Isothiocyanato tolane liquid crystals exhibit a high birefringence and relatively low viscosity. The phase transition temperatures, birefringence and visco-elastic coefficient of several high birefringence isothiocyanato tolane compounds are evaluated. Using these compounds, several eutectic mixtures were formulated. These new mixtures exhibit a wide nematic range, high birefringence (>0.4) and low viscosity.

The classic challenge faced by researchers dealing with liquid crystals is to control the LC molecular orientation and hence optimise the optical properties. Well known techniques for influencing LC texture include the use of surfactants or thin film alignment layers. The underlying limitation common to such techniques is that while excellent control of LC anchoring at the substrate surface is achieved, molecular alignment in the bulk of the LC is reliant entirely upon the cooperative effects and resulting elastic properties of the LC material. Generally, this has worked sufficiently well in practice, but unfortunately, the complete dependence on the intermolecular forces of the LC means that unencumbered, reversible switching is not always possible. Our group has taken a unique approach to influence LC orientation. Using glancing angle deposition (GLAD), highly porous thin films can be grown possessing isolated columnar microstructure whose shape can be tailored via substrate motion during film deposition. In particular, we can grow films of helical columns with controlled pitch and handedness. These films exhibit circular dichroism and optical activity similar to that seen in chiral LCs. The high porosity of GLAD films permits fluids such as LCs to be introduced into the pores, leading to a new type of hybrid optical material. Most significantly, initial work showed that when achiral LCs were embedded in chiral GLAD media, there was an enhancement of the circular dichroism and optical activity as the chiral GLAD film served to induce a chiral orientation in the LC. In this report, we start with a brief overview of the GLAD process and some relevant optical studies, leading to a review of GLAD/LC hybrid materials, switchable devices, and finally, a discussion of recent research optical characterisation and some ideas for future avenues of investigation.

A series of new disubstituted liquid crystalline polyacetylenes (LCPAs) with general molecular structures of -{(R)C=C[(CH₂)_m-Mes]}_n- and -[(C₆H₁₃)C=C(C₆H₄-Mes)]_n- (R = CH₃, C₆H₅, m = 3, 4, 9, Mes = mesogen) have been designed and synthesized. All the LCPAs are thermally stable and do not loss their weights when heated to a temperature as high as ~400°C. While a few polymers exhibit nematicity, most of them form enantiotropic S_A phase of monolayer structure. Upon photoexcitation, the polymers emit intense UV and blue lights with quantum yield up to 81%. Multilayer light-emitting diodes with a device configuration of ITO/PVK/PA/LiF/Al are constructed, which emits blue light with maximum luminance and external quantum efficiency of 119 cd/m² and 0.12%, respectively.

In a one-dimensional liquid crystal optical phased array (LCOPA), a liquid crystal layer is electrically addressed by an array of long, narrow electrodes. A spatially periodic voltage profile can be applied to the liquid crystals in order to induce a sawtooth-shaped index of refraction variation in the liquid crystal layer that will steer an optical beam in a fashion analogous to that of a blazed diffraction grating. In reality, an LCOPA is a thick, periodic, anisotropic structure with complicated polarization properties. The changes in polarization as a beam passes through an LCOPA can have negative practical effects, particularly in optical systems where LCOPAs and other polarization-sensitive optical elements are cascaded. This paper presents experimental measurements of the polarization state of the light diffracted by an LCOPA as well as a discussion of the origin of these effects.

Amorphous silicon/ferroelectric liquid crystal spatial light modulator technology is reviewed. The current performance of the CUED device is summarised, together with first results from two application demonstrators in 3D and high resolution display which have been recently constructed. The paper will conclude with guidelines for improved OASLM performance in dye-doped liquid crystal layers.

We investigate methods for actively tuning two-dimensional photonic crystal devices by modulating the index of refraction of the constituent materials. The index of refraction is modulated by infiltrating liquid crystals into a photonic crystal lattice of air cylinders in silicon. Moreover, the orientation of the liquid crystal molecules within the cylinders is actively modulated in order to induce a change in the dielectric tensor; thereby, tuning the optical properties of the photonic crystal. We validate and characterize the tunability of these devices both experimentally and with three-dimensional finite-difference time-domain method simulations. Furthermore, we integrate these tunable devices to demonstrate their enhancement of WDM photonic crystal applications.

