Chiral smectic-C Liquid Crystals Research Articles

Optical diffraction in chiral smectic-C liquid crystals

Optical diffraction in chiral smectic-C liquid crystals

Dispersion relation for electromagnetic propagation in stochastic dielectric and magnetic helical photonic crystals.

We theoretically study the dispersion relation for axially propagating electromagnetic waves throughout a one-dimensional helical structure whose pitch and dielectric and magnetic properties are spatial random functions with specific statistical characteristics. In the system of coordinates rotating with the helix, by using a matrix formalism, we write the set of differential equations that governs the expected value of the electromagnetic field amplitudes and we obtain the corresponding dispersion relation. We show that the dispersion relation depends strongly on the noise intensity introduced in the system and the autocorrelation length. When the autocorrelation length increases at fixed fluctuation and when the fluctuation augments at fixed autocorrelation length, the band gap widens and the attenuation coefficient of electromagnetic waves propagating in the random medium gets larger. By virtue of the degeneracy in the imaginary part of the eigenvalues associated with the propagating modes, the random medium acts as a filter for circularly polarized electromagnetic waves, in which only the propagating backward circularly polarized wave can propagate with no attenuation. Our results are valid for any kind of dielectric and magnetic structures which possess a helical-like symmetry such as cholesteric and chiral smectic-C liquid crystals, structurally chiral materials, and stressed cholesteric elastomers.

Configuration of a chiral smectic-C film with a circular inclusion

It was shown experimentally (P.V. Dolganov et al., Europhys. Lett. 76, 250 (2006)) and by numerical calculations (C. Bohley, R. Stannarius, Eur. Phys. J. E 23, 25 (2007)) that the c -director profile of a two-dimensional chiral smectic-C (SmC) film around a circular inclusion adopts dipolar rather than quadrupolar configuration observed in achiral SmC films. We give an analytical argument on how spontaneous bend inherent in chiral SmC liquid crystals influences the configuration of a SmC liquid crystal film around a circular inclusion imposing tangential anchoring. We find how the angle alpha between two surface defects seen from the center of the inclusion depends on the radius of the inclusion R and the strength of the spontaneous bend q . We show, however, that the contribution of the spontaneous bend to the free energy suffers from mathematical ambiguity; it depends on the mathematical treatment of the outer boundary even when it is at infinity. This might indicate that the shape as well as the treatment of the outer boundary of the film can significantly influence the equilibrium configuration of the c -director and the position of the surface defects.

Kinetics of the photoferroelectric effect in chiral smectic-C liquid crystals studied by time-resolved measurements of spontaneous electric polarization and director tilt angle.

The origin of the photoferroelectric effect in liquid crystals, where the spontaneous polarization of a chiral ferroelectric smectic-C* (SmC*) host phase is changed by the E,Z-photoisomerization of azobenzene dopant molecules, was investigated by kinetic studies on the molecular isomerization and the subsequent changes in the SmC* order parameters, the director tilt angle, and the spontaneous electric polarization. The photoresponse of a liquid-crystal mixture consisting of 5 mol% 4,4'-bis-[(2-methyl)butyloxy]azobenzene dissolved in the SmC* host phase FLC 6430 was studied at low UV-light intensities (lambda = 366 nm, 15 microW cm-2) using an electrooptical technique that measured the desired parameters with a time resolution of about 1 s. The time-resolved measurements of optical absorption, tilt angle, and spontaneous polarization during the thermal reisomerization after ending the sample irradiation showed that the photoinduced changes in the spontaneous polarization simultaneously followed the molecular isomerization with the same rate constant and activation energy, while the director tilt angle remained basically unchanged. Minor changes in the tilt are explained by the local heating of the sample due to the optical absorption. Since the photoinduced change in polarization was observed at constant tilt, we conclude that in the limit of low UV intensity the photoferroelectric effect originates from a photo-induced change of the bilinear coupling coefficient between the polarization and the tilt. In the molecular theory of chiral SmC* liquid crystals, the coupling coefficient is related to the bias of molecular rotations. This bias may be considerably disturbed by the formation of the bent Z-isomers during the photoisomerization.

Electrorheological Properties of Chiral Smectic-C Liquid Crystals Observed by Using Shear Horizontal Wave Propagation

An electrorheology (ER) effect of a chiral smectic- C (Sm- C *) liquid cyrstal is observed using a shear horizontal (SH) wave propagating in a liquid-crystal cell. Propagation loss and acoustic phase-delay in the liquid-crystal cell are measured and the propagation characteristics of the SH wave in the cell structure is numerically analyzed in the form of considering the stress tensor of Sm- C * phase. The experimental results are explained qualitatively in comparison with the numerical analysis results. A relative viscosity change under electric field is evaluated, which indicates that the ER effect in the Sm- C * liquid crystal is predominated by the orientations of the liquid-crystal directors and the Sm- C * layers. The present technique has the advantage of evaluating the ER properties without disturbing the Sm- C * layers.

