Effect In Cholesteric Liquid Crystals Research Articles

Localized Conical Edge Modes of Higher Orders in Photonic Liquid Crystals

Most studies of the localized edge (EM) and defect (DM) modes in cholesteric liquid crystals (CLC) are related to the localized modes in a collinear geometry, i.e., for the case of light propagation along the spiral axis. It is due to the fact that all photonic effects in CLC are most pronounced just for a collinear geometry, and also partially due to the fact that a simple exact analytic solution of the Maxwell equations is known for a collinear geometry, whereas for a non-collinear geometry, there is no exact analytic solution of the Maxwell equations and a theoretical description of the experimental data becomes more complicated. It is why in papers related to the localized modes in CLC for a non-collinear geometry and observing phenomena similar to the case of a collinear geometry, their interpretation is not so clear. Recently, an analytical theory of the conical modes (CEM) related to a first order of light diffraction was developed in the framework of the two-wave dynamic diffraction theory approximation ensuring the results accuracy of order of δ, the CLC dielectric anisotropy. The corresponding experimental results are reasonably well described by this theory, however, some numerical problems related to the CEM polarization properties remain. In the present paper, an analytical theory of a second order diffraction CEM is presented with results that are qualitatively similar to the results for a first order diffraction order CEM and have the accuracy of order of δ2, i.e., practically exact. In particular, second order diffraction CEM polarization properties are related to the linear σ and π polarizations. The known experimental results on the CEM are discussed and optimal conditions for the second order diffraction CEM observations are formulated.

PB27 コレステリック液晶におけるフレクソエレクトリック効果に及ぼす光学純度の影響(液晶化学材料,ポスター発表,2015年日本液晶学会討論会)

PB27 コレステリック液晶におけるフレクソエレクトリック効果に及ぼす光学純度の影響(液晶化学材料,ポスター発表,2015年日本液晶学会討論会)

Microscopic vs. macroscopic origin of the Lehmann effect in cholesteric liquid crystals

In a recent letter (EPL 97, 36006 (2012)), we have shown that the Leslie thermomechanical coupling cannot alone explain the Lehmann effect (namely the rotation of cholesteric droplets when they are subjected to a temperature gradient). This result was obtained by measuring in a compensated cholesteric mixture the "Lehmann coefficient" as a function of temperature both below and at the transition to the isotropic liquid. In this article, we detail these experiments and present new ones performed with other compensated mixtures and a diluted cholesteric mixture. The new results confirm the macroscopic origin of the Lehmann effect, in contrast to the Leslie thermomechanical effect that is clearly of microscopic origin.

Time Dispersion, Waves Irreversibility and Absorption Effects in Cholesteric Liquid Crystals

The interaction of light with Cholesteric Liquid Crystals (CLC) in the presence of time dispersion, magnetooptical activity and absorption is considered. The magnetooptical activity leads to the waves irreversibility, i.e. to the absence of center of symmetry for the surface of wave vectors and other surfaces, characterizing the direction dependence of optical properties (the nonequivalency of the forward and backward directions of propagation occurs). The waves irreversibility effects in CLC and their influence on the geometry of the Bragg reflection are considered.The time dispersion leads both to the splitting of region of diffraction reflection, and to the appearance of new region of diffraction reflection. Near the isotropic point it is possible to get a narrow region of diffraction reflection with tunable width and location.The results present also interest for the electrodynamics of media with waves irreversibility. They can be used no only in CLC optics but also in microwave electrodynamics of artificial ferromagnetic helical media.

Polarity sensitive bistable color effect in cholesteric liquid crystals with an asymmetric polymer network

A polarity sensitive bistable color effect has been observed in cholesteric liquid crystal—polymer mixtures where a gradient of the polymer network density is created by strongly absorbed UV light. The switching between two stable states is driven by electric pulses of the same amplitude and duration but with opposite electric polarity. The effect could be of great interest for practical applications in color display technology.

Electric-field controlled color effect in cholesteric liquid crystals and polymer-dispersed cholesteric liquid crystals

In cholesteric liquid crystals with positive dielectric anisotropy an electric-field induced texture with unusual properties was observed. This texture appears after the reduction of the electric field during the transition from the homeotropic nematic state into the planar cholesteric one. The angular and the electric-field dependence of the spectral characteristics indicate that this state possesses diffractive grating properties. A voltage controlled color shift and a laser beam deflection are possible. An electric-field controlled cholesteric diffractive grating texture was also observed in polymer-dispersed films. In this case the electric-field induced transition into the homeotropic state is not necessary.

Switching Characteristics of a Field-Induced Structure Near the Isotropic-Cholesteric Transition

Abstract The static and dynamic properties of the flexoelectric effect in cholesteric liquid crystals are presented. In a simple geometry, the application of an external electric field perpendicular to the cholesteric helix results in a linear electro-optic effect. The switching time for optical modulation is almost independent of the field strength. The temperature dependence of this optical switching follows the Arrhenius type behavior.

Electromechanical Effect in Cholesteric Liquid Crystals with Fixed Boundary Conditions

We describe a number of experiments to demonstrate the electromechanical effect in cholesteric liquid crystals. The Lehmann rotation phenomenon of drops with stress free boundary conditions is the most convincing effect produced by this coupling. In samples with fixed boundary conditions, the cross coupling produces a distortion of the profile of the azimuthal angles of the director. We have used a conoscopic technique to demonstrate this distortion in thick samples containing negative dielectric anisotropy materials. A strong linear electrooptic effect of electromechanical origin is produced in materials with a weak positive dielectric anisotropy. We provide an evidence that a twisted nematic cell without any chiral molecules also exhibits the electromechanical effect, due to the macroscopic chirality of the medium in such a cell.

