We describe the main features of current reflective and transflective liquid crystal displays (LCDs) for low-power mobile applications. The reflective LCD is relatively simple but not readable in dark environment, and the transflective LCD involves complicated fabrication processes. We demonstrate two types of transflective liquid crystal displays (LCDs) in a single gap geometry to overcome these problems. One type of our transflective LCD with a single gap has multimodes based on the homogeneous alignment and the hybrid alignment, with a low helical twisting power. This multimode transflective LCD was fabricated by a single-step exposure of the ultraviolet (UV) light through an array of metal reflectors used as an amplitude photomask which gives an alternating homogeneous and homeotropic LC configuration. The single-step UV exposure produces no variations of the cell gap. In such configuration, the electro-optical disparity between the transmissive region and the reflective region was found to be significantly reduced by the low helical twisting power of the chiral dopant. Another type having a single cell gap and a single LC mode was developed using a patterned retardation layer. In this transflective configuration, a single LC mode of the 60-degree twisted nematic LC was used for both transmissive and reflective applications. The patterned retardation layer was fabricated using a reactive mesogen by photo-patterning. The measured electro-optic characteristics of the single mode transflective LCD agree well with numerical simulations.

This work demonstrates the feasibility of using polymer-dispersed liquid crystal (PDLC) films as electrically switchable spatial filters in the optical signal process. The fabrication relies on the fact that the size of the LC droplet formed in a PDLC film is inversely proportional to the intensity of curing. Controlling the driving voltage on the PDLC sample can filter particular spatial frequencies in the Fourier optical signal process. A simulation is also performed, and the results are highly consistent with those of experiments.

We report on reorientation and two-beam coupling process in liquid crystal cells with different aligning polymer layers. Polymers such as poly(N-vinyl carbazole), pure and doped with fullerenes as well as standard polyimide layers were considered. Electric field distribution inside a liquid crystal cell was modelled and different penetration depths into the liquid crystal bulk were calculated depending on the modulation of surface electric field. The characteristics features of reorientation process were studied via measurements of capacitance with an increasing DC field. The evidence of strong screening was found in cells with photoconducting polymers, as well as in those with thicker polyimide layers. Surface screening layers could be discharged by illuminating a cell with light to induce selective reorientation of a liquid crystal director.

The photorefractive properties of SmC* phases are usually studied in samples which are not intrinsically photoconductive and in which light sensitization is achieved by doping. We propose two different approaches: the first one is the use of a liquid crystal which is intrinsically photoconducting and photorefractive without any doping. In this system a PR performance larger than what previously reported for doped systems was measured, as well as the amplitude of the photoinduced space-charge field. A further development of this research is the control of the polarization switching in a bistable surface-stabilized ferroelectric liquid crystal (SSFLC) device via a photorefractive (PR) mechanism. In this case, the space-charge field was generated on the surfaces of the SSFLC device, in two photoconducting layers coated on the sample substrates. The formation of a stable grating was observed.

Photoinduced reorientation of a copolymer liquid crystal (CPLC) and an alignment behavior of low-molecular-weight liquid crystals (LCs) on the resultant film are described. Adjusting the exposure energy of a linearly polarized ultraviolet (LPUV) light and the subsequent annealing control the reorientation direction of copolymer films both perpendicular and parallel to the polarization (E) of LPUV light, and the LCs align along the reorientation direction of the mesogenic groups of the CPLC film. In contrast, LCs align perpendicular to E of the LPUV light regardless of the exposure doses when using a copolymer film without annealing because non-reacted mesogenic groups control the alignment of the LC molecules. When the fabricated LC cell is annealed near the glass transition temperature of the CPLC film, the LC alignment behavior changes similar to that using the annealed film, where the reorientation of the mesogenic groups of the alignment layer in contact with the LC materials is generated. Since the LC alignment direction can be controlled both parallel and perpendicular to E, a new type of pure polarization grating with periodical LC alignment direction is demonstrated using a LC cell. The conversion of the polarization of diffracted light beams as well as a transmitted light beam is observed and the diffraction efficiency is 18%. These optical properties of the polarization grating exhibit a good agreement with the theoretical calculation.