Electrorheological Properties of Chiral Smectic-C Liquid Crystals Observed by Using Shear Horizontal Wave Propagation

An electrorheology (ER) effect of a chiral smectic- C (Sm- C *) liquid cyrstal is observed using a shear horizontal (SH) wave propagating in a liquid-crystal cell. Propagation loss and acoustic phase-delay in the liquid-crystal cell are measured and the propagation characteristics of the SH wave in the cell structure is numerically analyzed in the form of considering the stress tensor of Sm- C * phase. The experimental results are explained qualitatively in comparison with the numerical analysis results. A relative viscosity change under electric field is evaluated, which indicates that the ER effect in the Sm- C * liquid crystal is predominated by the orientations of the liquid-crystal directors and the Sm- C * layers. The present technique has the advantage of evaluating the ER properties without disturbing the Sm- C * layers.

Comparison between the switching dynamics of homeotropic and planar cells of chiral smectic-C liquid crystals

The molecular dynamics during the switching of homeotropic and planar cells of chiral smectic-C liquid crystals has been investigated using electrooptic measurements and second-harmonic-generation interferometry. Two ferroelectric liquid crystals with great differences in their molecular structures have been studied. It has been found that the molecular motion is rather different depending on the type of cell used. For planar cells, the azimuthal rotation of the molecules is limited within half the smectic cone and the molecules rotate in opposite directions in the two halves of the chevron structure. In contrast, for homeotropic cells there is no chevron structure and a uniform macroscopic optical indicatrix can be defined all over the sample during the whole switching process. However, the molecular reorientation takes place within numerous microdomains of size smaller than the optical wavelength. There are two types of microdomains that occur with different probabilities depending on the material. Within each domain the molecules rotate in opposite directions, and the molecules describe a complete cone during a whole switching period.

Berreman-matrix formulation of light propagation in stratified anisotropic chiral media

We critically analyze the problem of light propagation in stratified birefringent gyrotropic media by introducing a \(\) matrix formulation. As a test model we consider the case of short-pitch chiral smectic-C liquid crystals, for which simple descriptions in terms of an effective gyrotropic homogeneous model and of a microscopic periodic structure are available. By comparing reflectances and transmittances from finite and semi-infinite slabs, we show that the homogeneous description is unable to account for some of the properties of the underlying periodic structure, whatever small is the pitch with respect to the light wavelength.

Anomalous heat capacity of smectic-C and chiral smectic-C liquid crystals above the transition point

The anomalous parts of the specific heat capacity of smectic-C and chiral smectic-C liquid crystals above the transition point is calculated on the basis of the fluctuation theory of Landau. The present theory explains the available experimental results of heat capacity. The analysis shows that only the Gaussian model is sufficient to explain the anomalous heat capacity of smectic-C liquid crystals above the transition point.

Optical unwinding and reentrance phenomena in chiral smectic-C liquid crystals

We optically measure the molecular reorientation induced by an argon laser in homeotropically aligned chiral smectic-C films. Optical unwinding of the helix is observed and the critical unwinding intensity is determined. We have also developed a theory which not only explains the observed effect but also predicts a novel reentrance phenomenon of the unwinding state.

Modulations in the diffracted intensity in chiral smectic-C liquid crystals.

Recently [K. A. Suresh, Yuvaraj Sah, P. B. Sunil Kumar, and G. S. Ranganath, Phys. Rev. Lett. 72, 2863 (1994)] the intensity and polarization features associated with the optical diffraction in chiral smectic-C liquid crystal were studied in the phase grating mode. In the computations it was found that the diffracted intensity, as a function of sample thickness, has modulations of different length scales. In this paper, using a perturbative approach, we show that these modulations are a consequence of a coupling between different orders of scattering. \textcopyright{} 1996 The American Physical Society.

Melted-grain-boundary phase in chiral smectic-C liquid crystals near the triple N*A*C* point.

Melted-grain-boundary phase in chiral smectic-C liquid crystals near the triple N*A*C* point.

Electric-field-induced layer reorientation in ferroelectric liquid crystals.

We show that a sufficiently large ac electric field reorients the smectic layers irreversibly in chiral smectic-C liquid crystals. Using uniformly aligned samples, prepared by unidirectional buffing, it is demonstrated that at a given temperature the layers tilt away from the buffing axis, b^, by an angle equal to the molecular tilt angle of the smectic-C phase. In this new configuration the molecules lie along b^ and the layers are tilted. It is also found that a small dc bias field can control the direction of the tilted layers with respect to b^. Furthermore, microscopic observations suggest that the chevron layers, common in the ferroelectric smectic-C phase, are absent after the electic field treatment which eliminates zig-zag-type defects.

Dependence of the Tilt Angle on External Forces for Smectic-C And Chiral Smectic-C Liquid Crystals—Measurement of the Heat Capacity of DOBAMBC

Abstract A measurement of the specific heat of DOBAMBC is presented, and it is shown that a Landau expansion of the free-energy density with a sixth-order term is adequate to describe the measurement. The coefficients in the Landau expansion are estimated from this measurement. The dependence of the tilt angle on applied electric and magnetic fields is discussed for both smectic-C and chiral (ferroelectric) smectic-C liquid crystals. Numerical estimates of the Landau coefficients are used to show that the effect of external forces on the tilt angle is negligible except when being less than approximately 0.2 K from the transition to the smectic-A phase. The effect on the smectic-C-smectic-A phase-transition temperature is also discussed. Boundary effects can also affect the tilt angle, and an expression for the penetration depth of a boundary-induced disturbance is calculated. For thin, planar samples, a coupling between the two boundaries can induce a transition to the smectic-A phase. It is demonstrated ...