The very high birefringence of liquid crystals makes them attractive for photorefractive applications. However, the drawbacks of using liquid crystals as photorefractives include a small phase shift between the optical and refractive index gratings, coarse grating spacings with narrow beam intersection angles, operation usually restricted to the Raman-Nath regime, a need to apply an external electric field, and, with most geometries, a need to tilt the cell at an angle to the grating k-vector. In this paper, we describe two-beam coupling with hybrid photorefractive cells comprising a nematic liquid crystal layer adjacent to inorganic photorefractive windows. In this arrangement, the underlying photorefractive properties are determined by the inorganic windows while the liquid crystal molecules amplify the overall refractive index modulation. Using this technique we have obtained Bragg matched liquid crystal gain coefficients of more than 1600 cm^-1, grating periods of less than 300 nm and a wide range of beam intersection angles without the need to apply an external field.

Novel non-symmetric dimeric liquid crystal, 1-(4-cyanobiphenyl-4'-yloxy)-11-[2-(4-octylphenyl)pyrimidine-5-yloxy]undecane (8YP11OCB), has been prepared and the physical properties investigated by means of optical microscopy, differential scanning calorimetry (DSC), X-ray diffraction and dielectric measurements. The crystal structure has been determined to understand microscopic behavior of the compound. Electro-optical properties for 8YP11OCB were compared to those for 1-(4-cyanobiphenyl-4'-yloxy)-11-[4-(5-octylpyrimidine-2-yl)phenyl-4"-oxy]undecane (8PY11OCB). 8YP11OCB with smaller dielectric anisotropy than 8PY11OCB was found to show a lower threshold voltage in the smectic A phase than 8PY11OCB. Crystal structure of 8YP11OCB indicates that three types of core-core interactions exist and that the enthalpy gained by the specific interactions stabilizes the molecular packing with large free volume in the SmA phase.

An optical measurement method to determine the cell thickness of reflective liquid crystal (LC) cells is prepared using a polarization-converting device such as a circularly-homogeneously aligned LC (CH-LC) cell and a charge couple device (CCD) camera. Since the minimum point of the spatial light intensity distribution from the reflective LC cell is measured in real time through the CH-LC cell, and then the cell thickness can be derived by the Jones matrix analysis. This method has an advantage that the measurement of the cell thickness is not affected by the reflection component from the surface and interface of glass substrates.

We present the status of our research in liquid crystal display (LCD) photo-aligning. We describe the photo-induced order in azo-dye layers and show that photo-aligning methods can provide a controllable pretilt angle and anchoring energy of the liquid crystal cell, as well as its high thermo and UV stability. The photo-aligning of ferroelectric and vertical aligned nematic (VAN) mode is also discussed.

A polymer wall confined transmissive switchable liquid crystal grating is proposed and investigated by two-dimensional finite-difference time-domain (FDTD) optical calculation and liquid crystal director calculation for the first time. The results show how to get optimized conditions for high diffraction efficiency by adjusting liquid crystal parameters, grating geometric structure and applied voltage. The light propagation direction and efficiency can be accurately calculated as well as visualized at the same time.

Various types of tunable lasing in dye-doped liquid crystals with one dimensional periodic structure have been demonstrated. An electrical tuning of lasing wavelength has been demonstrated in a dye-doped chiral smectic liquid crystal mixture with a short pitch helical structure which is so-called ferroelectric liquid crystal (FLC). Waveguide configuration of FLC laser has also been proposed, also in which the lasing wavelength widely can be tuned upon the electric field. The electrically tunable lasing has been observed also in a focal conic structure of dye-doped cholesteric liquid crystal. This laser action is based on a helix micro-cavity in focal conic domains. Optically pumped distributed feedback lasing has been proposed in a dye-doped nematic liquid crystal (NLC) waveguide by holographic excitation, in which continuous tuning of the lasing wavelength is performed upon applying electric field. Electrical tuning of the wavelength of the defect mode lasing in a one-dimensional periodic structure has been demonstrated using a dye-doped NLC as a defect layer in the periodic structure. Lasing wavelength is widely tuned upon applying the electric field, which is due to the refractive index change in the defect layer caused by the field-induced realignment of the NLC molecules.

We present data that indicates much more uniform reflection grating notches are formed when using thiol-ene photopolymerization as compared to the typical multifunctional acrylate photopolymerization. The scaling behavior of both diffraction efficiency and notch bandwidth versus thickness is presented for both acrylate and thiol-ene-based polymer hosts. The latter follows predicted behavior over a much larger range of thickness than the former. We attribute these differences to a delay in the gel-point with respect to the reaction start. Since the bulk of shrinkage occurs before gelation, detrimental effects caused by anisotropic stress build-up and subsequent relaxation is minimized. Using real-time monitoring techniques, we monitor the blue shift of the notch wavelength and the bandwidth development as a function of time for both chemistries. We demonstrate that thicker samples with appreciable diffraction efficiency and narrow bandwidths can be obtained.