This paper summarizes recent progress toward an emerging, unconventional application of thermotropic liquid crystals (LCs) - their use as solvents for controlling the assembly of non-LC nanoscale building blocks. LCs offer a number of potential advantages compared to conventional isotropic solvents, including the ability to influence building block orientation and other structural properties. Strategies are reviewed for the exploitation of LC media to engineer order in a range of systems, including oriented organic monolayers, chiral films, and nanometer-scale particles.

To create polarized organic light emitting diodes (OLEDs), liquid-crystalline polymers (LCPs) containing oxadiazole and amine moieties in the side chain were design and synthesized. These LCPs emitted intense blue fluorescence with high quantum yield over 0.6 in a solution. Furthermore, polarized emissions of fluorescence were observed in a LC phase, and order parameters estimated by the polarized emission were about 0.2. They have proven to function as polarized blue-emitting materials for OLEDs.

Multifunctional acrylate formulations containing nematic liquid crystals have been shown to form holographic polymer dispersed liquid crystal gratings (H-PDLCs) easily using ultra-violet AND/OR visible photoinitiators. Laser wavelengths of 364, 476, 488, 514, 532 and 647 nm have been used for the fabrication of the gratings. Recently, the use of a thiol-ene based monomer system has been shown to overcome some of the adverse effects like post polymerization, voltage creep, and non-uniform shrinkage incurred when using highly functional acrylate monomers. However, Bragg reflection gratings have only been demonstrated utilizing ultra-violet (UV) (363.8 nm Argon ion) photopolymerization. Using UV irradiation and single prism geometry limits the upper end of the reflection notch wavelength. In this work, we report on new visible photoinitiator systems developed for the formation of reflective H-PDLCs using thiol-ene monomers. Using these new photoinitiator systems, reflection notches have been routinely written from the visible to the near infrared (IR) regions. The visible photoinitiator systems included the photoinitiator and radical generator titanocene organo-metallic complex (commercially known as Irgacure 784 (Ciba-Geigy), Rhodamine 6G, Pyrromethene, and a radical generating organic peroxide as coinitiator. Reflection gratings were written using laser wavelengths 442, 488, and 532 nm with diffraction efficiencies (DEs) above 70%. Angle tuning allowed for gratings with reflection notches in the near IR (900-1500 nm) to be written using these initiator systems. Rhodamine 6G was found to be more efficient than the other two initiators. We discuss here this new chemistry, the morphology, and electro-optical properties of the reflection gratings.

Self-assembly of coil-SGLCP-coil block copolymers provides a route to model liquid crystalline (LC) gels in concentrations ranging from bulk polymer to gels with as little as 5 wt % polymer in a small-molecule LC host. Triblock copolymers with LC-phobic endblocks associate in a nematic liquid crystal to create crosslinks. An SGLCP midblock allows for solubilization of the network. These materials have a precisely tailored network structure ideal for comparison to theory¹ of liquid crystalline elastomers. In addition, the gel responds readily to external fields since the concentration of polymer can be relatively low. In this report we discuss rheological measurements that demonstrate gelation at low polymer concentrations. We show that the association of the PS blocks at low concentration is driven by the presence of the SGLCP, rather than incompatibility between PS and 5CB. We then discuss the alignment of the gels via shear, magnetic fields, electric fields, and alignment surfaces. Finally, we present results on a distinctive striped texture observed in aligned gels when subjected to an applied electric field or normal force. The exceptional electro-optic and mechano-optic responsiveness of these gels coupled with thermally reversible gelation suggests they would be ideal candidates for use in large-area printable display technology.

Novel organic materials are constructed by attaching monodisperse oligofluorenes to a hole- or an electron-conducting core through a flexible spacer. These materials exhibit desirable properties for use in polarized and unpolarized light-emitting diodes, such as the ability to form morphologically stable glassy liquid crystalline and amorphous films with elevated glass transition temperatures, capability for full-color emission, tunability of charge injection and transport, and ultimately achieving superior OLED device efficiency and lifetime.

