Photoinduced two-dimensional (2-D) and three-dimensional (3-D) alignment behavior of polymer liquid crystals (LCs) with azobenzene moieties was investigated. Two-dimensional alignment of the polymer LCs was brought about on irradiation with linearly polarized light. The effects of various factors on the phenomena were discussed in detail, and experimental conditions and structural parameters of the polymer LCs were optimized. It was found that the response could be enhanced by two methods: chemically tailoring the structure of the polymer LCs and physically modifying the alignment procedure. Three- dimensional alignment of the azobenzene moieties was achieved on irradiation with unpolarized light. It was revealed that the azobenzene moieties were aligned along the propagation direction of the incident light. Photochemically inert mesogens underwent reorientation together with azobenzene moieties in the 2-D as well as 3-D alignment process.

Quasi-phasematched (QPM) Second Harmonic Generation (SHG) in poled polymer waveguides has been realized. Rib-waveguide devices were fabricated by lithographic techniques like multilayer spin coating and reactive ion etching, well known from semiconductor technology. Guest-host-systems [Cyanobiphenyl-Polymethylmethacrylat (CN-PMMA)] as well as co-polymers have been used as the NLO-waveguiding layer between Perfluorcyclobutane (PFCB) layers acting as lower refractive index claddings. Cut-off experiments yield waveguide losses of 3 dB/cm and endfire coupling losses of 3 - 5 dB. The QPM-period has been determined experimentally to (Lambda) approximately equals 20 micrometer for a 4 micrometer wide Rib- waveguide by recording Maker fringes. Periodically poled waveguides were realized using Corona poling in combination with corrugated surfaces. The period (Lambda) of the poling structure was varied continuously between 15 micrometer and 30 micrometer using a fan-shaped corrugation structure. SHG tuning curves exhibit a relative acceptance bandwidth of 3.7 times 10^-3 which indicates an effective interaction length l_eff over the full waveguide length of 4 mm.

Light-emitting diodes based on chiral substituted, liquid crystalline (pi) -conjugated poly(fluorene)s have been built up which emit bluish light with a significant circularly- polarized component, despite the sub-micrometer thickness of the active layer. After proper annealing absolute values of the dissymmetry factor exceed 0.15 in absorption, and up to 0.25 in emission are measured. Compared to the first demonstration of CPEL in a conjugated polymer, a 200-fold increase in the degree of circular polarization in emission is achieved. Since polyfluorenes have been proven to be efficient and stable emitters in polymer light-emitting diodes, this concept has the potential for the construction of large-area circularly-polarized light sources for use in LCD displays and all-plastic optical storage devices.

Development of advanced polymeric materials with both liquid crystallinity and light emissivity is of scientific interest and technological importance. In this study, we studied light emission from tetrahydrofuran solutions of a liquid crystalline polyacetylene, poly(11-{[(4'-heptoxy-4- biphenylyl)carbonyl]oxy}-1-undecyne), in the electrical field. The field exerts little effect on the photoluminescence of the polymer solution with a low concentration (0.10 mM). The photoluminescence of a concentrated solution (11.3 mM) is, however, noticeably quenched under an electrical field with a field strength of > 300 kV/m. When the field strength is increased to >= 367 kV/m, the bimodal emission spectrum of the solution changes to a monomodal one. Thus, both the emission intensity and spectral profile of the luminescence of the concentrated solution can be tuned by the electrical field, which is probably caused by the aggregate dissociation and mesogen realignment induced by the external stimulus.

Absorption spectrum, optical anisotropy, and photostability of liquid crystalline compounds are reviewed in connection with their molecular structures, providing explanation of superior optical properties of advanced materials. A novel class of substances is introduced to exhibit both high birefringence and good photostability. These features are indispensable to liquid crystalline materials for uses in modern displays.

A 3-terminal bistable twisted nematic liquid crystal display has been demonstrated. This display makes use of a combination of strong in-plane electric fields and vertical electric field for switching between the (phi) and the (phi) +(pi) twist states. The lifetimes of the two bistable twist states are infinite, which is a significant improvement over conventional bistable twisted nematic displays.

Holographic polymer-dispersed liquid crystal, a new class of composite electro-optic materials, has rapidly matured as a technology for electronically switchable Bragg gratings. Recent progress in H-PDLC materials science has improved basic understanding of the mechanisms of diffusion, nanodroplet formation, and the morphology underlying switchable holograms, leading to practical improvements in key parameters such as the switching ratio, speed and operating voltage and an ability to optimize formulations for varied device functions such a switchable lenses and switchable waveguide Bragg gratings. Combining lens, filter or holographic optical functions with switchability leads to reduced parts count and simplified designs for projection displays and compact wearable microdisplays. H-PDLC technology is now approaching the performance and stability requirements that will enable photonic applications such as low cost WDM switches for optical networks.

