The chiral smectic phases of two liquid crystals (LCs) were investigated, one of them is a high tilted, long pitch, near ortoconic LC mixture and the second exhibits broad temperature of the SmC alpha* phase. For both materials second-harmonic electro-optic (EO) spectra were studied. The particular LC phases were characterised by a specific temperature dependence of the frequency at which the real part of the second-order electro-optic response (EOR) possesses an extremum (f(ext)). The obtained values of f(ext) (correlated with the phase-type mode processes) were used to identify LC phases and temperature of phase transitions. The sensitivity of the used method allowed to identify the SmC gamma* and SmC alpha* phases for the studied materials, not detected before by DSC or dielectric measurements.[Graphics].

In this paper, we report on how flexoelectric and piezoelectric polarization components can be determined by a method based on simultaneous studies of dielectric and electrooptic properties of the chiral smectic liquid crystal in the regime of weak electric fields. As a rule, the measurements of spontaneous polarization are performed using switching experiments. The polarization measured in this way is not complete—it contains the piezoelectric component only. However, the knowledge of the entire local polarization of a single smectic layer is of great importance—it is necessary for correct determination of some material parameters, for instance elastic constants. Our experiments performed in a helical smectic mixture demonstrated that flexoelectric contribution to the local spontaneous polarization is significant in both ferroelectric and antiferroelectric phases. In the antiferroelectric phase, the flexoelectric polarization is less due to higher helical pitch.

Dielectric properties of chiral smectic liquid crystals characterised by the occurrence of the C(α)(*) phase were investigated in the frequency range 10 Hz-1 MHz. In the range of existence of this phase the observed relaxation spectrum is composed of two kinds of mode, and not of a single one, as commonly thought. Phase modes of the Goldstone type coexist in it with an amplitude type soft mode. The share of the soft mode in the global value of electric permittivity ε can be dominant and attain 90%. A possible explanation for that effect is sought in the similarity to chiral phases of the de Vries type.

Electro-optical measurements of the second, third and fourth harmonic frequencies have been performed in prototype antiferroelectric liquid crystal materials: 4-(1-methyl-heptyloxycarbonyl) phenyl 4′-octyloxybiphenyl-4-carboxylate (MHPOBC) and 4-(1-methyl-heptyloxycarbonyl) phenyl 4-(4′-octyloxyphenylcarbonyloxy) benzoate (MHPOPB). These two substances possess a very rich sequence of phase transitions between various helical smectic subphases. The analysis of the observed anomalous behavior of the real and imaginary parts of the second, third and fourth harmonics of the electro-optical response spectra can serve for identification of various liquid crystalline phases. The anomalies of the nonlinear electro-optical response of the investigated liquid crystals have been compared with the linear electro-optical measurements.

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We employed fluorescence confocal polarizing microscopy (FCPM) to visualize three-dimensional patterns of the director field in cholesteric and nematic liquid crystals (LCs). Two types of textures were investigated: fingerprint and schlieren, by both FCPM imaging and conventional polarizing optical microscopy (POM). Some kinds of spontaneous periodic distribution of the nematic director were presented.

The knowledge of the material parameters related to the microscopic structure of liquid crystals is one of the main problems in the construction of the liquid crystal devices. In the case of ferroelectric liquid crystals, the viscoelastic properties are very important as they determine the switching speed and the threshold voltage in displays. There exist several experimental methods for measurements of viscosity and elasticity constants in tilted smectic phases which exploit various phenomena for deformation detection, e.g., light transmission, polarization current, light modulation, dielectric constant, and helix deformation or helix unwinding. In this article, we compared the results of measurements obtained for the same material using various methods. The experiments proved that the correct bulk values of the mentioned material constants can be determined using thick, homeotropically aligned samples only.

