Pitch Of Cholesteric Liquid Crystal Research Articles

General Purpose Soft Template for Photonic Applications: From All-Optical to Electrical Reconfigurability

In this paper we report on the realization and characterization of a polymer template sculptured in photosensitive material on a chemical inert surface, devoted to micro/nano-confinement of a wide range of organic components, with self-arrangement properties at the nanoscale. The high quality morphology of the polymeric micropattern arrays is obtained by combining a nano-precision level optical holographic setup and a multi-step chemico-physical process. The general purpose template represents the basic platform to be filled with different soft composite materials: Due to their self organization capabilities, light responsive Liquid Crystals (LC) and short pitch Cholesteric LC have been investigated.

Photo-responsive doped cholesteric liquid crystals

This review deals with recent developments in the design of switchable dopants capable of altering the pitch of cholesteric liquid crystals under the influence of light. Cholesteric liquid crystals possess many potentially useful properties owing to the helical organisation of their mesogens. For many of these applications, dynamic control over the cholesteric pitch can be achieved using chiral dopants capable of changing their shape in response to an external stimulus. The first attempts at developing such responsive systems stem from the 1970s, but major advances have been reported in recent years (2003–present), which is the subject of this review. Efficient dopants showing large changes in helical twisting power upon photo-switching have been developed, often capable of inverting the sign of the helical organisation of the surrounding liquid crystalline host. Overcrowded alkene-based molecular motors and binaphthylazobenzene-based switches have emerged as two classes of highly efficient dopants, enabling manipulation of cholesteric pitch over wide ranges using low concentrations of dopant.

Continuous Spatial Tuning of Laser Emissions in a Full Visible Spectral Range

In order to achieve a continuous tuning of laser emission, the authors designed and fabricated three types of cholesteric liquid crystal cells with pitch gradient, a wedge cell with positive slope, a wedge cell with negative slope, and a parallel cell. The length of the cholesteric liquid crystal pitch could be elongated up to 10 nm, allowing the lasing behavior of continuous or discontinuous spatial tuning determined by the boundary conditions of the cholesteric liquid crystal cell. In the wedge cell with positive slope, the authors demonstrated a continuous spatial laser tuning in the near full visible spectral range, with a tuning resolution less than 1 nm by pumping with only a single 355 nm laser beam. This continuous tuning behavior is due to the fact that the concentration of pitch gradient matches the fixed helical pitch determined by the cell thickness. This characteristic continuous spatial laser tuning could be confirmed again by pumping with a 532 nm laser beam, over 90 nm in the visible spectral range. The scheme of the spatial laser tuning in the wedge cell bearing a pitch gradient enabled a route to designing small-sized optical devices that allow for a wide tunability of single-mode laser emissions.

Periodic anchoring condition for alignment of a short pitch cholesteric liquid crystal in uniform lying helix texture

Due to flexoelectricity, a cholesteric liquid crystal unidirectionally aligned in a sandwich cell, with its helix lying parallel to the confining cell substrates, exhibit a flexoelectro-optic response when subjected to an electric field applied perpendicular to the helix axis. The clarity of the flexoelectro-optic response is dependent on the quality of the uniformly lying helix texture of the cholesteric which often is difficult to achieve. Herein, a method for alignment of a short pitch cholesteric in such a texture by means of a periodic surface anchoring condition with periodicity matching that of the cholesteric liquid crystal is described.

Bandwidth-controllable reflective cholesteric gels from photo- and thermally-induced processes

A bandwidth-controllable reflective gel has been investigated from photo- and thermally-induced processes. Due to the pitch of cholesteric liquid crystal (CLC) composite increasing as the temperature rises, the CLC with short pitch is frozen by UV-curing the polymer network at low temperature and the CLC with long pitch is fixed by heat curing the polymer network at high temperature. A non-uniform pitch distribution of CLCs forms in the gel when temperature becomes low. It is demonstrated that the memory effect of the polymer network is an important mechanism for the resulting gel.

Cholesteric Emulsions for Colored Displays

A new colored electro-optical film obtained by means of an oriented cholesteric liquid crystal (LC) confined in droplets dispersed in an organic-monomer matrix is presented. The cholesteric LC pitch can be adjusted using a photosensitive chiral material in order to produce the three primary reflected colors (see Figure). Morphology, response times (τON and τOFF), and reflection spectra of such films are reported.

