Nonlinear-optical properties of nematic liquid crystal 5CB doped with a small amount of high-molecular polymer MEH-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]) have been studied. This liquid crystalline system was shown to exhibit a negative nonlinearity due to the light-induced director reorientation away from the light-field direction.

Holographic gratings of different periods were recorded in the photopolymer by two laser beams with opposite circular polarization and a nematic liquid crystal was brought in contact with them. The anchoring conditions for the nematic were modulated in a similar way and an induced, field controlled replica manifested a characteristic asymmetric diffraction for the beams of opposite circular polarizations with dramatically enhanced diffraction efficiency. In fact the situation is more complicated than expected and our modelling of the director distribution for a finite anchoring strength explains the experimental data by considering an additional modulated structure at the polymer-nematic interface.

When a nematic liquid crystal layer is brought in contact with a photosensitive polymer, on which a holographic grating is recorded by two laser beams with opposite circular polarization, due to a spatial modulation of the direction of the local optical axis, the photopolymer modulates the anchoring conditions for NLC in a similar way. As a result, a similar grating is induced in NLC with the enhanced diffraction efficiency and specific diffraction properties controlled by electric field.6 Here we show that, even in the case of a typical finite value of the anchoring energy, the numerical modeling confirms the characteristic asymmetric diffraction of the circularly polarized reconstructing beam and exchange by energy between the zero and first-order diffracted beams. Some other properties, such as reduced diffraction efficiency, appearance of higher diffraction orders and too-large optical phase retardation of a nematic cell at the areas occupied by gratings, can also be modeled by assuming some corrugation of the surface.

An investigation was carried out on a hybrid structure consisting of a photosensitive polymer in contact with a nematic liquid crystal (NLC) layer. Before assembling the structure, holographic gratings of different periods were recorded in the photopolymer by two laser beams with opposite circular polarization. Such gratings are known for their unique asymmetric diffraction in case of elliptically polarized light and can be used in optical polarization devices. Due to a spatial modulation of the direction of the local optical axis, the photopolymer modulates the anchoring conditions for NLC in a similar way. As a result, a new grating is induced in NLC with the enhanced diffraction efficiency (in a particular experiment it was four orders of magnitude higher than that of the photopolymer). The NLC grating also shows the characteristic asymmetric diffraction for the reconstructing beams of opposite circular polarizations. The diffraction efficiency is easily controlled by an external electric field.

Orienting influence of femtosecond laser pulses on nematic liquid crystals has been studied. We have shown experimentally that in both a nematic liquid crystal doped with anthraquinone dye D4 and nematic matrix "mixture A" (consisting of azoxy-molecules) the efficiency of the liquid-crystal director reorientation under the influence of femtosecond pulses is less than for continuous wave radiation. The mechanism responsible for this difference is associated with light-induced variation of the anchoring conditions.

A detailed study of electro-optical response has been carried out over wide range of the thickness of homeotropically aligned ferroelectric liquid crystal layers for in-plane electric field between inter-digitated electrodes. The field dependencies of switching times, optical transmission and contrast ratio have been measured. The physical mechanisms of the observed effects are discussed.

The switching dynamics for the “bookshelf” and “chevron” geometry in surface stabilized ferroelectric liquid crystals (SSFLC) has been investigated by numerical simulation and experimentally. We have studied the electrooptical switching in FLC cells oriented by different alignment layers and proposed a new anchoring model for surface stabilized states. According to the model, the difference between zenithal and azimuthal anchoring strength with account of electric properties of the alighnment layers and FLC is of crucial significance for the existence of the bistable states. We formulated general criteria (necessary and sufficient conditions) for the inverse and normal bistable switching. These criteria involve all the principal parameters of the alignment layers and FLC material.

A new type of reversible orientation first-order transitions in a nematic liquid crystal (NLC) in the optical and low-frequency electric fields is studied experimentally and theoretically. It is shown that these transitions, characterised by a distinct bistability of the director field both upon variation of the light beam power P (the low-frequency electric-field voltage U = const) and the electric-field voltage U (P = const), are related to a finite diameter of the light beam and the oblique incidence of the beam on the NLC.

