K. Nowicka

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 reliable method for detecting phase transitions in liquid crystals with polar order

We present a method for detecting phase transition in liquid crystal based on the analysis of the second harmonic component of electro-optic response spectra. The effectiveness of the method is shown using the well-known prototype antiferroelectric liquid crystal material, 4-(1-methyl-heptyloxy carbonyl) phenyl 4′-octyloxybiphenyl-4-carboxylate (MHPOBC). In contrast to the usual procedures, the temperatures of the phase transitions are determined in simple way during one experiment, even for phases of low thermal stability.

Director distribution and surface anchoring potential in Grandjean-Cano wedge

Theoretical and experimental studies of the director distribution of a chiral nematic liquid crystal placed in a wedge-shaped cell with weak surface anchoring were performed. We measured local value of pitch as function of position inside each Grandjean-Cano zone by registering selective reflection spectra from narrow stripes cut from each Cano zone. We also measured the rotation of the polarisation plane as function of thickness of the wedge sample. These data allowed determination of the angle between the local director and the easy direction on the surface. The obtained director distributions in individual Cano zones were compared with the results of theory and thus used for restoration of the actual shape of the surface anchoring potential.

Nonlinear electro-optical spectroscopy of liquid crystals possessing polar order

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.

Field-induced dynamics of ferroelectric liquid crystals with elastic interfacial confinement

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.

Determination of the bulk rotational viscosity coefficient in a chiral smectic C* liquid crystal

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

Non-linear electro-optical effects in the study of the helical smectic liquid crystals

Measurements of the non-linear electro-optical effects for the well-known prototype liquid crystal material (MHPOBC) are presented. The method to identify liquid crystalline phases and to determine temperatures of phase transitions based on the analysis of the second harmonic component of electro-optical response spectra is used. Applying that method, the values of the frequency () at which the second harmonic electro-optic response (EOR) possesses an extremum are determined for each smectic phase. We suggest that this characteristic frequency correspond to the phase-type mode processes. Furthermore, we show that the usually neglected results on heating can be useful in discussions of dynamical behaviour of second harmonic EOR in case of smectic phases.

High permittivity of chiral tilted smectic phases in a binary mixture

Characterization of structure and properties of the bicomponent mixture of partially fluorinated chiral three ring ester with an chiral alkyl compound by applied different methods such as the differential scanning calorimetry, the electro-optic and the dielectric spectroscopy, and the helical pitch measurement is presented. In particular, we investigate the influence of the texture changes in order to determine the phase transitions in the mixture under investigation.

Linear and non-linear electro-optical measurements in the study of polar liquid crystal phases

ABSTRACT The article shows that the recently presented technique based on the temperature dependence of the characteristic peak frequency (fext) of the real part of second harmonic electro-optic (EO) response can be complementary to the Debye type relaxation method, usually used to study the dynamical behaviour of liquid crystals (LCs). The most important advantages of this method are the sensitivity – no pretransitional effects observed, and simplicity – no fitting procedure necessary. The problem is illustrated with the experimental first and second harmonic EO spectra recorded for LC material with broad temperature range of the helical smectic subphases. The obtained values of the frequency rate of each characteristic response of every polar phase are correlated with the particular collective modes. GRAPHICAL ABSTRACT

Improvement of the blue phase stability in chiral nematic liquid crystal mixtures

ABSTRACT The method of widening of the blue phase (BP) temperature range is presented. By means of polarising optical microscopy, we demonstrate that the temperature regime in which BP is stable is greatly enhanced by the cell thickness changes in mixtures of nematic liquid crystal (LC) and chiral dopant.