional/hexafunctional counterparts, but require higher voltages to switch. Scanning electron microscope investigations are also undertaken.

While the liquid crystal industry is primarily driven by the display industry, increasingly important applications in science and engineering have emerged such as beam steering, wavefront modulation and polarization switching and control. We will discuss some of the differences in construction techniques needed to produce a precision optical device rather than a flat panel display along with development work being carried out at Meadowlark Optics in some of the above areas. These include polarization switches capable of greater than 5000:1 contrast and high efficiency beam steering for precision interferometer gauges.

We present a review of the basic mechanisms for laser induced refractive index changes associated with director axis reorientation in nematic liquid crystals. In methyl-red doped nematic liquid crystal, an index change coefficient of more than 10 cm²/GW has been obtained, allowing us to perform image conversion and optical limiting processes at very low optical intensities. In particular, we demonstrated a very large dynamic range twisted nematic liquid crystal limiter that will protect sensor from cw or long-pulse laser jamming.

A large number (more than 20 different kinds) of new polyacetylenes with general molecular structures of -[HC equals C(C₆H₄-mesogen)]_p- [poly(arylacetylene) type] and -{HC equals C[(CH₂)_n-mesogen]}_p- [poly(alkylacetylene) type] are designed and synthesized. Pendant interaction and backbone rigidity in the polymers are tuned through systematic molecular engineering endeavor, and liquid crystalline polyacetylenes (LCPAs) with novel mesomorphic, optical, and electronic properties are successfully developed. The rigid polyacetylene backbones enable ready alignments of the LCPA molecules by simple mechanical perturbations. Upon photoexcitation, the LCPAs with the poly(alkylacetylene) skeleton structure emit strong blue light clearly observable by naked eyes under normal room illumination conditions. The shape and position of the emission peaks and the color of the emitted light can be manipulated by the application of external electrical fields. The LCPAs exhibit excellent intrinsic photoconductivity in the visible spectral region in the undoped (pure) states, and doping with electron acceptor/donor further increases the photoconduction efficiency of the LCPAs.

A graded index type lens can be attained by utilizing the liquid crystal molecular orientation effects in an axially symmetrical electric field which is produced by the circular hole-patterned electrode. The liquid crystal (LC) microlens has a variable focusing property and is easy to make large scale of lens array because of its simple structure. Excellent lens properties can be obtained by optimizing the electrode structure, driving voltage and etc., and the focusing spot size is as small as the diffraction limit. We can expect a new type of lens as an active device for light control by the LC microlens. However, time response of the LC microlens is usually very slow and it has a large aberration caused by the liquid crystal molecular orientation. In this paper, fabrication and fundamental properties of the LC microlens are briefly reviewed, and then the improvement of response and optical properties are discussed. The response and recovery time can be extremely reduced by introducing the polymer stabilization technique using UV curable LC materials. In addition, the aberration can be eliminated by introducing divided electrode structure. The new electrode structure can also provide free space focusing and deflection properties to the LC microlens.

The 2 X 2 Jones matrix and 4 X 4 Mueller matrix methods are used as computational tools for LCD quiescent state optical mode design. Matrices of a general twisted nematic (GTN) LC cell and some basic optical components are given. The transmission or reflection of a GTN cell can be calculated easily by using these matrices. A parameter space approach of designing LCDs is described, by which many GTN optical modes are revealed with information on their transmittance/reflectance, dispersion and also cell gap tolerance. The 4 X 4 Mueller matrix is introduced both as an alternative to the Jones matrix calculation and an efficient tool for investigating GTN cell polarization transformation. A polarization transformation switching scheme is proposed to explain all GTN display operations. New bistable twisted nematic modes are invented by considering horizontal switching of linear polarization preservation modes.

Photoresponsive liquid crystalline polymer networks with macroscopically uniaxial molecular orientation were prepared by polymerizing mixtures of liquid crystalline mono and di- acrylates, and an azobenzene compound in a homogeneous glass cell at a nematic phase. The polymer networks showed an enantiotropic phase transition from an anisotropic phase to an isotropic phase as well as high transparency. The photoisomerization of the azobenzene compound resulted in a change in a birefringence. The photochemical change in the birefringence was investigated by using a Xe lamp and a single pulse light from a Nd:YAG laser at light sources. We observed a response time in a range of a few microseconds and a decay time in a range of a few microhundreds of seconds. The optical switching behavior depended on the cross-linking density of the liquid crystalline polymer networks.

Near-edge x-ray fine structure investigations were performed on buffer polyimide alignment layers for liquid crystal (LC) displays as well as on thin n-octyl-cyanobiphenyl (8CB) LC films evaporated in-situ onto the alignment layers. The full tensorial liquid crystal order parameter can be derived from the x-ray dichroism as a function of the substrate conditions, the layer thickness, and the thermal history. We compare a data set taken on poly-pyromellitimido- oxydianiline polyimide to a data set where highly oriented pyrolithic graphite was used as a substrate. While on graphite the aromatic rings are aligned parallel to the substrate, this s to a much lesser extent the case on polyimide. On polyimide, the LC molecular main axes are parallel to the substrate but the aromatic rings are about as often facing the substrate with the ring plane, as they do with the edge. During a brief heat treatment, most of the LC-material evaporates. On graphite, the remaining, rather tightly physisorbed layer is very well ordered, whereas on polyimide substrates the remaining layer is less ordered than thicker films.

