Behavior Of Ferroelectric Liquid Crystals Research Articles

Photo-Aligned Ferroelectric Liquid Crystal Devices with Novel Electro-Optic Characteristics

This paper examines different applications of ferroelectric liquid crystal devices based on photo-alignment. Successful application of the photo-alignment technique is considered to be a critical breakthrough. A variety of display and photonic devices with azo dye aligned ferroelectric liquid crystals is presented: smart glasses, liquid crystal Pancharatnam–Berry phase optical elements, 2D/3D switchable lenses, and laser therapy devices. Comparison of electro-optical behavior of ferroelectric liquid crystals is described considering the performance of devices. This paper facilitates the optimization of device design, and broadens the possible applications in the display and photonic area.

Relaxation Dynamics of Ferroelectric Liquid Crystals in Pulsed Electric Field

In this contribution we report a theoretical study of relaxation processes in surface-stabilized ferroelectric liquid crystals with spontaneous polarization. The influence of pulsed electric field on the behavior of ferroelectric liquid crystal in the SmC* phase, which is placed in a thin cell with strong anchoring of SmC* molecules with the boundary substrate, is studied. In the vicinity of the substrate interface, temporal dependence of the azimuthal motion of the director induced by electric field is obtained. The response to the external distortion of ferroelectric liquid crystal confined between two microstructured substrates is the occurrence of periodic temporal formation of solitons connected with the distortion of the director field n in the sample bulk. The interplay between microstructured substrates and director distribution of the ferroelectric SmC* phase is explained by the Frenkel–Kontorova model for a chain of atoms, but adapted for the continuum problem.

A dielectric mode in electroclinic liquid crystals

The dielectric properties of electroclinic and ferroelectric liquid crystal materials have been investigated in the frequency range of 100Hz–10MHz. A dielectric mode has been predicted in electroclinic liquid crystals near the transition temperature of Sm-C*–Sm-A phase. It has been observed that the investigated material has nonlayer shrinkage near the transition temperature of Sm-C*–Sm-A phase and shows anomalous behavior of dielectric spectra, tilt, and texture which is entirely different from the behavior of ferroelectric liquid crystals (FLCs). The dielectric relaxation frequency and tilt angle are almost constant with respect to temperature near the transition temperature under high bias field, which is characteristically different from FLCs. The high dielectric permittivity near Tc owing to the presence of mode contribution leads to the fact that there is an intralayer phason variation few degrees before transition. The mode has been named random mode due to its origin from randomization of tilt near Tc. Tilt randomization has been considered as an order parameter because in both the phases molecules are tilted but it is disordered in Sm-A phase making its intralayer polarization zero.

Simulation of electrooptical effects and dynamics of ferroelectric liquid crystals

The behavior of ferroelectric liquid crystals in an external electric field is simulated numerically. The equations that describe the dynamics of the director of a liquid crystal are derived within the continuum theory of elasticity with due regard for compressibility of smectic layers, finite anchoring energy, and dielectric properties of orienting coatings and external elements of a real electric circuit. These equations make the basis for simulation of the electrooptics of ferroelectric liquid crystals. The specific features and mechanisms of the surface-stabilized bistability and hysteresis-free electrooptical switching (the V-shape effect) are discussed.

Switching behaviour of ferroelectric liquid crystals probed by optical second-harmonic generation

Ferroelectric properties of thin (1.5–4 μm planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using electro-optic measurements, second-harmonic generation (SHG) and SHG interferometry. A switching behaviour of the FLC cells in external dc electric fields is observed. It is characterised by rotation of the polarisation plane of the transmitted light and by changes in the SHG intensity, phase and anisotropy dependences, which are attributed to a collective motion of the system as a ferroelectric uniform state with C2 symmetry.