Effect of flexoelectric coupling on helix distortions in cholesteric liquid crystals

The static and dynamic properties of the flexoelectric effect in cholesteric liquid crystals are presented. The application of an external electric field, below the critical strength for helix unwinding, perpendicular to the cholesteric helix, results in a linear electro-optic effect. The experimental results are in good agreement with theoretical predictions made under the assumption of the uniform distortion of the helix due to the flexoelectric coupling with the field. The temperature dependence of the dynamic response to the field is found to be the Arrhenius type. It is suggested that the surface boundary conditions play a role in the properties of the flexoelectric effect.

Fast linear electro-optic effect based on cholesteric liquid crystals

Flexoelectric effect in cholesteric liquid crystals can be used to produce optical switching on a microsecond time scale. In a simple geometry, this effect produces a light modulation the amplitude of which is linear in field. The switching time from one optical state to the other is predicted to be independent of the magnitude of the electric field. We confirm this prediction experimentally in a room-temperature cholesteric liquid crystal.

Recent developments in the physics of liquid crystals

Some of the more recent advances in the field of liquid crystals are summarized, with emphasis on the experimental developments rather than their theoretical implications. A major part of the discussion is devoted to newly discovered phases and phase transitions, particularly those that are of special interest to condensed matter physicists. Observations are also described of two new phenomena, which, although predicted by the continuum theory, have eluded experimentalists for many years: (a) the appearance of a regular network of singular points at the nematic-isotropic interface and (b) the Lehmann rotation effect in cholesteric liquid crystals under the action of a d.c. electric field.

New theoretical results for the Lehmann effect in cholesteric liquid crystals.

We point out for the first time that the classical Lehmann effect, which arises when a temperature gradient is applied parallel to the helical axis of a cholesteric droplet, is not only due to a dynamic cross-coupling between director orientation and temperature, but has also a static contribution. In addition it is demonstrated that the analogous effect in an external homogeneous electric field is purely dynamic, in an external concentration gradient it is a mixture of a static and a dynamic contribution, whereas in a density gradient there are only static cross-couplings involved. The latter possibility has apparently never been discussed before.

Thermomechanical Effect in Cholesteric Liquid Crystals

Abstract We discuss some possible experimental geometries for studying Leslie's thermomechanical effect in cholesteric liquid crystals. The discussion includes the effect of a temperature gradient (i) along and (ii) perpendicular to the helical axis. The feasibility of observing the effect and of extracting the thermomechanical coefficients is examined.

Thermomechanical Effects in Cholesteric Liquid Crystals

Thermomechanical effects in cholesteric liquid crystals are considered in the coarse-grained approximation of Leslie's theory. In certain situations one finds a close analogy with superfluid hydrodynamics. Effects of permeation have also been discussed.

Brillouin zone effect in cholesteric liquid crystals

Sound wave propagation is discussed in cholesterics using a simplified form of the Leslie theory. It is found that the damping constant depends on the angle between the propagation vector q and the helical axis, and its exhibits a band gap when the component of q along the helical axis equals the reciprocal lattice vector q 0= π P where P is the pitch.

Solute and Temperature Induced Pitch Changes and Pretransitional Effects in Cholesteric Liquid Crystals

The effect of non-chiralic solute molecules on the pitch of cholesteric mixtures of cholesteryl chloride and cholesteryl nonanoate has been studied as a function of the composition and the temperature. The occurrence of a pretransition to the smectic state at low CC content is established. In the pretransitional state the pitch decreases exponentially with increasing temperature and is extremely sensitive against the addition of most organic molecules. In the normal cholesteric state the pitch increases with increasing temperature. In this state only a very limited number of nonchiralic molecules exert an appreciable influence on the pitch.

Electrohydrodynamic Effects in Cholesteric Liquid Crystals under ac Electric Fields

When an electric field is applied perpendicularly to a cholesteric planar texture, periodic perturbations arise. The frequency dependence of the threshold of this instability is reported for nematic-cholesteric mixtures with both positive and negative dielectric anisotropy. In the latter case a "conduction" and a "dielectric" regime are observed, as in nematics. Our results are in agreement with a recent extension to ac excitations of Helfrich's theories for cholesterics.

Electrohydrodynamic and Dielectric Instabilities of Cholesteric Liquid Crystals

A model is proposed for electrohydrodynamic alignment effects in cholesteric liquid crystals. We derive and discuss a formula for the threshold voltage of such alignment, starting from the planar texture between two conducting plates. An improved formula for the threshold of the dielectric deformation of this orientation pattern is also given.

Some thermal effects in cholesteric liquid crystals

On the basis of conservation laws proposed by Ericksen (1961), constitutive equations are formulated for cholesteric liquid crystals. To motivate the choice, an entropy inequality of the type discussed by Müller (1967) is employed. Apart from minor differences, the hydrostatic theory of Ericksen (1962) is obtained, and proposals are made for the non-equilibrium terms. With this theory, the spinning phenomenon noted by Lehmann (1900) is investigated, and it is shown that solutions similar to his observations arise in a natual way.

Voltage Effects in Cholesteric Liquid Crystals

Abstract Liquid crystal mixtures were sandwiched between two transparent electrodes on glass and a direct or alternating potential was applied. The light scattering property of the liquid crystal changes upon application of voltage, the intensity varying from 1 to 10% of that scattered by a MgO diffuse surface. The maximum increase in intensity due to voltage was a factor of about six. The maximum wavelength shift was 40 nm to the blue by 80 V applied to a layer 10 μ thick. The response time varied from 0.03 to 0.5 sec.