Yu. P. Panarin

The investigation of the relaxation processes in antiferroelectric liquid crystals by broad band dielectric and electro-optic spectroscopy

Wide band dielectric and electro-optic spectroscopy of an antiferroelectric liquid crystal (AFLC) has been carried out over a range of frequencies from 1 kHz to 1 GHz. The AFLC sample under investigation possesses a variety of different ferri-, ferro-, and antiferro-electric phases. Dielectric spectroscopy of the LC cells in the antiferroelectric phases reveals both collective and individual dynamics of molecules. In the antiferroelectric SmCA phase, three dielectric relaxation processes are found in the absence of the bias and an additional relaxation process appears under the bias field. A solution of the dynamical equation of the director subject to a weak alternating field for the anitferroelectric helix has been found. A comparison of the results of electro-optic and dielectric spectroscopy with a theoretical study enables a determination of the origin of the relaxation processes in antiferroelectric phases. The mechanism for the distortion of the antiferroelectric helix has been determined using die...

The investigation of the relaxation processes in antiferroelectric liquid crystals by electro-optic spectroscopy

Electrooptic spectroscopy of an antiferroelectric liquid crystal is carried out over a range of frequencies from 1 Hz to 100 kHz. In the antiferroelectric SmCA phase two relaxation processes are found, one at the fundamental frequency of a mode and the second at twice the frequency of a different mode. A comparison of the results of the electro-optic spectroscopy with a theoretical study of the motion of the director of an antiferroelectric helix subject to a weak alternating field enables a determination of the origin of the relaxation processes in antiferroelectric phases.

An investigation of the field-induced ferrielectric subphases in antiferroelectric liquid crystals

We report results of detailed investigations of the dielectric response of two antiferroelectric liquid crystal materials over a wide range of frequencies as a function of bias voltage and of temperature. On the basis of extensive measurements of the tilt angle, the spontaneous polarization, and the optical transmittance as a function of voltage, we conclude that there appears to exist a previously undetected, new field-induced phase (referred to as the X phase) with uniform monodomain structure. This new phase seems to be stable over a wide range of applied voltages and possesses effective values of spontaneous polarization and tilt angle within 70-80% of their maximum values. Although the results appear to rule out the presence of the SmCalpha * phase, nevertheless, the new phase has some characteristics similar to those of the SmCalpha * phase. The differences between the characteristics of the two phases are detailed.

Investigation of the TGBA* phase in a ferroelectric liquid crystal using dielectric spectroscopy

The dielectric response of a ferroelectric liquid crystal with the TGBG* phase has been investigated over the frequency range 0.1-106 Hz. The dielectric properties as a function of temperature and direct bias voltages of the TGBA* phase exhibit behaviour different from that in the smectic A (SmA) phase. From the dielectric spectra, two relaxation processes, instead of only one process (the soft mode) previously known, have been found for the helical TGBA* phase in the absence of the direct bias voltage. These are assigned to (a) the soft mode and (b) a relaxation process in the grain boundaries. The latter process is found for the first time. The dielectric response for the SmC*-TGBA* transition suggests a weakly first-order phase transition. The effect of direct bias voltage on the dielectric spectra of the TGBA* phase differs from that of the SmA phase in terms of (a) the relaxation frequency of the soft mode decreases instead of increasing with increase in the bias voltage-suggested to arise because the TGB helix is unwound by the electric field, and (b) a relaxation process is found to emerge for higher bias voltages. The latter is assigned to the deformation process of the structure because the TGB helix is disturbed by the electric field.

Observation of a possible random ferroelectric liquid crystal phase

A random ferroelectric tilted smectic phase has been observed and investigated for the first time. In this phase, the molecular director in smectic layers is tilted with respect to the smectic layer normal but the tilt directions in different smectic layers are randomly oriented. This structure is confirmed by electrooptics, effective birefringence measurements and dielectric spectroscopy.

Liquid Crystal Based Optical Switches

Optical switches using two electro-optical effects in liquid crystals are proposed and investigated for incorporation in a switch matrix for optical networks. These two devices employ selective reflection in cholesteric layers and total reflection at the border between glass and nematic liquid crystal. Prototypes of these switches have been designed and their parameters have been investigated. Initial results suggest the switching contrast ratio of 38.5 and 42 dB, respectively, with insertion loss about 2.3…2.9 dB for polarized light.

On the V-shaped switching in antiferroelectric liquid crystals

Abstract The electrooptic response of a thin cell of antiferroelectric liquid crystal is investigated as functions of frequency and temperature. It was found, that under specific conditions the FLC cell exhibits the V-shaped switching, the parameters of which are strongly dependent on the frequency of applied electric field. The frequency dependence of electrooptic response has been modelled by taking in account the phenomenon of ion transport. The conditions for a hysteresis-free V-shaped switching were found from the model.

Pyroelectric properties of ferroelectric liquid crystal cells with chevron, bookshelf, and helical structures

The pyroelectric properties of ferroelectric liquid crystal (FLC) cells possessing helical and surface stabilized structures have been investigated. Pyroelectric properties of a FLC cell depend on the applied voltage, the internal smectic layer structure, and the ratio of the cell thickness to the material pitch d/p0. For surface‐stabilized FLC (SSFLC) cells, hysteresis of the pyroelectric signal was observed for both the chevron and the bookshelf structures and is explained by the intermediate domain states that arise from a distribution in the pretilt azimuthal angle. For the chevron structure a reverse hysteresis loop is observed which is explained by a stronger competitive effect of the ionic field compared to the surface anchoring forces. Cells containing helical structures are studied and for cells with a large d/p0 ratio a linear response of the signal to applied voltage is found. Possible implications of the results obtained for SSFLC cells for which the change in the spontaneous polarization in t...

Experimental study of de Vries properties in antiferroelectric smectic liquid crystals.

Results of the experimental study on different antiferroelectric liquid crystal (AFLC) materials are presented using a number of techniques such as the optical birefringence, electro-optics and the measurements of optical thickness of free-standing films. Despite differences in the molecular structures of the various AFLC materials studied, these are found to exhibit a de Vries type of smecticA* (SmA*) properties in a temperature range higher than SmC* . This correlation leads to the conclusion that these two classes of liquid crystals are related to each other. Furthermore, we suggest that these arise from the same physical mechanism, namely the existence of the weak synclinic (or reduced anticlinic) correlations between the neighbouring molecular tilt directions.

Flexoelectric Behavior of a Bimesogenic Liquid Crystal

The effective flexoelectric coefficient and flexoelectric polarization are investigated for a bimesogenic liquid crystal CBC11CB with a twist bend nematic phase (Ntb). The effective flexoelectric coefficients for CBC11CB in the ordinary nematic phase (N) are obtained to be much larger in comparison to conventional calamitics, and is almost twice higher for CBC11CB than that reported for another odd symmetric bimesogen. The flexoelectric polarization (Pf) in the N phase is found to be almost 2.5 times higher than in rod-like molecules. Additionally, Pf is obtained to be much larger in the Ntb phase than in the nematic phase.