Twist of cholesteric liquid crystal cells: Stability of helical structures and anchoring energy effects

We consider helical configurations of a cholesteric liquid crystal (CLC) sandwiched between two substrates with homogeneous director orientation favored at both confining plates. We study the CLC twist wave number q characterizing the helical structures in relation to the free twisting number q(0) which determines the equilibrium value of CLC pitch P(0) = 2 pi/ q(0) . We investigate the instability mechanism underlying transitions between helical structures with different spiral half-turn numbers. Stability analysis shows that for equal finite anchoring strengths this mechanism can be dominated by in-plane director fluctuations. In this case the metastable helical configurations are separated by the energy barriers and the transitions can be described as the director slippage through these barriers. We extend our analysis to the case of an asymmetric CLC cell in which the anchoring strengths at the two substrates are different. The asymmetry introduces two qualitatively different effects: (a) the intervals of twist wave numbers representing locally stable configurations with adjacent helix half-turn numbers are now separated by instability gaps; and (b) sufficiently large asymmetry, when the difference between azimuthal anchoring extrapolation lengths exceeds the thickness of the cell, will suppress the jumplike behavior of the twist wave number.

Widely tunable ultraviolet-visible liquid crystal laser

Quasicontinuous tuning of a dye doped cholesteric liquid crystal (CLC) mirrorless laser in the ultraviolet-visible wavelength range is demonstrated using a single original device based on CLC as resonator and several resonant dyes. The thought is to combine the CLC pitch gradient and the distribution of different dyes. In the same cell, six dyes are combined in order to nearly cover the whole wavelength range from ultraviolet (370 nm) to red (680 nm). Some of the used dyes work as emitter, while others work in the Forster regime to decouple the excitation and emission processes. The relevant aspect of the device is that a simple translation of the cell respect to the same pump beam enables fine tuning of the laser wavelength in almost all the visible range, up to the ultraviolet.

Discontinuous change in the helical pitch of cholesteric liquid crystals by photoisomerization of a chiral azobenzene molecule

This paper describes the discontinuous change in the helical pitch of a cholesteric liquid crystal (ChLC) by means of the photoisomerization of chiral azobenzene molecules under homogenous alignment conditions. A mixture of E44, R811 and Azo was prepared in the ratio 68/28/4, respectively. R811 and Azo have opposite twisting abilities such that they induce right- and left-handed helices, respectively when added to E44. The mixture was injected into a glass cell having a 2 or 5 µm cell gap, and treated for homogeneous molecular orientation. The wavelength of selective reflection from the ChLC was shifted to shorter wavelengths by the trans-cis photoisomerization of Azo. The change in the helical pitch was not only discontinuous, but also dependent on the cell thickness. The discontinuous change in the helical pitch was estimated to be almost the same as the half turn of the helical pitch in each cell gap, and was dependent on the number of helical half pitches in the glass cell. The homogeneous alignment condition affects the photochemical change in the helical structure of the ChLC system.

Influence of the Surfaces on the Nucleation Process During the Cholesteric-Nematic Transition

The nucleation phenomenon in short pitch cholesteric liquid crystals (LC), taking place during the relaxation from the electric field-induced cholesteric-nematic (CN) transition, has been investigated for various boundary conditions. The dynamic hysteresis properties have been studied by direct optical methods. For the first time it is shown the strong evidence that the nucleation is not exceptionally a bulk phenomenon and the supporting solid surfaces may act as nucleation sites. It is confirmed also that the nucleation velocity is not a function of the type of molecular alignment at the liquid crystal/solid interface rather than it depends on the uniformity of the surface interactions with liquid crystal molecules. This result is of importance for time scale optimization in modern fast driving schemes for cholesteric LCDs.

Molecular Mechanism of Anomalous Increase in the Helical Pitch of Cholesteric Liquid Crystals Induced by Achiral Dopants

A dopant-induced anomalous increase in the helical pitch of dicholesteryl 10,12-docosadiynedioate 1 is explained by generation of smectic clusters in the cholesteric phase where the dopants act as promoters of smectic domains. The dopants, having a mesogenic core and the proper length of alkyl chains on both their molecular sides, such as 4,4‘-dialkylazobenzenes, 4,4‘-dialkylazoxybenzenes and 4,4‘-dialkylbiphenyls, considerably increase the pitch. However, dopants having benzene or a binaphthyl structure in the molecular center, an alkyl chain at one side of the molecules and Z-isomers of azobenzene and azoxybenzene derivatives, slightly decrease the pitch by acting as impurities that decrease the transition temperature of the host. An X-ray diffraction study of the cholesterics revealed that the pitch increases with an increase in size and amount of the smectic domains induced by the appropriate dopants.