A detailed study of the dynamics of electro-optical response has been carried out over the whole temperature range of the antiferroelectric B2 phase of a compound with bent-core shape molecules, a homolog (n=14) of the series 4-chloro-1,3-phenylene bis[4-(4-n-alkylphenylimino)benzoates]. Two types of stripe domains were observed with opposite handedness and simultaneous clock and anticlock motion of the director in the neighboring domains. The temperature dependence of the interlayer potential has been found from the threshold of the transition from the ground antiferroelectric (AF) state to the field-induced ferroelectric (F) state. The rotational viscosity gamma(phi) has been calculated from the dynamics of the field-induced azimuthal director switching between F-F and AF-F states and free relaxation of the director from F to AF state. The electro-optical response was also observed below the AF-F threshold. The latter was attributed to the soft-mode distortion of the molecular tilt angle in the vicinity of the transition from the B2 phase to the isotropic phase.

We report on the observation and investigation of the light-induced director reorientation due to the change in the orienting properties of the boundary surfaces of the homeotropically aligned NLCs in the samples containing pure (nondoped) nematic matrices and in the samples doped with the conformationally active azo and stilbene dyes, as well as the conformationally stable anthraquinone dyes. The specific features of the light-induced memory are established, among which an effect of light-initiated director self-reorientation--the continuation of director reorientation upon switching off theillumination--is the most interesting one.

An interaction of the director of NLCs, doped with mono- and diazodyes and showing the sign-inversion nonlinearity, with the simultaneously applied light and low-frequency electric fields has been studied using the technique of the aberrational self-action. Mathematical simulation of this interaction was performed. A fair fit to the experimental data was obtained.

A detailed study of the field induced polarization switching has been carried out over the whole temperature range of the antiferroelectric B-2 phase of a banana-shaped compound, the tetradecyl homologue (n=14) of the series 4-chloro-1,3-phenylene bis[4-(4-n-alkylphenylimino)benzoate]s. The fine structure of the current response oscillograms was attributed to the field switching of two antiferroelectric subsystems, most probably the racemic and homogeneously chiral domains which, according to the Boulder group, appear as a result of the spontaneous breaking of mirror symmetry. The temperature and field dependences of polarization as well as switching times and the corresponding viscosity of the substance have been measured.

The temperature dependencies of surface polarization in the nematic (N) and smectic A (SmA) phases have been measured for both the planar and homeotropic orientations of 4-octyloxy-4′-cyanobiphenyl (8OCB). Polarization was determined in the field- off regime from the pyroelectric response of the cell to a short laser pulse, and the temperature increment was calculated from the in situ measurements of the light induced birefringence. The surface polarization in the N-phase for both orientations is directed from an interface into the bulk at similar magnitudes (about 0.5–0.8 pC/m) but its change at the N–SmA transition was qualitatively different for the two cases. The experimental data are consistent with the quadrupolar model of the order-electric polarization. The field-off measurement technique also allowed for the measurements of the z-component of the flexoelectric polarization of 8OCB in both the N and SmA phases using a hybrid aligned nematic cell. The sum of the flexoelectric coefficients (e1+e3) in both phases was shown to be positive (e1+e3 ≃7.4 pC/m at 40°C), which is opposite to the case of 4-pentyl-4′-cyanobiphenyl (5CB). On the molecular scale, the difference is accounted for by the different distribution of electric charges in molecular quadrupoles forming the corresponding liquid crystalline phases.

The temperature dependences of the surface polarization have been measured at the interface of a conductive glass with both the homogeneously and homeotropically oriented nematic liquid crystal p-methoxybenzylidene-p(')-butylaniline. The polarization was found in the field-off regime from the pyroelectric response of a cell to a short laser pulse, absorbed in the bulk of the liquid crystal. The temperature increment was calculated from the measurements of the birefringence induced by the same light pulse. It has been shown that the surface polarization at the homeotropic (m(h)) and planar (m(p)) interfaces is directed from an interface into the bulk and from the bulk to an interface, respectively (with a magnitude m(h) approximately -0.3 pC/m and m(p) approximately +0.2 pC/m at 25 degrees C). The experimental data may be explained in terms of the quadrupole model of the order-electric polarization with account of some additional contribution from molecular dipoles. The same technique also allows for the measurements of the z component of the flexoelectric polarization using a pyroelectric response of a hybrid (homeoplanar) aligned nematic cell and proper subtracting of the surface contributions. The flexoelectric polarization has been shown to be opposite to the sum of the surface terms m(h)+m(p) and directed from the planar to homeotropic interface. This means that the sum of the flexoelectric coefficients e=(e(1)+e(3)) is positive (e congruent with 1.7 pC/m at 28 degrees C). The temperature dependence of e has been shown to involve a combination of both the quadrupolar and dipolar contributions.