A variable optical attenuator (VOA) at λ=1.55 μm using a sheared polymer network liquid crystal (SPNLC) is demonstrated. SPNLC is fabricated by mixing ~15 wt % photo-polymerizable monomer in a LC host. To polymerize the SPNLC cell, a two-step UV curing process was adopted. Before shearing, the cell scatters light strongly, but after shearing the cell becomes highly transparent in the near IR region. Using an E7-based SPNLC, the attenuation of our VOA is rather insensitive to wavelength over the ITU C-band. The rise time and decay time were measured to be 35 μs and 205 μs, respectively, at room temperature. Such a response time is at least one order magnitude faster than the state-of-the-art nematic competitors. Comparing with other polymer-stabilized liquid crystals, the SPNLC exhibits a lower driving voltage and negligible light scattering loss in the IR spectral region. A reflection type, polarization-independent VOA with ~240 μs response time and -32 dB dynamic range was demonstrated at room temperature and 35 V_rms voltage.

Recently discovered stressed liquid crystals (SLCs) are of a great interest because they provide largest phase retardation shift achievable within shortest time interval. This effect was accomplished by decoupling the speed of a liquid crystal layer from its thickness. SLCs easily switch 5 microns of the phase retardation at sub-millisecond speeds while 50 microns requires only several milliseconds. SLCs are therefore able to modulate the IR light with the frequencies higher than 10 kHz. The SLCs are polymer/liquid crystal composites; however, their electro-optic properties differ significantly from previously developed polymer dispersed liquid crystals and polymer network/stabilized liquid crystals. The applied stressed force aligns the domains, eliminating scattering and hysteresis at the same time. The phase shift is highly linear with the applied voltage, greatly simplifying the drive electronics. The SLCs pose intriguing basic scientific questions and may be used in a lot of new electro-optical applications (micro-displays, diffractive optical elements, beam steering devices).

A new phenomenon was found for the alignment change of hexagonal columnar (Col_h) mesophase of a typical discotic liquid crystal of triphenylene derivatives, which is caused by a polarizing infrared laser irradiation with a wavelength to excite the vibrational mode of the selected chemical bond in the mesogen. A series of experiments has revealed that the polarization direction of incident laser is correlated to the molecular alignment generated by the irradiation, where the molecules are reoriented in a way that the molecules no longer absorb the incidence. The resultant new domain has a certain level of uniformity for the alignment from an optical point of view and remains for several hours to minutes depending on the temperature against the isotoropization. The manual scanning of the beam successfully gives uniform domains with a shape and one can generate a character "H", for example, with lath-like shaped uniform domains in the mesophase film. It was found that such new domains could be freezed into the polymer film by photopolymerization when the mesogens have polymerizable groups in the molecular flamework. This strongly indicate that the combination of the infrared control of the molecular alignment of Col_h mesophase and the following photopolymerization could realize highly functional polymer films where the structured domains with a variety of shapes and alignments are assembled in the film. Recently, it was also demonstrated that a homeotropic domain could be generated using a circularly polarized incidence at 6.18 μm which corresponds to the wavelength to excite the aromatic C-C stretching band, being coincident with the relation of the alignment to the polarization direction of incidence.

Several types of liquid crystal (LC) lenses with variable focusing functions by applying an external electric field have been reported. We propose a new type of LC lens using the molecular orientation effects and resulting elastic force of LC director. The hole-patterned alignment area is treated for LC molecules to align parallel or perpendicular to the substrate coated with transparent electrodes, and outside area is treated to be an inverse alignment state. The surface of another substrate is coated with a homeotropic alignment layer. When the diameter of the hole pattern is nearly equal or less than the LC thickness, the distribution of the refractive index becomes bell-shaped and the LC cell behaves like an optical lens in the absence of the applied voltage. The focal length can also be varied by applying a voltage across the transparent electrodes.

Most liquid crystal display (LCD) devices use two ITO-glass substrates in order to confine the fluidic LC. To align the LC molecules, the inner surfaces of the substrates were coated with a thin polyimide (PI) layer. These PI layers are mechanically buffed in order to produce uniform molecular alignment. To reduce weight, the single-substrate approach has been explored recently in which the LC device consists of a substrate and a thin polymer film. The major technical challenge is how to align the LC molecules on the polymer film side. In this paper, we demonstrate a new single-substrate IPS-LCD. The LC cell consists of an anisotropic LC polymer film and an interdigitated ITO-glass substrate. The anisotropic film not only behaves as a substrate but also helps align the LC molecules. Compared to the LCD using two glass substrates, the new device has almost the same bright state and the same dark state. Our new device exhibits a higher contrast ratio (~514:1) because of good LC alignment. The driving voltage is low, and the response time is reasonably fast. The measured rise time is ~8ms and decay time is ~63 ms using a 12-μm cell gap and E7 LC mixture. This technology is particularly attractive for making single-substrate displays and also has potential for a double-layered guest-host display and a flexible display using IPS LCDs.

Liquid crystal (LC) devices including displays, beam-steering devices, electrically- and optically-controlled spatial light modulators, are widely used in a variety of applications. Some important operational properties of these devices, such as spatial resolution and diffraction efficiency, are severely limited by the influence of fringing electrical fields, generated between adjacent pixel electrodes. This work combines the results of three recent studies encompassing computer simulation, the development of an approximate analytical model and its experimental verification. The approximate analytical model ties the fringing-field-dependent broadening kernel, to the physical LC Cell properties. In particular, it is shown that, the broadening of the phase profile due to the fringing field is proportional to the LC cell thickness. These results are found to be in an excellent agreement both with high-precision computer simulations and experimental results. Finally, the phase broadening kernel is found to be independent of the particular shape of the phase profile, allowing the model to be used for other LC device architectures such as LCDs.

Molecular rotational dynamics is an old problem that continues to be relevant today. In the continuing search for new optical materials and phenomena, both from an application and scientific standpoint, a detailed understanding of molecular rotational dynamics is often of crucial importance. An example is the so-called Janossy effect, where it was found that the already large optical reorientation of liquid crystals (LC) could be greatly enhanced by doping a small amount of absorbing dye. We report here our recent effort studying the Janossy effect in isotropic-phase LC. The current explanation of the Janossy effect assumes a change in guest-host interaction upon photoexcitation of the guest molecules, which would manifest as a difference in rotational dynamics between solute ground and excited states. While there are many available experimental techniques to selectively probe the solute excited state, probing the solute ground state is often difficult, due to interferences from the excited solute and solvent molecules. We have developed an optical pump/probe technique that employs two successive pump pulses that are adjusted to allow selective measuring of the solute ground state rotational dynamics. Application of the technique to the dye-LC system that exhibits the Janossy effect shows a large difference between rotational diffusion rates of the ground and the excited state of the dye molecules. Combined with results from optical pump/probe of the LC host, information on the rotational dynamics of the dye yields a better understanding of the mechanism of the Janossy effect.

Until recently, it has not been possible to determine, with any real certainty, a complete picture of "chirality" (absolute configuration, optical rotation direction, and helical twisting power) for new chiral compounds without first synthesizing, purifying, characterizing, and testing every new material. Recent advances in computational chemistry now allow the prediction of certain key chiral molecular properties prior to synthesis, which opens the possibility of predetermining the "chiroptical" properties of new liquid crystal dopants and mixtures for advanced optical and photonics applications. A key element to this activity was the development of both the chirality index (G₀) by Osipov et al., and the scaled chirality index (G_0S) by Solymosi et al., that can be used as a "figure of merit" for molecular chirality. Promising correlations between G_0S and both circular dichroism (CD) and the helical twisting power (HTP) of a chiral dopant in a liquid crystal host have been shown by Neal et al., Osipov, and Kuball. Our work improves the predictive capabilities of G_0S by taking into account the actual mass of each atom in the molecule in the calculations; in previous studies the mass of each atom was assumed to be equal. This "weighted" scaled chirality index (G_0SW) was calculated and correlated to existing experimental HTP data for each member of a series of existing, well-known chiral compounds. The computed HTP using G_0SW for these model systems correlated to the experimental data with remarkable accuracy. Weighted, scaled chiral indices were also calculated for the first time for a series of novel chiral transition metal dithiolene dyes for near-IR liquid crystal device applications.

Spectral and polarization properties of microstructured photonic crystal fibers filled with nematic liquid crystals characterized by either extremely low (of the order ~ 0.05) or higher (of the order ~ 0.3) material birefringence have been investigated. The photonic crystal fiber used as a host material was manufactured in Lublin, Poland and the nematic liquid crystals were introduced into the micro holes of the photonic crystal fiber by the capillary effect. Due to anisotropic properties of the obtained microstructured photonic liquid-crystal fiber, switching between different guiding mechanisms as well as novel spectral and polarization phenomena have been observed.

The development and industrialization of some liquid crystal materials: the ester with strong positive dielectric anisotropy for TN LCD, the tolan with large birefringence, the alkenyl with large k33/k11 elastic ratio and the azine with large birefringence and high N-I transition temperature for STN LCD, and the fluorinated liquid crystals of fused ring systems for TFT LCD, are reviewed. The work on new fluorinated naphthalene with negative dielectric anisotropy for VA LCD is introduced.

We developed electrically-switchable two-dimensional diffractive gratings with periodic refractive index modulation arising from layers undulations in cholesteric liquid crystal. Two-dimensional layer undulations occur above the threshold voltage when a planar cholesteric cell of thickness much larger than the cholesteric pitch is subjected to an electric field. The periodic structure of the layers undulations and corresponding spatial modulation of an average refractive index in the plane of a cell allows us to produce diffraction patterns with a square-type arrangement of diffraction maxima. The cholesteric cell can be switched by pulses of ac voltage between two states: one with flat layers of a planar cholesteric texture and another with square lattice of periodic director modulation associated with layer undulations that produces two-dimensional diffraction patterns. The periodicity of the developed two-dimensional phase gratings and intensities of the diffraction maxima can be tuned by changing the applied field magnitude. The diffractive properties of gratings are practically independent of the polarization state of the incident beam and can be optimized for different wavelengths by appropriately choosing the cholesteric pitch, cell thickness, and surface treatment. The potential applications include beam steering devices, optical waveguides, devices for splitting monochromatic beams and beam multiplexing.

In this paper, we present electro-chromatic switching of a cholesteric gel, which exhibits a shift in the reflection band with the application of an electric field, but preserves the intensity of the reflected light. The reflecting color of the cholesteric gel was controlled by the strength of an electric field. When optimized to reflect visible light the reflecting color of the gel reversibly shifts to shorter wavelengths with increasing voltage. The reflection intensity remains the same over a range of voltage and begins to decrease with further application of the electric field, eventually leading to homeotropic alignment. We have systematically investigated the mechanism of the switching by studying the optical spectra and measuring the dielectric constant of a cholesteric gel as a function of voltage. Using a simple model that includes tilting and untwisting of a helical structure, we show that uniform electro-chromatic switching is mostly initiated by a helix tilting mechanism, whereas decrease in the intensity of reflection is mainly caused by helical untwisting.

We report a new class of colossal nonlinear effects observed at the nematic-isotropic temperature transition. Large diffraction efficiency (30%) and colossal nonlinear refractive index (n₂≈200 cm²/W) has been measured in randomly oriented thin (1 μm) films of 5CB liquid crystals doped with small quantities of the azo dye methyl red (1% wt). Additionally, the dye reduces the phase transition temperature of the liquid crystal by almost 4 degrees C. We also found that ITO-coated glass substrate gives larger diffraction efficiency than those where only glass was used.

The analog switching mode (sometimes referred to as V-shaped switching mode) of the ferroelectric liquid crystal cell is a recently developed type of liquid crystal cell in which the molecular director can be arbitrarily positioned with high speed on the surface of a cone depending on the steering voltage over the cell. This changes the orientation of the slow and fast axes as well as the amount of the birefringence. We show that theoretically, it is possible to use a V-shaped switched ferroelectric liquid crystal cell to achieve near lossless analog phase modulation between zero and π radians for a special ellipticity of the polarized input light. We also fabricated a cell which slightly deviates from the ideal (tilt cone half-angle 38° instead of 45°) for which near-lossless transmission was obtained, manifested as a < 4% modulation of the amplitude, and a continuous phase modulation between 0 and ~0.8π radians; the values agree very well with numerical simulations.

We consider a nematic liquid crystal (NLC) cell with planar structure of director sinusoidally modulated in transverse direction in the cell plane. If thickness of the cell is such that it locally acts as a half-wave plate, thin screen approximation predicts complete diffraction of normally incident light into +1st and −1st diffraction orders. We numerically study the case when this periodically aligned structure is made using anchoring at surfaces. Proper direction of anchoring can be achieved by illumination of photosensitive polymer coating by interference pattern of pair of circularly polarized waves. We found that the structure is unstable when its thickness exceeds a certain critical thickness. Calculations show that for a number of commonly used NLC's this critical thickness lies between 0.66 and 0.86 of the physical period. Such thickness allows for diffraction angles up to 21°. Friedericksz transition voltage of this structure depends on its thickness. This dependence can be well described by an ellipse in the voltage-thickness coordinates. Propagation of a plane wave through the periodically aligned NLC is described using coupled-mode approach. We estimate the contrast ratio versus the influence of walk-off effects and deviation of equilibrium structure from perfectly sinusoidal. Estimations for reasonable set of NLC's parameters show that contrast ratio can be 1000:1 and higher. As a result of this analysis, the transverse size of the cell can be estimated to be less or about 0.25 mm, which suggests it for use as a pixel in projection displays.

We studied the electro-optical property and photorefractive effects in a semiconductor CdSe nanorod doped neamtic liquid crystal [NLC] system. The nonlinear index coefficient is measured to be n₂=2.05×10⁻²cm²/W, which is 10 times larger than that of an equivalent pure liquid crystal system. Electro-optical switching investigation shows that the Freedericksz transition voltage of this system is also noticeably lower than that of un-doped NLC. These enhanced electro- and nonlinear optical properties are attributed to the photoconductivity of CdSe nanorods and the enlarged electric conductivity and dielectric anisotropies of the doped system. An AC field assisted photorefractive effect in CdSe nanorod doped liquid crystal system has also been studied.

We propose optical phase-control devices using a liquid crystal (LC) material without applying a voltage, and demonstrate a composite alignment of LC molecules on a substrate with a patterned photo-resist film, where LC molecules are partly aligned parallel and perpendicular directions divided into very small areas on the substrate. The patterned film such as randomly distributed circular areas of small diameter size is fabricated by using a photo-mask and a photolithography technique. The LC cells are prepared using the locally composite alignment substrate and a perpendicular alignment substrate. The optical phase of the transmission light through the different pattern density regions was measured. Then, it is found that the optical retardation of the higher pattern density region is larger than that of the lower one.

Stressed liquid crystals (SLCs) have been applied in fields such as optical phase array non-mechanical beam steering applications, adaptive optical tip-tilt correction, and fast displays because SLCs are capable of switching large phase shift in sub-millisecond time ranges. SLCs consist of liquid crystal micro-domains dispersed in a stressed polymer matrix. In this paper, we propose a model of close-packed, shaped liquid crystal droplets inside a sheared polymer matrix based upon the measurements of polarizing microscopy, fluorescence confocal microscopy, and visible-near-infrared spectroscopy. The light scattering of SLC films results mostly from the index mismatch between adjacent liquid crystal domains instead of the index mismatch between polymer matrices and liquid crystals as in traditional polymer dispersed liquid crystals. We show how the light scattering of SLC cells is greatly reduced upon shearing because the liquid crystal domains are aligned along the direction of shearing. The stretching of polymer matrices and the reshaping of liquid crystal domains upon shearing are confirmed by fluorescence confocal microscopy. The calculations of the electro-optic responses are based on the balance between the elastic torque and the electric field torque. Our experimental results support the calculations.