Dielectric behavior of ferroelectric liquid crystals in the vicinity of the transition into the hexatic phase

Collective molecular dynamics was studied in ferroelectric smectic C* and hexatic smectic I* phases by the dielectric method. In the frequency range 1 Hz to 10 MHz one relaxation process was observed in both phases. In the SmC* phase the relaxation frequency of this process linearly decreases and its dielectric strength increases when approaching the SmC*–SmI* phase transition temperature TI. In the SmI* phase the relaxation frequency is one order of magnitude lower and further slightly decreases on cooling. A phenomenological theory has been developed which describes static and dynamic dielectric properties near TI and satisfactorily explains the experimental results. The observed relaxation has been attributed to fluctuations of the bond orientational order coupled to the molecular tilt. A parameter describing the softening of this mode is determined by fitting theoretical results to the experimental data. The model also describes an observed anomalous increase in the spontaneous polarization as a result of the change in the strength of bond orientational order when passing to the SmI* phase.

Thickness dependence of polarization and response characteristics in thin FLC films

The effects of cell thickness on the pretransitional behaviour of ferroelectric liquid crystals are studied experimentally and theoretically for thin cells. The temperature and field dependences of the tilt angle, polarization, polarization to tilt angle ratio, dielectric permeability and relaxation frequency are considered in detail in the framework of a simple model for the surface tension. Apparent values of the phenomenological coefficients in the free energy expansion and some bulk and surface material parameters are estimated.

FINITE SIZE EFFECTS IN THE DYNAMIC BEHAVIOR OF SMECTIC C* PHASE STUDIED BY LIGHT SCATTERING

Abstract The dynamic behavior of ferroelectric liquid crystals in thin cells with planar boundary conditions and the helix unwound, was studied by the quasielastic light scattering. The autocorrelation function of the scattered light is non-exponential. From the angular dependence of the measured relaxation rate, the lowest orientational mode eigenvalue was determined. From this value the surface anchoring energy for SCE9 on nylon surface was determined to be 5×10−5 J/m2

Theory of Pattern Formation in a Freestanding Ferroelectric Film in a Rotating Electric Field

Abstract A free-standing ferroelectric film subjected to a rotating electric in-plane field has previously been investigated under a polarizing microscope [1]. Depending on the strength and rotation frequency of the external field, various kinds of pattern formation were observed, e.g. ring systems which can shrink or grow. This contribution provides a theoretical examination of the effect. It is based on the usual Landau theory [2]. In particular, it is shown that the ring systems observed can be described as inversion walls, which exhibit soliton-like behavior1. The analytical results are supported by numerical simulations. As a result, a qualitative and, to some extend, also a quantitative understanding of the complex behavior of ferroelectric liquid crystals in the case of inhomogeneously aligned smectic layers has been achieved.

Guided Optical Waves in a Ferroelectric Liquid Crystal Layer: A Birefringence Analysis of Molecular Orientation on the Switching Process

Abstract Guided optical waves are very sensitive to the alteration of optical properties of dielectric media. In this report, we demonstrate the use of guided waves for studying dynamic behavior of ferroelectric liquid crystals. Propagating light in the anisotropic medium suffers a birefringent effect, which causes coupling of p- and s-polarized light. Theoretical calculations, based on the Maxwell equations, successfully describe this phenomena, using a dielectric tensor diagonal in molecular coordinates, which is transformed to the laboratory coordinate system by three Euler angles. The waveguide measurements are able to probe the molecular orientation and movement of the liquid crystal molecules in the three dimensional coordinate system. This method has been used to examine the switching behavior in the SmC* phase. The bistable states and the path of the molecular swing under alternating external electric fields are well characterized by the reflected intensity of single and double frequency modulation.

Dielectric, Electrooptical and Magnetic Properties of Metallomesogens

Abstract The dielectric behaviour of ferroelectric liquid crystals of copper (II) Schiff bases complexes were investigated by dielectric spectroscopy. The spontaneous polarization Ps and the switching time T were determined by means of the polarization reversal method. Temperature dependent magnetic susceptibility measurements were carried out in the temperature range of 4.2 - 450 K to examine the magnetic properties of the copper (II) complexes.

