Non-linear electrooptic effect in antiferroelectric liquid crystal

Electrooptic phenomena caused by weak electric fields, much lower than those needed for the helix unwinding, in helical smectic liquid crystals were studied in thin planar samples. The investigations were performed in chiral liquid crystal 4-(1-methyl-heptyloxycarbonyl) phenyl 4′-(3-butanoyloxy propyl-1-oxy) biphenyl-4-carboxylate which exhibits antiferro-electric properties. We have found that electric field applied to a helical smectic liquid crystal caused two effects. First, the helix was deformed and the position of effective optic axis changed by an angle proportional to the field strength. The second effect, quadratic in field, causes the change in the shape of the indicatrix. As a consequence, the relative changes in the light intensity caused by external electric field consist of two components. The first component represents the modulation with the fundamental frequency and the second one with the doubled frequency (second harmonic of the electrooptic effect). The ab- solute values of the first- and second-order electrooptic coefficients have been determined and their temperature dependence discussed.     

A simple molecular theory for photo-induced smectic phase

The response of liquid crystals to light is very important for applications of liquid crystals in display and memory devices. Recently experiments have been carried out on liquid crystals doped with photoactive azo compounds. It is seen that UV rays incident on such systems can lower the nematic isotropic transition temperature T _NI . Also, in some mixtures, a photo-induced smectic phase is observed. This is attributed to the change in the trans (longer) isomer to cis (shorter) isomer of the photoactive dopant. We have earlier developed a molecular mean-field model assuming the medium to consist of inter-converting anti-parallel and parallel pairs to explain the molecular origin of “two lengths”. The model was used to explain double re-entrance, the effect of electric field on T _NI , etc. This model is modified to include the change of trans to cis isomer which is equivalent to an increase of fraction of parallel (shorter) pairs. The calculated phase diagram with respect to incident UV radiation energy shows an induced smectic phase. This is in qualitative agreement with experimental trends.     

Bistability of a “chevron” smectic C* liquid crystal in an external electric field

The system of nonlinear equations describing a surface-stabilized ferroelectric smectic C* liquid crystal in the chevron geometry has been investigated by numerical methods in the framework of the continuum model of liquid crystals. Stable orientational and structural configurations have been studied, and the results obtained have been compared with those derived using simplified models. The height of the potential barrier separating two stable configurations has been determined, and the transition between them under the action of the external electric field has been examined. It has been demonstrated that this effect has a threshold character. The dependence of the threshold field on the film thickness has been analyzed.     

Nonlinear repolarization processes in ferroelectric liquid-crystal thin films

The dynamics of spontaneous polarization switching of the ferroelectric smectic C* in a variable electric field are examined theoretically and experimentally with the help of polarized light scattering. The observed effect of quasiresonant scattering both in freely suspended smectic films and in ordinary electro-optical cells is interpreted within the framework of the nonlinear model of isolated movable kinks in the director orientation distribution. It is shown that the maximum of the scattering intensity at the characteristic frequency of the applied electric field disappears at low temperatures and for small thicknesses of the smectic film. The dependence of the “resonant” frequency on the electric field amplitude, the proximity to the phase transition temperature, the film thickness and thickness of the ferroelectric domains, and also various material parameters is found. Estimates are made of such important characteristics as the dielectric anisotropy, viscosity, and elasticity of the smectic films. The effect of film thickness on the density distribution of the polar anisotropy energy in the film and on the corresponding shape of the moving orientation front within the film are discussed. Zh. éksp. Teor. Fiz. 111, 919–937 (March 1997)     

Molecular-statistical approach to a behavior of ferroelectric,antiferroelectric and ferrielectric smectic phases in the electric field