PD01 コレステリック液晶ネットワークの光スイッチング : 選択反射波長のデジタル的なシフト(フォトニクス・光デバイス)

PD01 コレステリック液晶ネットワークの光スイッチング : 選択反射波長のデジタル的なシフト(フォトニクス・光デバイス)

Optical properties of short pitch cholesteric liquid crystals

The optical properties of short pitch cholesteric liquid crystals are theoretically analysed in the framework of an effective medium theory. It is shown that such properties are well described by a very simple homogeneous model, defined by an effective permittivity tensor having uniaxial symmetry, that is expanded in a power series of the ratio p/λ0 between the helix pitch and the light wavelength. The linear term is identically zero. This fact gives short pitch cholesterics very unusual properties for their optical activity, which is related to the scaling terms as (p/λ0)m, with m odd and greater than 1. Some problems related to the presence of these terms and concerning boundary effects are discussed but not fully resolved. The limits of validity of the different approximations, obtained by considering only a limited number of terms, are also found.

Effects of doped dialkylazobenzenes on helical pitch of cholesteric liquid crystal with medium molecular weight: utilisation for full-colour image recording

The effects of small amounts of achiral photochromic dopants, 4,4′-di-n-alkylazobenzenes, with various lengths of substituted alkyl chain (CnH2n + 1 ∶ n = 4, 7, 11, 12, 13, 14, 15, 16) on the helical pitch of a cholesteric liquid crystal (CLC) with ca. 1000 molecular weight were investigated in order to utilise the cholesteric reflected colour of these combinations for a rewritable colour image recording. The (E)-azobenzenes (~2 wt%) in the host CLC induced pitch changes dependent on temperature, the concentration of the azobenzenes and the length of substituted alkyl chain on the azobenzenes. In the low region of the cholesteric temperature (below 100 °C), some (E)-azobenzenes remarkably lengthened the pitch. Contrary to this, upon irradiation, E–Z photoisomerization of all the azobenzenes in the CLC phase induced consecutive pitch shortenings in response to the formation of the Z isomers. The changed pitch (reflected colour) could be fixed in cholesteric glassy solids. In the glassy solid state, the photo and thermal isomerization of the azobenzenes could not influence the cholesteric pitch any longer, therefore, the colour was stably stored. However, since the molecular arrangement of the CLC and the conformation of the doped azobenzenes completely returned to the initial state upon re-heating to isotropic temperatures, the stored colour could be easily erased. Based on these fundamental investigations, it is suggested that the mixed systems of the medium-molecular-weight CLCs and the 4,4′-dialkylazobenzenes will be effective materials for a rewritable full-colour image recording in a photon mode.

Blue phases of cholesteric liquid crystals as thermotropic photonic crystals

The study of dye-doped low pitch cholesteric liquid crystals in their blue phases as an example of tunable "weak" photonic crystals is proposed and demonstrated. The presence of the blue phases in cholesterics can be tuned with temperature, and this allows for an easy in situ comparison of the emission and/or absorption of the dyes with or without an enwrapping lattice of disclination lines. The fluorescence emission of the dyes is shown to be affected by the presence of the blue phases. Although unlikely to be suitable for real applications (due to the natural low refractive index contrast), these systems may represent unique examples of tunable photonic crystals. It is proposed that single crystals of dye-doped blue phases should provide a very interesting testing ground for the study of optical emission anisotropies which can, on the other hand, be controlled by an external parameter.

