A. S. Zolot'ko

Laser Induced Reorientation of Nematic Liquid Crystals

Abstract A significant increase of the divergence and appearance of a fringe system has been observed by illuminating of MBBA (p-n-methoxybenzilidene-p-butylaniline) and OCB (octyl-cyano-biphenyl) nematic liquid crystalline samples with a collimated beam of an argon ion laser. The dependence of this effect on laser power, beam polarization, and angle of incidence has been studied in homeopolar and planar sandwich-like cells of 50-150 μm thickness, in the nematic phase. At a homeopolar cell of MBBA (150 μm thickness), at normal incidence a threshold laser power of 45 m W was found. The phenomena can be explained as deformation of the orientation by the Fredericks effect due to light fields. The observations are in good agreement with the theoretical predictions including an estimation of the laser power threshold.

Interaction of light with a NLC–dendrimer system

The interaction of light with a nematic liquid crystal (NLC) doped with dendrimers containing terminal light-absorbing azobenzene fragments and a low-molar-mass dye identical to the fragments was studied in detail. Both the dendrimers and the dye induce orientational nonlinearity in a transparent nematic host. For the fifth-generation dendrimer, the nonlinearity is negative (director is rotated away from the light field direction). For the first-generation dendrimer and the dye, the nonlinearity sign depends on the light propagation direction with respect to the NLC director, light polarisation and external electric field. The results obtained show that complication of the molecular structure of the dopant, i.e. passing from the ‘free’ dye molecule to the first- and fifth-generation dendrimers, results in an increase of the efficiency of the orienting effect of the light.

Light-Induced Director Reorientation in Nematic Liquid Crystals Doped with Azobenzene-Containing Macromolecules of Different Architecture

The results of the studies of the orientational optical nonlinearity of a nematic host doped with high-molar-mass compounds of different molecular architecture such as a comb-like polymer and a dendrimer containing azobenzene chromophores are presented. The nonlinearity induced by these compounds was found to be higher than that induced by the low-molar-mass dyes similar in structure to the light-absorbing constituents of the polymer and dendrimer. Possible reasons for the nonlinearity enhancement for macromolecular dopants are discussed.

Light Diffraction by Laser Beam Created “Channels” in Nematic Liquid Crystals

Abstract Light diffraction was found and studied on “Channels” i.e. cylindrical regions of isotropic liquid in the nematic phase of a liquid crystal, created along the path of a laser beam by heating. Reorientation of the director is shown to take place at the boundary of these channels.

Orientational Interaction of a Light Beam and NLCs Subjected to External DC Field

Orientational interaction of a light beam and nematic liquid crystals subjected to an external electrical dc field has been studied. It is shown that the intensity distribution in the aberrational pattern appeared in the cross section of the transmitted beam can be highly asymmetric, unlike the conventional, rather centrally symmetric, ring system. The director-field distortion produced by the combined action of light and dc fields is independent of the light polarization. The results are explained by the surface photorefractivity.

Reorientation of director of nematic liquid crystals, doped with azodyes, under light and low-frequency fields

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.

Optical Director Reorientation in NLCs Doped with Light-Absorbing Codendrimers of Different Generations

The orientational optical nonlinearity induced in NLCs by light-absorbing carbosilane codendrimers with statistical distribution of terminal aliphatic and azobenzene fragments has been studied comparatively for different (from the second to the fourth) codendrimer generations. It was found that all the codendrimers induce the negative nonlinearity (director rotates away from the light field thus decreasing the refractive index of the extraordinary wave). The orientational response increases with the generation number. The optical bistability is manifested for NLCs doped with the codendrimers of the second and third generations.

Peculiarities of Light-Induced Memory in Nlcs

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.

Orientational Light Interaction with Nematic Liquid Crystal Doped with MEH-PPV Polymer

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.

Light Self-Action in NLCs with Orientational and Thermal Nonlinearities

The properties of a light beam passed through an NLC layer with positive orientational nonlinearity and then reflected from the NLC–substrate interface have been experimentally studied. The divergence and the intensity profile of this beam were found to be quite different from those of the beam transmitted through the NLC cell. It is shown that the aberrational pattern in the cross-section of the reflected beam and the dynamics of its formation can be used to study the orientational nonlinearity even in the presence of thermal effects.