The polarization measurement of a nematic liquid crystal has been performed by a pyroelectric technique using short pulse of a Nd:YAG laser to create temperature increment. The temperature increment has been evaluated independently by monitoring temperature dependent birefringence for a He-Ne laser beam. The sign and absolute magnitude of surface polarization have been evaluated for both the planar and homeotropic orientation of the nematic liquid crystal at a solid substrate. The flexoelectric polarization in a hybrid cells has also been measured using the same pyroelectric technique. The value of the sum of the flexoelectric coefficients (e(1)+e(3)) for 5CB is negative and about - 13pC/m at room temperature.

The light-induced director reorientation in nematic liquid crystals doped with dyes has been studied by the aberration self-action technique. The sign of the orientational optical nonlinearity of NLC doped with conformationally stable anthraquinone dyes is shown to be determined by the structure of dye molecules and independent of the conditions of the experiment. For conformationally active diazo- and monoazodyes the nonlinearity sign depends upon the propagation direction, polarization and energy flux density of the light wave. It can also be changed under action of an external electric field. The experimental data obtained agree with the recently proposed mechanism of collective molecular reorientation derived from a non-central character of molecular interaction and the anisotropy of correlation function and with nonlinearity sign inversion originating from the photo-conformational activity.

The study results of optical, photoluminiscent and scintillation properties of a liquid crystal 4-pentyl-4′-cyanobiphenyl are presented. The scintillation light output of this liquid crystal is about 35% of crystal anthracene, its main decay time constants are 4 and 14ns, and the maximum of light emission spectrum is about 400nm. The light output of a dissolution of green emitting light scintillation dopant R6 in the liquid crystal is about 120% of crystal anthracene. The light output of the frozen dissolution measured at −112°C is about 2.5 times higher as observed at +20°C. In the uniaxially oriented liquid crystal, the predominant intensity direction of emitted light is pointed perpendicular to the liquid crystal director and an appreciable part of the emitted light is elliptically polarized. The possibility to use scintillation properties of liquid crystals is considered both for the improvement of existing particle detector characteristics and for the creation of new gated particle detectors.

An alternative, pyroelectric effect based technique was developed for the direct measurement of the surface polarization in homogeneously and homeotropically oriented nematic cells as well as for the measurement of the flexoelectric polarization in hybrid aligned nematic cells. The pyroelectric response was measured with a modulated beam of a diode laser, absorbed by a dye incorporated in a standard liquid crystal 4-pentyl- 4(')-cyanobiphenyl. The dye provided a gradient of the temperature increment along the cell normal, which allowed for the separation of pyroelectric contributions from both the opposite surfaces and the bulk. The signs and absolute magnitude of the polarizations mentioned have been found over the entire temperature range of the nematic phase.

The results of pyroelectric measurements in SmA phase of ferroelectric liquid crystal are presented. The pyroelectric coefficient as a function of temperature and applied bias voltage was measured and the polarization calculated. The experimental results are explained in terms of a simple theoretical model for the appearance of the macroscopic polarization in the SmA phase of a ferroelectric liquid crystal. The deviation of the smectic layers normal from the rubbing direction at the orienting surfaces and strong anchoring energy of the interactions of LC molecules with the surface are the reasons for the polar order in the SmA phase.

The results of studies of optical, photoluminescence, and scintillation characteristics of 4-amil-4'-cyanobiphenyl liquid crystals are presented. Those crystals Showed a scintillation light yield of similar to 35% of anthracene, fluorescence decay times of 4 and 14 ns, and a maximum of the emission spectrum of similar to 400 nm. The light yield of a solution of a liquid crystal and the R6 green scintillation additive is similar to 120% of anthracene. The light yield of a frozen solution at -120 degrees C was found to be 2.5 times higher than at 20 degrees C. An oriented liquid crystal has an appreciable level of emitted-light polarization. The maximum emitted light intensity in an oriented liquid crystal is perpendicular to the direction along which most long molecular axes are oriented. The possibilities of employing scintillating liquid crystals for improving the characteristics of existing particle detectors and designing new controllable detectors are considered.