Photopolymerization of liquid-crystalline (LC) monomers produces polymer films with a spatial control over the molecular organization. A powerful tool for creating even more complex molecular architectures than by LC order alone is photo-induced diffusion during polymerization of these monomers. Photo-induced diffusion during polymerization of chiral-nematic monomers yields a cholesteric network in which the helical pitch gradually changes over the cross-section of the film. The polarization selective reflection band can thus be made much wider than those of single pitch materials and may expand the whole visible spectrum. Performing photo- induced diffusion on a length-scale of half the cholesteric pitch by using a liquid-crystalline photoinitiator yields a cholesteric network with a deformed helix. Helix deformation gives higher order reflections and a built-in optical retardation. When the deformed helix is combined with a pitch gradient over the film thickness, the built-in retardation can be used for wide-band cholesteric polarizers that directly generate linearly polarized light without an additional quarter-wave foil.

Three azobenzene compounds with chiral moieties were prepared. A cholesteric phase was induced by mixing each chiral azobenzene in a host nematic liquid crystal. The twisting power of the chiral azobenzene compounds was decreased by ultraviolet irradiation to cause trans - cis photoisomerization of the azobenzene compounds. In addition, a compensated nematic phase was induced by mixing of a chiral azobenzene and a non-photochromic chiral compound having opposite chiral ability to the chiral azobenzene in the host nematic liquid crystal when the twisting powers of both chiral compounds were balanced. The photoisomerization of the chiral azobenzene broke the balanced state in the twisting power, resulting in the transformation of the compensated nematic phase into a cholesteric phase. Effect of the trans-cis photoisomerization on the phase structure of liquid crystals containing the chiral compounds will be discussed.

Thermotropic nematic and chiral-nematic poly(p-phenylenes) were prepared into well aligned solid films. Dilute solution photoluminescence spectroscopy revealed that, with excitation at 350 nm, the conjugated polymer backbone was the predominant light emitter. Linearly polarized photoluminescence, with (lambda) _ex equals 350 nm, and FTIR linear dichroism, performed on a uniaxially aligned film, yielded the orientational order parameter, S, pertaining to the conjugated polymer backbone and nematic pendants, at 0.62 and 0.73, respectively. The supramolecular structure in a uniaxially aligned film was also elucidated by these measurements; the polymer backbone and the nematic pendants were found to be collinear and lie predominantly along the buffing direction, i.e. along the director of the film. For emission outside the selective reflection region, also referred to hereafter as the resonance region, S was estimated at 0.67 for the conjugated backbone within the framework of a recent theory. At a decreasing chiral mole fraction, films prepared with chiral- nematic copolymers showed a selective reflection region in the ultraviolet to visible and infrared region. Moreover, the chiral-nematic films were found to consist of a left-handed helical stack of quasinematic layers with (S)-(-)-1- phenylethanol as the chiral moiety. For emission within the resonance region, the highest degree of circular polarization ever reported for neat conjugated polymers was observed. The left-handed component of the emission was found to be strongly suppressed within the resonance region and enhanced at its edges in close agreement with the results obtained when a rod- like luminophore was doped into a chiral-nematic host. In sharp contrast, regardless of the polarization state of incident light, propagation alone through a chiral-nematic film did not result in handedness reversal of the transmitted light. Viewing angle measurements revealed that the right- handed component of the emission follows Lambert's cosine law closely regardless of emission wavelength. The left-handed component of the emission, on the other hand, was found to be strongly dependent on both emission wavelength and viewing angle. This dependence can be explained, at least in part, to the shift of the resonance region to shorter wavelengths with increasing viewing angle.

Glass-forming liquid crystals (GLC) are a new class of materials suitable for use in a wide variety of latching optical and photonic applications. Applications range from physically small devices for latching fiber optic devices, such as switches and attenuators, to physically large devices, such as corrective optics for deployable space-based optical systems. Previously, we demonstrated the ability to electronically change and then latch the birefringent characteristics of an optical device. Recent data indicates that not only does the chemical design of a GLC material impact the electro-optic properties of a latching device, but stereochemistry also plays a significant role. This paper presents static and dynamic optical data taken on a set of four similar GLC materials. Based on the results of this study, we have developed a qualitative understanding of the structure-property relationships, leading to GLC materials that are suitable for use in latching electro-optic devices.

Smectic-A liquid crystals exhibiting a large electroclinic effect are important for applications in view of their analog gray scale capability. In most of these materials, large electroclinic tilt angles are accompanied by buckling effects due to layer compression. This layer buckling is easily observed in an optical microscope as periodic stripes and drastically reduces the high contrast ratio necessary for optical devices. We have performed optical and x-ray scattering studies on a chiral, organosiloxane smectic-A liquid crystal. It is found that while the optical tilt angle exhibits a large dependence on the field, reaching values of about 31 degrees (for 5 V/micrometer applied field), the layer spacing shows only a very weak field-dependence, suggesting that the molecules have a nonzero tilt even with no applied field, and that the primary effect of the field is to induce long range order in the direction of the molecular tilt. This important result -- large field-induced optical tilt without a layer shrinkage -- has led to the development of materials with 256 gray -- large field-induced optical tilt without a layer shrinkage -- has led to the development of materials with 256 gray levels.

A nematic liquid crystal, 4-(trans-4- pentylcyclohexyl)benzonitrile, has been physically gelled by hydrogen-bonded network formation of low molecular weight additives, that is, three amide compounds. Electro-optic measurements in twisted nematic (TN) cells have been performed for the resultant gels that exhibit liquid-crystalline gel states at room temperature. Each of the anisotropic gels formed by the three gelling agents, exhibiting microphase- separated structures, shows different electro-optic responses. One of the gels responds to an electric field more than twice as fast as the single liquid crystal component. Network aggregates with different morphologies are observed for each of the gelling agents. More finely dispersed fibrous networks would contribute to the faster electro-optic response. Thermal transition behavior and composite structures of the anisotropic physical gels have been examined by polarizing optical microscope observation and differential scanning calorimetry.

Photorefractive polymer-dissolved liquid crystal composites (PDLCCs) were investigated for use in real time holography. Our photorefractive materials consist of a functionalized copolymer, low-molar-mass liquid crystal mixture (E7) and 2,4,7-trinitro-9-fruorenone (TNF) as a sensitizing dye. The copolymer consists of mesogenic 4-cyanobenzoate and N- carbazoyl side groups and made charge-transfer complexes by adding TNF molecules, which is favorable for photoconductive effects. Since the copolymer has mesogenic side groups, E7 can dissolve the copolymer without phase separation and the resulting PDLCCs show the mesophase. The holographic gratings were created by means of the frequency-doubled YAG laser (532 nm) and the diffracted intensity was monitored with the He-Ne laser (633 nm). The high diffraction efficiency of 39% was achieved with a grating constant of 2.6 micrometer and an applied dc field of 0.3 V/micrometer.

The occurrence of surface relief gratings in azobenzene polymers under illumination with two superimposed laser beams is a puzzling effect which was discovered only few years ago. The origin of the large mass transport which occurs far below the glass transition temperature is not yet understood. We have performed experiments with cw holography. Due to low laser fluence, thermal effects can mostly be neglected. We have studied the relation between the light induced orientation of the chromophores in the bulk of the material and the observed surface relief gratings. We have performed a number of studies: Surface relief gratings were characterized under different polarization geometries of the writing beams, furthermore the grating period was varied. We could show that there is a competing behavior between the diffraction efficiencies by the volume grating and the surface relief, respectively. In order to analyze the orientational distribution function of the azobenzene chromophores, we have performed confocal Raman microspectroscopy in different regions of the surface relief grating. Our results are not in agreement with the current theories for the formation of surface reliefs.

Porous, chiral thin films with controlled microstructure fabricated by glancing angle deposition (GLAD) exhibit unique optical properties. The observed optical activity and circular birefringence in these films have been compared to those of cholesteric (or chiral) liquid crystals. Porous GLAD films have been previously demonstrated as alignment 'backbone' structures for liquid crystals (LC) embedded in the pores of the films, leading to a new class of composite optical materials. GLAD films with chiral, or 'helical,' microstructure have been found to impose a chiral nematic-like molecular ordering in non-chiral nematic liquid crystals. The addition of nematic LCs to the films was found to enhance significantly the chiral optic response compared to that of the film alone. Recently, we demonstrated electro-optic switching of the LC component in optical devices based on GLAD-LC composites. In an unaddressed state, the GLAD film induces chiral nematic-like alignment in the embedded LC, with the GLAD film controlling pitch and handedness of the aligned LC. In an addressed state, the LC molecules align parallel to the field and by index matching, the chiral optic response of the device vanishes. In this work, we present extensive optical characterization of the GLAD-LC composite materials including measurements of optical rotation and circular dichroism, switching behavior, and analysis of structure- property relationships.

All-optical holography of azobenzene-liquid-crystal (ALC) doped nematic liquid crystals (NLC) is investigated by means of polarization and intensity grating methods. The holographic grating is formed upon a total optical power as low as 200 (mu) W/cm² without any external biases, and can be switched within hundreds of milliseconds. The polarization dependence of the probe beam on the diffraction intensity reveals that the grating formation is attributed to reorientation and order-parameter change of the NLC molecules induced by the photoisomerization of the ALC molecules.

We found that in nematic liquid crystals doped with methyl-red dye or an azobenzene liquid crystal, the orientational optical nonlinearity can be dramatically enhanced. A concise review of the fundamental mechanisms involved and some application in all-optical image processing, optical limiting and switchable storage lens holograms is presented.

Recently, fast electro-optical switching in planar nematic liquid crystals using Multipass and also Fabry-Perot modulators has been reported. By applying short driving- voltage pulses of 150 V microsecond switching times have been achieved with nematic planar cells (NPC) and even sub- microsecond switching in case of nematic Fabry-Perot modulators (NFPM). In the present report the performance of these modulators depending on the thickness, the temperature, the amplitude and the width of driving pulsed voltage is discussed with respect to realization of nematic Q-switch modulators for solid state lasers. A special ITO-electrode configuration has been developed to minimize the electrical time constant of the modulators. Results of using those fast nematic modulators as active Q-switches in flash lamp pumped Nd:YAG solid state lasers are shown. Three different resonator configurations have been designed. The realized Q-switch lasers emit pulses of 40 - 70 ns duration (FWHM) and energies between 10 - 25 mJ.

For more than two decades, liquid crystal (LC) technology has proved to be highly reliable in information display, military, and medical applications. The unique characteristics of liquid-crystal-based components make them ideally suited for application to the telecommunications industry where they are currently being developed for high-bit-rate optical switches and other novel products. Previous application of liquid crystal components tended to focus on the visible part of the optical spectrum. This paper focuses on the electro-optic properties and applicability of the LC twisted nematic mode for optical switching components in the near IR.

This paper presents the current status of alignment techniques for a new class of liquid crystalline material being developed for both passive optical filtering/polarizing and latching electro-optic applications. This new glassy liquid crystal (GLC) material has the unique property of being electro-optic and fully latching. That is, in one state, the material has the properties of a conventional nematic liquid crystal, capable of being aligned with either an electric or magnetic field; while in its other state, it is an optical quality solid that maintains the molecular alignment set while in the fluid state. Molecular alignment of nematic GLC films is a critical technology necessary to develop high-performance, novel latching devices. The alignment of the nematic pendant component of GLCs directly correlates to the optical contrast, switching speed (turn-on time), and decay speed (turn-off time) of an active latching device. There has been little prior research conducted to assess conventional LC alignment techniques for use with GLCs. The processing and effectiveness of multiple alignment techniques will be discussed.

The materials and process technology necessary to fabricate free-standing, circularly-polarizing thin films based on chiral polymer liquid crystalline materials has recently been demonstrated. Free-standing membranes with thicknesses on the order of 10 microns and diameters in excess of 7 cm have been fabricated. The spectrally selective films possess exceptional optical and mechanical properties, exhibiting polarization contrast in excess of 250 with out-of-band transmissions greater than 95%. The theory, materials, processing techniques and spectral performance of these filters are presented.

We have investigated the single components and binary component mixtures of the liquid crystal E7 in NOA65 and UV1 thiol-ene pre-polymers. E7 is composed of K15 (4-pentyl-4'- cyanobiphenyl), K21 (4-heptyl-4'-cyanobiphenyl), M24 (4- octyloxy-4'-cyanobiphenyl), and T15 (4-pentyl-4'- cyanoterphenyl). The single liquid crystal components and binary liquid crystal mixtures that were investigated include K15, K21, K15-K21, K15-M24, and K21-M24. The liquid crystal/pre-polymer phase diagrams were developed using thermally induced phase separation to determine the temperature of phase separation. Then, PDLC samples were prepared using photo-polymerization induced phase separation, and were polymerized at varying increments above the liquid crystal/pre-polymer phase separation temperature to determine how the morphology changes with the polymerization temperature, the liquid crystal component, and liquid crystal percentage. Polarized optical microscopy was used to determine the phase separation temperatures for the liquid crystal/pre- polymer samples and the nematic-to-isotropic transition temperatures for the PDLC samples. Laser light transmission measurements were performed to determine the electro-optic properties of the PDLC samples.