We demonstrate multi-beam holographic lithography and temporal multiplexing techniques to create complex two- and three-dimensional structures in holographic polymer-dispersed liquid crystal (H-PDLC) materials. H-PDLCs are a variant of PDLCs formed under holographic conditions. The holographic image is typically a two-beam interference pattern, resulting in an array of liquid crystal (LC) droplets and solid polymer planes that act as a Bragg grating. Applying an external electric field can reversibly erase the resulting refractive index modulation. Using multi-beam holographic lithography, two-dimensional square lattices and three-dimensional FCC lattices have been created. The structures of the created lattices have been confirmed using a scanning electron microscope. We demonstrate the switchability of the lattices and tunability of the photonic bandgaps upon application of an electric field. Instead of simultaneously exposing a sample to multiple laser beams, we have developed a technique for creating complex structures by time-sequentially exposing the sample with multiple holographic configurations. By time-sequentially exposing the sample, two switchable reflection gratings are formed in a single film. The reflectance of the resulting gratings is well controlled by the individual exposure time. The optical performance can be modeled using a 2x2 matrix method based on the reaction diffusion equation.

We studied the formation of gratings by means of the photoinduced alignment change of polymer liquid crystals (PLCs) in a glassy state. To obtain high diffraction efficiency (η), we prepared a novel PLC containing an azobenzene group directly connected with a tolane moiety (3AT) which has large birefringence (Δn). It was found that the value of birefringence in 3AT was extremely high (Δn = 0.39). When two s-polarized writing beams (Ar⁺ laser, 488 nm) were interfered on the surface of the 3AT film in the glassy state, multiple diffraction beams were observed immediately, resulting from the grating formation. We explored the effect of light intensity on the grating formation and the relation between the alignment change and η. The value of η showed a maximum when the change in photoinduced alignment was almost completed. Furthermore the value of η was relatively high (η = 20%) even though the alignment change of mesogens was small.

We report theoretical and experimental studies of supra-photorefractive nematic liquid crystals doped with C60 and/or Carbon nanotubes. Theoretical estimate shows that the nonlinear refractive index change coefficient n₂ in such systems can be >> 1 cm²/Watt. Experimentally, we have observed n₂ of ~ 10 cm²/W, with typical nematic response times.

The hybridization of different types of materials frequently leads to specific performances and structure formation. Our current interest is focused onto dynamic performances photofunctional polymers assisted by hybridized liquid crystal molecules. In this paper, two current topics undergoing in our laboratory are presented. In the former part, mnolayer formation of hydrophobic polysilanes assisted by polar calamitic liquid crystal molecules is described. This is the first example of monolayer formation of a fully dydrophobic polymer on water. The latter part shows the marked acceleration of lateral mass migration promoted by photoirradiation in azobenzene-containing polymers. Both systems utilize common and general features of liquid crystals, namely, dynamic properties (fluidity) maintaining orientational order in the molecular assemblies. We show herein the high validity of liquid crystals for thin film processing of polymers.

The defined preparation of anisotropic films of functionalized polymers is important for different fields of optical applications. A promising way to create such films is based on the interaction of photochromic polymers with linearly polarized light. Different mechanisms are known to generate anisotropy. The best studied process is the photoorientation in the steady state of the E/Z photoisomerization of azobenzene moieties. But azobenzene based systems have some disadvantages. For this reason the light-induced orientation of photochromic liquid crystalline polymers without azobenzene moieties is presented. In the case of the studied photochromic LC polymers the light-induced anisotropy is used as a starting point for the thermal development of the order by a thermal post-orientation step based on the self-organization of LC polymers. This two-step orientation process combining a photoinduced and a subsequent thermal ordering process is a very promising way to orient efficiently LC polymers containing photochromic moieties. The light induced and the thermal developed order of four different polymers containing a mesogenic, a photochromic and a fluorescent stilbene side group were compared with respect to the composition of the polymers. This orientation procedure results in films with anisotropic fluorescence properties in the case of two of the investigated polymers.