A guided light wave can be modulated by coupling it into a ferroelectric liquid crystal (FLC) waveguide. The FLC, which changes its optical properties under the application of an electric field, acts as active medium. With a carefully chosen aligned configuration, the large change in refractive index induced by the electric field, gives a good contrast ratio in the optical response. The optical and electro- optical properties of liquid crystals can be modified by the propagating light and these effects can be enhanced by the addition of a suitable absorbing dye in the liquid crystal. Thus changes in the guiding conditions of the light can be induced also by the guided light itself.

The second harmonic generation (SHG) of liquid crystalline main-chain type polymers were investigated. The sample polymer is a copolymer of 2-hydroxy-6-naphtoic acid with 4- hydroxy-benzoic acid, in the composition ratio (equals 30/70). The nonlinear optical coefficient d_exp of the samples prepared by mechanical extension at the liquid crystalline temperature is about 5 pm/V. Quasi-Phase Matching seems to be performed by piling the mechanically oriented thin films with alternatively directed machine direction. More than 8 times SH intensity value of that for 1 sheet of film was observed. For this trials, films being immersed in equal refractive index liquid crystals were used.

Spatial light modulators (SLMs) are the key components for real time electro-optical signal processing, computing, holographic optical memory storage, free space optical interconnect, information processing and data conversion systems. We have used our liquid crystal SLM technology to develop LC-SLMs for beam modulation in femto-second laser systems.

We have studied the three-component liquid crystal mixture reported by Inui et al. to exhibit `thresholdless antiferroelectricity', as well as some other materials with the same V-shaped (linear) electro-optic switching behavior. We find that thresholdless behavior is obtained in the absence of antiferroelectricity. The characteristic response of these materials is due to the field-induced switching of a twisted smectic C* structure stabilized by strong polar surface interactions. The twist is confined to thin surface regions by polarization charge effects leaving the bulk of the cell uniform, which gives almost complete extinction at zero field. Model calculations of the electro-optic response are in good agreement with the experimental data. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric but the cell is maintained in the ferroelectric state by hysteresis.

Two guest dyes are used to change dramatically the torque exerted by DC and optical frequency electric fields on ferroelectric and nematic liquid crystal hosts. The particularity of the photoisomerization mechanisms of these dyes is responsible for the unusual behavior of the guest- host compounds. Corresponding molecular model is provided by describe the photoexcitation of dyes and its influence on the host material.

The irradiation with linearly polarized light generates optical anisotropy in isotropic films of photochromic amorphous and liquid crystalline polymers due to photoorientation. The photochemically induced orientation of the photochromic side groups causes a co-operative orientation of the non-photochromic parts of the polymers such as the non-photochromic side groups. The orientational order photogenerated in the glassy state acts as an initializing force to align LC polymers by thermotropic self-organization on annealing above Tg. The combination of both ordering principles can result in a significant amplification of the light-induced in-plane anisotropy or, results alternatively in a homeotropic alignment of the side groups. The dependence of the photoorientation process and its thermal amplification on the liquid crystallinity, aggregation and the irradiation conditions is discussed.

A light-induced drop in the chiral reflection coefficient of a nonabsorbing chiral nematic (cholesteric) liquid-crystal mirror is observed experimentally in the region of selective reflection under the action of a high-repetition-rate, high- power laser beam, circularly polarized. The independence of the effect from the average power density (and its dependence only on the peak power density) allow it to be attributed to an increase in the pitch of the cholesteric helix to such a degree that it is completely unwound in the strong field of the light wave, an effect previously observed only in static and low-frequency electric and magnetic fields. The kinetics of this first experimental observation of a chiral helix unwinding by the optical field, is a step changing of the reflectivity of the cholesteric liquid-crystal mirror in the strong field of the light wave over the time under the action of high- repetition-rate, pulsed, laser radiation. On account of the specially chosen irradiation conditions, the changes produced in the pitch of the helix by the field of the light wave can be accumulated over time, so that an athermal mechanism can be invoked to explain the nonlinear reflectivity drop of a cholesteric mirror.

The multi-sensing system for detecting external electric fields and decomposition products of SF₆ is proposed by using a nematic liquid crystal (LC) cell sandwiched with two cholesteric LC cells and a corner cube prism coated with dye films. The electric field and the decomposition products can be measured as changes in optical retardations of the nematic LC cell at a selective reflection band of the cholesteric LC and as the absorption changes of the dye films at different wavelength regions outside the selective reflection band, respectively. Furthermore, we discuss the dependence of the sensitivity to the electric field on the molecular orientation and the anchoring strength of the alignment layer in the nematic LC cells.

The optical properties of the liquid crystal (LC) polarization-converting device, which can convert a uniformly-distributed polarization state to a radially- distributed one (a radially polarized light), are measured. The availability of such the unique device for optical wavelet transforms is discussed experimentally and theoretically. It is found from the observation of Fourier images that the radially polarized light can be regarded as a 1D Haar wavelet and a circular Haar wavelet in the case with and without an analyzer, respectively. Therefore, we can select two types of Haar wavelets by attaching or removing the analyzer. To discuss the edge extraction properties, the wavelet transforms are implemented by using computer simulations. It is confirmed that the radially polarized light produced by the LC polarization-converting device is useful for extracting edge features of 1D and 2D images in the case with and without the analyzer, respectively.

Design, construction and performance characteristics of the optically addressed transmission mode SLM based on the Chalcogenide Glass Photoconductor-Liquid Crystal structure is discussed in this paper. Utilization of an SLM of this type for adaptive signal processing is proposed. It allows one to significantly simplify the structure of a multi- channel adaptive RF signal processor.