Excitation of solitary waves and their propagation in surface-stabilized ferroelectric liquid crystal cells under alternating external electric field is investigated both theoretically and experimentally. The effect of solitary waves on electro-optic response spectra is analyzed for different amplitudes of applied fields, temperatures, and sample thicknesses. It is shown that solitons can only be excited within narrow ranges of frequencies of the sufficiently strong electric fields. The minimal frequency, at which soliton waves appear in ferroelectric smectic liquid crystals, is found to be related to the material constants of these systems. It is proved that measuring this threshold frequency gives the possibility to determine one of the material parameters, if the others are known. In this way, the intra-smectic-layer elastic constant is found for systems with the chevron geometry.

The viscoelastic properties belong to the most important properties of chiral smectic liquid crystals. In this paper different methods for determination of viscoelastic properties of chiral smectic liquid crystals are compared. All of them belong to small deformation methods. The importance of the director alignment in the sample for determination of the bulk elasticity and viscosity coefficients is described. Using a typical ferroelectric in the chiral smectic C* phase we demonstrated that the correct bulk values of mentioned material constants can be determined using thick, homeotropically aligned samples only.

Propagation of solitary waves activated in thin ferroelectric liquid crystal cells under external, sinusoidally alternating electric fields is investigated using the electro-optic technique. It is shown that solitons give contributions only to the loss component of the response spectrum, within rather narrow ranges of frequencies and in sufficiently strong fields. The limit frequency, at which the amplitude of the velocity of the solitary waves is greatest, is found to be related to material constants of liquid crystals. Measuring this threshold frequency provides the capability to determine the elastic constant of surface stabilized liquid crystalline materials in the bookshelf or chevron layer geometries.

An electro-optic method for determining the twist elastic coupling between smectic layers in antiferroelectric liquid crystals with a helical superstructure has been introduced. This method is based on a calibration procedure which enables a modulation of light intensity under an alternating applied electric field to relate to a respective modulation caused by mechanical oscillations of a sample. The elastic constant of antiferroelectric liquid-crystalline materials has been obtained by applying the method to a liquid crystal which displays a direct transition from antiferroelectric C*(a) phase to the smectic A phase.

A method is described for determination of bulk values of a twist elastic coefficient for smectic c-director in chiral smectic liquid crystals with a helical structure. The method was applied to 4-methylbutyloxy phenyl-4-octyloxy-benzoate (C8) in the chiral smectic C* phase. The measurements were performed using optical detection in a small deformation limit. In contrast to the usual methods, initial deformation of the helix (caused by strong surface interactions) was avoided by using homeotropic aligned thick samples. The critical temperature dependence of the measured coefficient was observed. The relation between the measured parameter and the smectic C order parameter is presented.

Dynamic properties of surface stabilized ferroelectric liquid crystals driven by an alternating external electric field and confined inside measuring cells by air are studied. Using the electro-optic response method, it has been possible to register response spectra of small parts of liquid crystals near surfaces of their contact with air. This has allowed the direct determination of the dependence of changes of such partial electro-optic response spectra on the distance of illuminated liquid-crystal areas from the contact surfaces. The interfacial dynamic processes are found to enhance considerably the electro-optic response over relatively large distances from contact surfaces, within a wide range of field frequencies. Experimental data obtained from the used approach to determine the fragmentary electro-optic response indicate that the approach can be very effective for studying various interfacial phenomena in thin liquid-crystal systems.

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.

Electric- field induced motions of zigzag walls forming defects in the chevron structure of surface stabilized ferroelectric liquid crystals are investigated. An explicit experimental evidence, including direct microscopic observation, is reported that the walls display viscous creep motions at small length scales, being dependent on the amplitude U and frequency f of an applied voltage. The relaxation- to- creep transition is analyzed using both the electro- optical response and dynamic hysteresis data recorded at different values of U and f. It is shown that, at any fixed temperature below the chevron- to- bookshelf transition point, there exists a critical line in the U- f plane, separating the relaxation and creep dynamic regions. In contrast to field- activated nonlocal excitations discovered in various disordered media, the creep motions of zigzag walls are found to occur not only at low or very low field frequencies.