The Critical Behaviour of Nonlinear Dielectric Constants of Ferroelectric Liquid Crystals in the Smectic A Phase

Abstract The critical behaviors of the linear and higher-order nonlinear dielectric constants of ferroelectric liquid crystals (FLCs) are studied in the SmA phase near the SmA-SmC* phase transition. The critical exponents of the linear, third-order and fifth-order dielectric constants experimentally obtained are 1, 4 and 7, respectively. These exponents and the signs of nonlinear dielectric constants are in good agreement with those calculated theoretically from the phenomenological theory of Landau type. The influence of the order of phase transition (first or second) on the critical behavior of FLCs is also studied.

Behavior of ferroelectric liquid crystals in external fields.

It is shown that the reentrant Sm-${\mathit{C}}^{\mathrm{*}}$ phase in an external magnetic as well as in an external electric field in typical ferroelectric liquid-crystalline compounds, such as p-(n-decyloxybenzylidene)-p'-amino-(2-methylbutyl) cinnamate (DOBAMBC), has the same physical origin as the anomalous temperature dependence of the pitch p. The observed temperature dependence of the critical electric field ${\mathit{E}}_{\mathit{c}}$ is described well within the extended Landau model using the same values of the model parameters as in the magnetic-field case [Phys. Rev. E 48, 455 (1993)]. In DOBAMBC a simple relationship between ${\mathit{E}}_{\mathit{c}}$, the tilt \ensuremath{\Theta}, the polarization P, and the pitch p in a zero field, ${\mathit{E}}_{\mathit{c}}$\ensuremath{\propto}${\mathrm{\ensuremath{\Theta}}}^{2}$/(${\mathit{Pp}}^{2}$) is found to be valid at all temperatures except for ${\mathit{T}}_{\mathit{c}}$-T\ensuremath{\lesssim}0.1 K. At temperatures 0.2 mK${\mathit{T}}_{\mathit{c}}$-T2.5 mK, where the Sm-${\mathit{C}}^{\mathrm{*}}$\ensuremath{\leftrightarrows}Sm-C transition is first order, the stability limits of both phases are determined: the associated hysteresis is found to be small.

Soliton switching in ferroelectric liquid crystals and their transient electro-optic response

The switching behavior of ferroelectric liquid crystals (FLCs) is studied both experimentally and theoretically. Experimentally the electro-optic response of multiwavelength-thick FLC cells revealed transient peak and dip characteristics. This behavior could not be explained at all according to the standard picture of a switchable birefringent plate, nor according to transient light scattering associated with ferroelectric domain reversal. Solutions of a numerical model of the FLC electric and elastic torques, which results in the sine-Gordon equation, showed the propagation of kink-antikink pair solitons. The nonuniform director structures corresponding to these solitons are sufficient to explain the experimental behavior of the transient electro-optic response with the wavelength, the field, and the polarizer-cell-analyzer orientation. The electro-optic response is found to be highly sensitive to the fixed boundary dipole orientations.

Theoretical investigation of the behavior of ferroelectric liquid crystals in a magnetic or in a high-frequency electric field.

A theoretical explanation of the phase diagram of ferroelectric liquid crystals in an external magnetic or high-frequency electric field is given. The investigation is based on a rigorous treatment of the problem within the phenomenological model, being an extension of the Pikin-Indenbom model [Ferroelectrics 20, 151 (1978)]. The phase transition from the modulated Sm-${\mathit{C}}^{\mathrm{*}}$ phase to the homogeneously ordered Sm-C phase is studied numerically. The relevant parameters of the model are chosen in the best way to fit the experimentally obtained critical-field lines on two ferroelectric liquid-crystalline compounds. As a result, the observed relationship between the critical field and the helical pitch in zero field is theoretically confirmed. The critical-field line is investigated in the vicinity of the Lifshitz point where the metastable states appear. In both compounds, the tricritical point is found to be very close to the Lifshitz point, which seems to disagree with measurements showing hysteresis at all temperatures. This contradiction is removed by showing that, in a sample of a finite length along the helical axis, a slow pitch-relaxation process exists. Consequently, during the critical-field measurements, the system is probably not in an equilibrium, which prevents the continuous phase transition from taking place.