The fundamental theoretical approach derived in A.V. Emelyanenko et al., Phys. Rev. E 74, 011705 (2006) is complemented by a consideration of the influence of the homogeneous electric field on Sm- C _A ^* , biaxial intermediate phases, and Sm-C ^* . The crucial role of the induced polarization is investigated for the first time. The evolution of any tilted smectic phase in the electric field is found to meet the two thresholds. The first threshold corresponds to the unwinding process, and the second one corresponds to the phase transition into the bi-domain structure of Sm-C ^* , where the tilt plane has some contribution either along or against the electric field, while the average direction may still be perpendicular to the electric field. The tilt plane in the monodomain (conventional) structure preceding the second threshold is the same in every unwound phase, and is perpendicular to the electric field. No 3D distortion in Sm- C _A ^* is predicted on application of the electric field. The entire electric-field-temperature phase diagrams including the possibility of existence of the maximal number of tilted smectic phases are plotted and compared with the experimental ones. The numerical calculations in the framework of this fundamental study are done with help of AFLC Phase Diagram Plotter software developed by the author and available at his web page.     

Studies of thin smectic C films by X-ray reflectivity

The smectic order in thin and ultra thin films (150–600Å) of the chiral ferroelectric liquid crystal mixture ZLI-3654 is studied using the X-ray reflectivity technique. The spin cast films on various substrates (float glass, Si wafer, polymer coated glass, etc.) order spontaneously with smectic layering parallel to the substrate surface. A simple model which assumes a sinusoidal density modulation can describe well the experimental reflectivity profiles. The X-ray reflectivity provides a method to evaluate the phases of the structure factor. We demonstrate, for the first time, that is possible to extract the molecular tilt angle, , in ferroelectric liquid crystals from X-ray reflectivity measurements of ultra thin films. The temperature dependence of the tilt angle in the smectic C^* phase are almost independent of the film thickness (down to 200 Å) and are similar to those in the bulk.     

Microsphere manipulation using ferroelectric liquid crystals

We propose a strategy for a micromanipulation method using SSFLC (surface stabilized ferroelectric liquid crystals). By adjusting the frequency of the applied ac electric field, the surface layers that cannot follow an applied ac electric field are constructed in SSFLC. In addition, by applying a sawtooth wave voltage, net flow along the smectic layer is generated. The flow direction is reversed by changing the polarity of the sawtooth wave. Consequently, the particles dispersed in SSFLC can be driven bidirectionally along the smectic layer. The particle velocity depends on the temperature, amplitude, and frequency of the applied voltage.     

Uniform rotation of smectic layers of a ferroelectric liquid crystal in an asymmetric electric field

An asymmetric ac electric field with amplitudeE=10 V/μm gives rise to a matched rotation of the normal to the smectic layers, while a sinusoidal fieldE=1 V/μm is used to study the rotation in the method of modulation total internal reflection ellipsometry, which makes it possible to probe the region of a ferroelectric liquid crystal (FLC) next to the electrode (h≈0.7 μm). It is shown that the angle of rotation of the normal to the smectic layers near the surface of the electrode varies reversibly as a function of the polarity and number of electric pulses applied, just as in the interior region. The characteristic dynamic properties of thin layers of nematic liquid crystals, such as an anomalously long relaxation time and a high-frequency relaxation process, are observed in thin FLC layers.     

Photonics Devices Based on Hybrid Approach Combining Liquid Crystals and Sol-Gel Waveguides

In this article, the analysis of both an integrated electro-optic switch and a continuously tunable filter based on a Bragg grating in planar waveguide with a liquid crystal overlayer is reported. The fast and bistable switching of smectic C* in the surface-stabilized liquid crystal structure has been utilized in order to investigate the possibility of realizing an integrated electro-optical switch, whereas the soft-mode of smectic A* liquid crystals, allowing a continuous modulation of extraordinary refractive index, have been utilized in order to design an integrated wavelength filter in the wavelength range of interest for optical communication. Moreover, the performance of an electro-optical router, working at the wavelength of 1.55 μm and based on the multimode interference effects together with the electro-optic effect of smectic A* liquid crystal, is theoretically and numerically discussed.     