An analysis of consistency of dielectric and optical response methods is carried out for surface stabilized ferroelectric liquid crystals (SSFLC) with chevron geometry. The consistency is found both theoretically and experimentally for weak external electric fields of intermediate frequencies, for which the response of SSFLC is dominated by collective relaxation processes due to azimuthal reorientation of molecules arranging chevron layers. The methods are experimentally shown to lack consistency within very low, relatively low, and high field-frequency ranges. The disagreement appearing at relatively low frequencies is argued to be a consequence of different recording by dielectric and optical techniques the dynamics of zig-zag walls, forming defects in chevron structure.

The chiral liquid crystalline (S)-4-(methylheptyloxycarbonyl) phenyl 4-(4′-octyloxyphenylcarbonyloxy) benzoate is distinguished by the exceptionally large temperature range of its phase. This allowed us to study in detail the director tilt angle and electric permittivity of in comparison to the temperature T and the electric field strength E. We basically observed that the SmCα temperature range is reduced at increasing electric field strength and completely disappears beyond a certain critical field. Tilt angle measurements were supplemented by dielectric studies, which accurately indicated the field-dependent phase transition temperatures and led to the accurate phase diagram in the T, E-plane.

The dielectric response of surface stabilized ferroelectric liquid crystals with chevron layer structure is studied within low and intermediate frequency ranges, characteristic for collective molecular excitations. By analytically solving the dynamic equation for collective molecular fluctuations under a weak alternating electric field, it is demonstrated that chevron cells stabilized by both nonpolar and polar surface interactions undergo at medium frequencies two Debye relaxation processes, connected with two chevron slabs, on opposite sides of the interface plane. This result is confirmed, experimentally, making use of the electro-optic technique. Based on qualitative arguments supported by microscopic observations of zigzag defects at different frequencies and amplitudes of the external electric field, it is shown that, at low frequencies, the electro-optic response of chevron samples is determined by three kinds of motions of zigzag walls. The first two dynamic categories are related to collective relaxation processes at weak fields, within smectic A layers forming zigzag walls, and drift or creep motions of thick walls occurring at stronger field amplitudes. Dynamic processes of the third kind correspond to sliding of zigzag walls, which appear at yet stronger field amplitudes, but below the switching threshold.

Weak external electric field E causes two effects in chiral smectic liquid crystals: linear change of the optic axis direction and modi. cation of the shape of the indicatrix. The relative changes in the light intensity consist of two components. The first component represents the modulation with the fundamental frequency f and the second one, with the doubled frequency 2f ( second harmonic). We carried out measurements of the electrooptic modulation at both the first and second harmonics frequency. The applied calibration procedure allowed for expressing the experimental results as angular quantities independent of experimental conditions. Therefore we were able to determine the absolute values of the coefficients describing both effects of the electric field on the optical properties of the sample. We studied the temperature dependence of the first and second order coefficients in the ferroelectric smectic C* and paraelectric smectic A phases. The experiments delivered an information on the structure of tilted smectic liquid crystals and its dependence on the electric field.

A method of determining the local polarization in chiral smectic liquid crystals is proposed. The method consists of simultaneous measurement of the electric permittivity and depth of electro-optic modulation in a helical liquid crystal using weak ac electric field. The spontaneous polarization of a single smectic layer determined in this way contains both the flexoelectric and piezoelectric component. On the other hand, the polarization measured using strong electric fields in a switching experiment contains the piezoelectric component only. The comparison of polarization obtained using strong and weak fields makes it possible to determine the value of flexoelectric polarization. This kind of measurement has been performed for two ferroelectric liquid crystals having quite small absolute values of the spontaneous polarization (few nC/cm(2)). It turned out that in both cases, the flexoelectric polarization is of the same order of magnitude as the piezoelectric one.