Time-resolved FT-IR study of ferroelectric liquid crystals with SiO obliquely evaporated alignment layers

Abstract Time-resolved FT-IR spectroscopy was performed to monitor the behaviour of ferroelectric liquid crystals on a microsecond time scale. A bookshelf geometry was obtained by alignment with SiO obliquely evaporated layers in a CaF2 cell with 2 μm gap. Six kinds of FLC mixtures were examined, all of which were three-component systems consisting of chiral molecules of 5 wt% and two kinds of phenyl pyrimidine molecules of 47·5 wt% each. All IR spectra were dominated by vibrational modes from the phenyl pyrimidines. Typical vibrational modes from core and alkyl chain parts are selected in order to compare the movement of the alkyl chain as a function of the chiral components. A novel parameter α = (intensity of largest peak at 2925 cm−1/intensity of smallest peak at 2925 cm−1)/(intensity of largest peak at 1432 cm−1/intensity of smallest peak at 1432 cm−1) for evaluating the alkyl chain movement is introduced. It was found that the length of the rigid or flexible parts of the chiral molecules and the par...

Influence of the crystallinity of the alignment layer on the bistability of the surface-stabilized ferroelectric liquid-crystal effect

The influence of the crystallinity of the alignment layers on the electro-optical behavior of ferroelectric liquid crystals has been quantitatively demonstrated. The degree of crystallization of thin nylon 6.6 alignment layers was varied using thermal anneal methods and determined by Fourier transform infrared spectroscopy. The reaction of an obtained memory state of surface-stabilized ferroelectric liquid-crystal samples to small disturbing electric fields decreased when the degree of crystallization of the incorporated nylon 6.6 alignment layers was higher.

Microscopic model of the spontaneous polarization in ferroelectric liquid crystals

Abstract A microscopic model is introduced which describes the coupling between the tilt of molecules from the normal to smectic layers in ferroelectric liquid crystals and the rotation of a molecule around its long axis. The single particle potential consists of a dipolar and a quadrupolar term. The microscopic model is shown to be consistent with the extended thermodynamic model, which takes into account the bilinear as well as the biquadratic coupling between the tilt of molecules and the induced transverse polarization, and which describes the anomalous thermodynamic behaviour of ferroelectric liquid crystals in the smectic C* phase. It is shown that close to the transition to the smectic A phase the thermodynamic description is exact, but overestimates the spontaneous polarization at lower temperatures. The results are compared with experimental data. It is demonstrated that the quadrupolar order is larger than the dipolar order except very close to the transition.

The characteristic dielectric behaviour in ferroelectric liquid crystals at a phase transition and the contribution of the soft mode

Abstract The characteristic dielectric behaviour of ferroelectric liquid crystals with a large spontaneous polarization has been studied as functions of the D.C. bias field, frequency, cell thickness and applied pressure. Under the condition in which the contribution of the Goldstone mode is suppressed, a sharp peak in the temperature dependence of the dielectric constant is clearly observed at the transition between SA and S*C phases T S C *s A. The relaxation of the soft mode is observed both in the SA and S*C phases by eliminating the contribution of the Goldstone mode under a D.C. bias field. Another relaxation is also observed in the S*C phase around several kHz in addition to that of the soft mode and the Goldstone mode. The pressure effect on the soft mode was also studied.

Interaction between flc:s and electric fields: Flexoelectric effects and surfaces

Abstract In ferroelectric liquid crystals there are besides the ferroelectric polarization a number of different flexoelectric interactions, but as in nematics, these effects are difficult to study experimentally. Here the interpretation of the flexoelectric effects will be discussed. The flexoelectric energy terms are so closely connected to the boundary conditions that it is questionable if and when it is possible to give the coefficients bulk values. Why not then accept some of them as linear interaction terms between the boundary conditions and the electric field? The self energy of the ferro- and flexoelectric vector fields will give contributions to the elastic energy and to the elastic behavior of ferroelectric liquid crystals. Could there also be similar effects in nematic liquid crystals?