Van der Waals interactions in cholesteric liquid crystals

Microscopic calculations of the pitch of cholesteric liquid crystals are based on a few types of interactions between molecules: $(1)$ short-range repulsive, $(2)$ direct Coulomb, and $(3)$ long-range van der Waals interactions. Recently, it was shown that first two types cannot be treated in the frame of mean-field approximation. Here we show that, contrary to common belief, an accurate evaluation of the intermolecular dispersion forces contributing to chiral ordering requires consideration of biaxial correlations between molecules which are neglected in the mean-field approximation. We found that in the presence of biaxial correlations chiral interactions depend very weakly on the anisotropy of the local (i.e., atomic) polarizability. Instead, the chiral interaction between two molecules is dominated by the character of biaxial correlations, the isotropic part of local polarizability of one molecule, and a chiral parameter of the other molecule, which characterizes the chiral molecular geometry and is similar to that found previously for steric interactions.

Reversible Control of the Pitch of Cholesteric Liquid Crystals by Photochromism of Chiral Fulgide Derivatives

Abstract When an enantiomer of resolved indolylfulgide (1) or either of the (R)-binaphthol-condensed indolylfulgides (2, 3) was mixed with a nematic liquid crystal (5CB), the cholesteric phase was induced. Their cholesteric pitches were reversibly changed by photoirradiation. Whereas the twisting ability of the cholesteric pitch (βM) was larger for the colorless form than for the colored form in the case of 1, it was larger for the colored form than for the colorless form in the cases of 2 and 3. In addition, βM of 1 is generally smaller than that of 2 and 3. These phenomena were explained in terms of the helical conformation of the hexatriene moiety of fulgides and the chirality of the binaphthol moiety. The cholesteric sense of a fulgide-doped liquid crystal was changed between plus and minus reversibly by the pre-doping of the chiral formaldehyde acetal of 1,1′-bi-2-naphthol ((S)-4).

Calorimetric Study of the Diastereomeric Interactions of Enantiomeric Iso-octyl-benzoyloxybenzoate Solutes with a Chiral Liquid Crystal

Precision differential scanning calorimetric (DSC) analysis of the effect of doping cholesteryl nonanoate with the enantiomers of 4-(4-n-hexyloxybenzoyloxy)-isooctyl benzoate (R811 and S811) has been performed. Depressions of the enthalpies of both phase transitions, from crystalline to cholesteric and from cholesteric to isotropic phases, were found to be ∼ 1.5 times greater for the samples of cholesteryl nonanoate doped with left-handed S811. Depressions of the temperatures of phase transitions are also more pronounced in the case of S811. Thus, the left-handed enantiomer S811 affects the molecular packing of cholesteryl nonanoate much more strongly than its right-handed counterpart R811. Remarkably, the difference between the enthalpy of melting of samples containing only 1 wt% of either enantiomer reaches ∼ 20% of the enthalpy of fusion of pure cholesteryl nonanoate, indicating strong diastereomeric discrimination in the system. This calorimetric manifestation of diastereomer discrimination of S811 and R811 in cholesteryl nonanoate is in agreement with earlier observed nonsymmetrical effects of these enantiomers on the pitch of cholesteric liquid crystals; that is the left-handed S811 enantiomer constricts the pitch nine times more strongly than the right-handed R811 expands the helix.

BISTABLE CHOLESTERIC REFLECTIVE DISPLAYS: Materials and Drive Schemes

▪ Abstract Cholesteric liquid crystals possess a helical structure and exhibit two stable states at zero field: the planar texture and the focal conic texture. In the planar texture, they reflect circularly polarized light, whereas in the focal conic texture, they scatter light in forward directions. They can be switched from the planar texture to the focal conic texture by a low-voltage pulse and switched from the focal conic texture to the planar texture by a high-voltage pulse. The wavelength of the reflected light is easily adjusted by varying the pitch of cholesteric liquid crystals. They can be used to make reflective displays that do not need a back light and have a good readability under room-light conditions. We first review the optical properties of bistable cholesteric reflective displays and discuss the techniques used to achieve high contrast and large viewing angle. We then discuss the transitions among the cholesteric textures and the drive schemes used to address bistable cholesteric reflective displays.

Experimental Study of the Growth of the Cholesteric Finger Pattern

The growth process of the cholesteric finger pattern, appearing in a large pitch cholesteric liquid crystal sandwiched between glass plates, has been experimentally investigated. Depending on the cell gap and the external electric field, several patterns of different kinds are observed. The characteristic feature of the patterns is extracted from the observed image and then the dynamical behavior of the patterns is discussed. Moreover, the formation process of the dendritic finger pattern is analyzed on the basis of the Avrami model.