73Ge NMR spectra in germanium single crystals with different isotopic composition

We have studied the influence of isotopic disorder on the local deformations in Ge single crystals from both experimental and calculation points of view. The nuclear magnetic resonance (NMR) spectra of⁷³Ge nuclei (the nuclear spin equals 9/2) in perfect single crystals of germanium with different isotopic content were measured at temperatures 80, 300 and 450 K. Abnormal broadening of the spectrum was found to occur when the magnetic field was aligned along the [111] axis of a crystal. The observed specific angular dependence of the quadrupole broadening was attributed to isotopic disorder among atoms of germanium sited around the⁷³Ge NMR probe. Local lattice deformations in germanium crystal lattice due to isotopic impurity atoms were calculated in the framework of the adiabatic bond charge model. The results obtained were applied to study random noncubic crystal field interactions with the nuclear quadrupole moments and corresponding effects in NMR spectra. Simulated second and fourth moments of resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions agree qualitatively with the experimental data.     

Electroluminescence in lead titanate single crystals

Electroluminescence in PbTiO₃ single crystals is studied with variation in applied electric field, frequency (20 Hz to 5 kHz) and temperature. The EL onset depends on the rate at which the dipole switches. Extremely sharp upward rising nature of the pulses of micro second duration suggest that there is a self maintained discharge in the dielectric due to secondaryγ _p mechanism. Frequency dependence of EL suggests that both the secondary mechanisms, viz. theγ _p andγ _i are active after the application of a high field and the critical field at which this occurs decreases with increase in the frequency of the applied voltage. Similarly the onset voltage decreases with increase in frequency. The temperature dependence of EL at the applied frequency of 50 Hz shows that the onset voltage is intimately connected with the coercive field of the crystal and it is minimum at the Curie point. The study suggests that EL occurs in the bulk and there is a breakdown in the dielectric due to an avalanche formation.     

Non-steady-state photoelectromotive force in SiC crystals irradiated by reactor neutrons

The effect of the non-steady-state photoelectromotive force in 6H-SiC crystals irradiated by reactor neutrons has been investigated experimentally. The dependences of the signal amplitude on the phase modulation frequency, spatial frequency, light intensity, and amplitude of an external alternating-current (ac) electric field have been analyzed. The unusual frequency dependence of the signal has been explained in terms of the two-level semiconductor model taking into account shallow traps. The photoelectric parameters of the crystals have been determined for the light wavelength λ = 532 nm.     

Microwave specific loss power of magnetic fluids subjected to a static magnetic field

This paper reports on the frequency dependence of the magnetic and electric power dissipation in a magnetic fluid sample, in the microwave frequency range (0.5 to 8GHz), at various values of the static magnetic field (0 to 167.8kA/m). The computation of the power dissipation relies on the experimental values measured for the complex dielectric permittivity, ɛ = ɛ′ - iɛ″, and the complex magnetic permeability, μ = μ′ - iμ″, over the same frequency range. The results show that the magnetic power dissipation is much larger than the electric one for the investigated sample. At a specific frequency, f (Hz) , the power dissipation, p, depends on the external magnetic field, and exhibits a maximum. The result obtained suggests the possibility of controlling the energy absorption in the microwave range by means of the application of an external magnetic field.     

Dynamics of domain-wall motion in ferroelectric liquid crystals in an electric field

The director reorientation in smectic liquid crystals with ferroelectric properties has been considered in the case where the interaction of liquid-crystal molecules with the surface leads to a partial unwinding of the helical structure of the liquid crystal and the reorientation occurs as a result of the domain-wall motion. The dependences of the velocity of domain-wall motion on the electric field strength, electric field variation frequency, boundary conditions, spontaneous polarization, and viscosity of the liquid crystal have been determined. It has been demonstrated that an increase in the electric field variation frequency or the polar part of the anchoring energy and the spontaneous polarization of the liquid crystal at a constant field frequency results in an increase of the velocity of domain-wall motion. As a consequence, the time of the electro-optic response of the liquid crystal in weak electric fields (from 0.4 to 2.0 V/μm) decreases by a factor of more than three.     