Optical performance of the LCD is strongly affected by optical films such as polarizers and retardation films. The characteristics of polarizers, which determine the contrast ratio and the hue balance of the LCD, need to be improved, in particular, to meet the current requirements on high-performance LCDs. Most polarizers for LCDs are made from polyvinyl-alcohol (PVA) films and iodine compounds by the wet dyeing method. Polarizers with high polarization efficiency and transmittance can be obtained by optimizing the dyeing and stretching conditions. In transmissive LCDs, blue decolorization in black images need to be improved by controlling the chemical species of PVA-iodine complexes. The authors analyzed the dichroic ratios of chemical species by separating each absorption peak, and presented a guide for the neutralization of the hue balance of polarizers. The characteristics of reflective LCDs, on the other hand, are largely dependent on the optical design of circular polarizers and quarter-wave plates, which constitute the circular polarizers. The important design elements of reflective LCDs include wavelength dispersion and viewing angle characteristics. The authors have contributed to the improvement of the performance of reflective LCDs by enhancing the characteristics of polymer films using stretching and optical lamination technologies.

This article describes light-induced reorientation effects on a homeotropical dye-doped liquid crystal (DDLC) cell. The photo-excited azo dye, methyl red (MR), diffuses and adsorbs onto the substrate, thus forming a ripple structure. The adsorbed dye and laser-induced ripple structure then reorient the liquid crystal molecules and induce a holographic grating. Initially, the LC directors are reoriented primarily by the adsorbed dye. However, given sufficiently large ripple groove amplitude, the torque imposed by the ripple grooves overcomes that owing to the adsorbed dyes, and the LCs are realigned along the groove direction.

Both commercial and military applications (e.g., free-space IR communications and sensor protection) exist for guest-host liquid crystal (LC) devices operating in the near- to mid-IR region. Progress in this area has been hindered by the severe lack of near-IR dyes with good solubility in the LC host, low impact on the inherent order of the LC phase, good thermal and chemical stability, and a large absorbance maximum tunable by structural modification over a broad range of the near-IR region. Transition metal complexes based on nickel, palladium, or platinum dithiolene cores show substantial promise in meeting these requirements. These new dye complexes are extraordinarily stable, possess liquid crystalline phases in their own right with the proper terminal functional groups, and can have melting points below room temperature. The latter property is especially significant for producing liquid crystal/dye mixtures with both high dye concentration and good resistance to phase separation. Because they are zerovalent, they can exhibit high solubility in LC hosts (up to 10 wt%). The λ_max in these materials can range from 600 nm to 1600 nm, depending on structure. With enantiomerically enriched terminal substituents, nickel dithiolenes can induce a chiral mesophase in a nonchiral nematic host. This finding opens the possibility of generating novel LC mixtures with two degrees of tunability: an electronic absorbance band tunable by synthesis, and a selective reflection band tunable by temperature or applied electric field. Such a materials system would be particularly advantageous in sensor protection for dealing with frequency-agile laser threats.

Picosecond fluorescence anisotropy and lifetime measurements are used to investigate the orientational dynamics and steady state order of the fluorescent probe oxazine 4 in the nematic and isotropic phases of 5, 6 and 7 cyanobiphenyl. Variation of the excitation polarization angle β with respect to the nematic director allows the preparation of both cylindrically symmetric and asymmetrically aligned probe distributions whose relaxation dynamics are sensitive to both θ and φ motions yielding two characteristic relaxation times: τ₂₀ (pure θ-diffusion) and τ₂₂ (predominantly φ-diffusion). Analysis of the fluorescence intensity decays for excitation polarisation angles of β=0° and β=54.7° allows a determination of the effect of local field and differential reflection losses without the measurement of sample refractive indices. A striking feature of oxazine 4 dynamics in the approach to the nematic-isotropic phase transition temperature (T_NI) is that whilst θ diffusion shows a characteristic Arrhenius temperature dependence, the rate of diffusion in the φ coordinate is reduced as the system becomes less ordered. In the isotropic phase over a 50°C temperature range above T_NI the fluorescence anisotropy is characterised by two correlation times consistent with restricted rotational diffusion (intra-domain relaxation τ_f) within a slowly relaxing (pseudo-domain) structure (τ_s). The temperature dependence of τ_f and τ_s was in good agreement with recent theoretical models for intra- and inter-domain relaxation.

The study of thermally induced holographic gratings written in NLC by the laser beams at telecommunication wavelength of 1500 nm is presented. Thermally induced gratings are especially interesting as having at least one order of magnitude faster response time than orientational gratings. However, their formation in the range 1300-1600 nm is limited due to small absorption of NLCs and absence of effective doping. The formation of thermal grating in our cell is initiated by absorption in the cell walls coating. The tuning of liquid crystal temperature and orientational enhancement of thermal grating allowed us to reach 3% level of diffraction efficiency at low writing beams intensity (23 Wcm^-2) and small thickness of material (0.02 mm). Holographic liquid crystal devices can be used in all-optical switching, beam routing, automatic wave front correction, phase conjugation etc.

Porous thin film structures have been fabricated by physical vapor deposition at an incident flux angle that was typically greater than 80°. This deposition technique, often called glancing angle deposition (GLAD), was used to create thin films composed of isolated helical columns. Modification of the deposition parameters was used to control the porosity, the handedness, and the pitch of the helical structure. The high porosity of the GLAD film (>50%) permits fluids, and in particular liquid crystals (LC), to be incorporated into the voids of the nanostructure. We present the results of a study assessing the effect of film material, chiral morphology, and liquid crystalline material on the optical performance of helical GLAD films. Films fabricated from TiO₂, a high refractive index material, exhibited strong optical rotation of linearly polarized light and selective reflection of circularly polarized light. By increasing the number of turns of the helix the chiral optical response was enhanced, and by tailoring the pitch of the helical columns, the wavelength-dependence of the reflection band was tuned to preferentially reflect red, green, or blue light.

First, a new method of voltage application is proposed. With the new drive method, disclinalion lines do not appear in the cell. Secondly, a liquid crystal lens with focus moveable along and off the axis is reported. The movements of the focus are controlled by the potentials of electrodes and sub-electrodes in the cell.

Stokes parameter methods to determine cell parameters such as a pretilt angle, thickness and twist angle in reflective liquid crystal (LC) cells have been developed by measuring Stokes parameters of reflected light at plural wavelengths. The cell thickness can be determined by measuring one-wavelength Stokes parameters including a near-infrared wavelength, and both the cell thickness and twist angle can also be determined by calculating the Stokes parameter at two wavelengths. Furthermore, these cell parameters containing the pretilt angle as well as cell the cell thickness and twist angle can be determined by using Stokes parameters at three wavelengths. These methods can be applied to determine two-dimensional (2D) cell parameter distributions of the reflective LC cell by using a high resolution CCD camera.

A multilayer structure realizing an optical switch with ferroelectric liquid crystal, polymeric buffers and waveguides has been analyzed at the wavelength of 1550 nm, focusing on fabrication and design tolerances. The used liquid crystal FELIX-M4851-025 from Clariant is aligned by Nylon6 and embedded between two polymeric waveguides. ITO is deposited on quartz substrates to apply voltage to the cell and polymeric buffers are employed to reduce ITO absorption losses. The polymers consisted of poly(pentafluorostyrene-co-glycidyl methacrylate). Light is switched between the optical waveguides by reorienting the liquid crystal. Optimization in terms of optical losses and extinction ratios was carried out by varying layer thicknesses, refractive indices of waveguides and buffers, and the angle α between the normal to the smectic layers and the propagation direction. An optimized device with α = 51°, refractive indices of 1.475 and 1.462, thicknesses of 3 and 6 μm for waveguides and buffer, respectively and 4.4 μm for the liquid crystal layer, exhibits an extinction ratio of 59.6 dB with losses as low as 0.8 dB for a length of only 174 μm. A device design using single mode channel waveguides for optical switching matrices has been also carried out.

A method of fabrication of twisted nematic device is described utilizing the monomolecular chemisorbed layer on at least one internal transparent electrode of capillary cell. The layer is capable of aligning and re-aligning the liquid crystal molecules upon UV irradiation. The preliminary uniform LC orientation with high contrast of the order of 1000 occurs after filling the LC material into the cell, which is irradiated before filling with small UV dosage of polarized light of the order of 1 mJ/cm². The obtained orientation can be further re-aligned and desirable local twist configurations can be obtained using the second irradiation with UV light having polarization different from that of the first irradiation.

The giant optical non-linearity of liquid crystals was used to develop a device for measuring intensity of laser beams. The sensitivity of the device was substantially increased by substitution of one of glass substrates of the liquid crystal cell by a photosensitive semiconductor. It is shown that this is due to decreased anchoring energy of LC molecules on the semiconductor surface. The peculiarities of giant optical non-linearity in a liquid crystal cell with one semiconductor substrate are investigated. The discovered regularities allow supposing that the observed decrease in anchoring energy is caused by the semiconductor surface states recharging at light generation.

We report on a comprehensive study to measure the interactions between polymer and elongated liquid crystal droplets. These polymer dispersed liquid crystal (PDLC) materials are well suited for scattering polarizer applications. A tensile strain is applied to the PDLC, an optical polarizer element is created. Using solid-state deuterium nuclear magnetic resonance we report on the degree of orientational order at the liquid crystal-polymer interface as a function of strain, which is correlated to insitu tensile-optical measurements.