Piezoelectric effect in liquid crystals

A piezoelectric effect has been observed in a cholesteric liquid crystal layer (an electric field effect type, a mixture of cholesteryl oleyl carbonate, cholesteryl chloride and cholesteryl nonanoate) [1]. The layer, subjected to shear vibration, generates an alternating electric potential of the same frequency as the exciting vibration. In the present paper an extension of these investigations is reported to cases with various mixing ratios and also to one of smectic liquid crystals (lecithin with water). For a cholesteric liquid crystal of a particular mixing ratio, the strength of the electric generation is found to reach as high as 450 mV(p-p) for a vibratory shear displacement of 1 μm(p-p) under a certain molecular orientation preparation. Small electric generation is also observed for the above smectic liquid crystal but not for the nematic liquid crystals (MBBA and EBBA). A simple continuum elasticity model can qualitatively explain the behaviour of this electric potential generation.     

Structure of π- and 2π-Walls in Smectic films

The structure of ??- and 2??-walls in smectic films was reconstructed from optical reflectivity measurements. Investigations were made in free standing films of nonpolar Smectic-C and ferroelectric Smectic-C* liquid crystals. ??-walls are observed in magnetic field and 2??-walls in electric field parallel to the film plane. For the first time the distribution of molecular orientation across the walls was determined. Peculiarities of the wall structure related to the anisotropy of the film elasticity were found. The structure of the walls is well described by the theory taking into account the anisotropy of two-dimensional elasticity of smectic films.     

Electroclinic effect in free-standing smectic elastomer films

The coupling of the mesogenic tilt in smectic liquid crystals to external electric fields in the layer plane (electroclinic effect) provides an opportunity to control the smectic layer thickness with electric fields. In ordered ferroelectric smectic elastomers it is possible to achieve a macroscopic electromechanical response. The effect is particularly pronounced near the smectic A-to-smectic C^* phase-transition temperature. In this work, the electrostriction of weakly cross-linked smectic elastomers is studied by means of optical interferometry of thin films. Its observed magnitude corresponds to an induced tilt of approximately 7.7°, yielding a layer contraction of 1%, in a 3 MV/m electric field. This value is in agreement with earlier measurements of tilt susceptibility in a structurally similar polymer, but is considerably smaller than previously reported data on elastomers with a comparable chemical structure. PACS 61.30.Vx; 83.80.Va; 77.65.Bn     

Hydrogen bonds in single crystals of para and meta-aminobenzonitrile as revealed by the Zeeman effect on the nuclear quadrupole resonance of 14N

The |e ² qQ/h| and η values for both the cyano and the amino-nitrogen of the title compounds are reported at room temperature as well as at 273 K and 77 K. From Zeeman effects on single crystals of both compounds the orientation of the principal axes of the electric field gradient (efg) on both nitrogen nuclei, as well as a number of data on the crystallographic and molecular structure, are obtained. A reliable interpretation is given of the experimental resonance frequencies, which are also safely assigned to the right nitrogen atoms. By studying the dependence of the resonance linewidth on the orientation, with respect to the efg principal axes, of the Zeeman field B ₀, the spatial distribution of the hydrogen nuclei around the ¹⁴N nuclei is determined. In this way hydrogen bonds are shown to exist in both single crystals. An analysis based on the Townes and Dailey theory allows an estimate to be made of the possible variation of the population of the cyano-nitrogen lone-pair orbital when involved in hydrogen bonds.     

Numerical study of deformations induced by ac electric field in insulating flexoelectric nematic layers

The influence of the frequency f of applied ac electric field on the time dependence of electric field induced deformations of homeotropic nematic layers is studied numerically. Three kinds of nematic liquid crystals were considered: