Light-induced rotation of dye-doped liquid crystal droplets.

We investigate both theoretically and experimentally the rotational dynamics of micrometric droplets of dye-doped and pure liquid crystal induced by circularly and elliptically polarized laser light. The droplets are dispersed in water and trapped in the focus of the laser beam. Since the optical torque acting on the molecular director is known to be strongly enhanced in light-absorbing dye-doped materials, the question arises whether a similar enhancement takes place also for the overall optical torque acting on the whole droplets. We searched for such enhancement by measuring and comparing the rotation speed of dye-doped droplets induced by a laser beam having a wavelength either inside or outside the dye absorption band, and also comparing it with the rotation of pure liquid crystal droplets. No enhancement was found, confirming that photoinduced dye effects are only associated with an internal exchange of angular momentum between orientational and translational degrees of freedom of matter. Our result provides also direct experimental proof of the existence of a photoinduced stress tensor in the illuminated dye-doped liquid crystal. Finally, peculiar photoinduced dynamical effects are predicted to occur in droplets in which the molecular director is not rigidly locked to the flow, but so far they could not be observed.

Light-induced changes of optical and electrical properties in bent-core azo compounds.

We have studied the optical and electrical properties of two bent-core substances with an azo linkage in their cores. Pump-probe laser studies, direct textural observations, and spectrophotometric recordings show an initial decrease of light transmission, which at larger light intensities (approximately 1 mW/mm2) is followed by a bleaching. Simultaneously the electrical properties (electric conductivity, antiferroelectric polarization, switching threshold, and switching time) decreased monotonically with increasing light intensities. The monotonic decrease of electrical properties indicates that the darkening and bleaching have the same origin, namely, the photochemical isomerization of the azo linkage from the trans to the cis isomer. The material with cis isomer has a lower clearing point and phase separates from the trans-rich domains. Initially the size of the separated isotropic domains is below the visible range, which causes increased scattering. As the size of the isotropic domains increase the scattering disappears and the transmittance becomes the average of the transmittances in the polar tilted smectic and isotropic phases.

Gliding of liquid crystals on soft polymer surfaces.

Magnetic-field-induced reorientation of nematic liquid crystals on polymer layers is studied near the glass transition temperature of the polymer. Kinetic curves for different field strengths and temperatures are presented. A model is developed which takes into account the structural rearrangements in the polymer induced by its interaction with the anisotropic potential of the liquid crystal. Simulations based on the model are in good quantitative agreement with the experimental data.

Optical generation of inversion walls in nematic liquid crystals.

An approach to generate inversion walls in nematic liquid crystals using optical reorientation is described. Planar cells with small pretilt are exposed to an oblique laser beam, which reverses the director tilt angle within the irradiated area. During the electric Freedericksz transition an inversion wall surrounding the laser spot is formed. A theoretical relation is provided for the critical laser intensity necessary to produce an inversion wall. Experimental results in a dye-doped nematic are in satisfactory agreement with the theory.

Anomalous intensity dependence of optical reorientation in azo-dye-doped nematic liquid crystals.

The intensity dependence of optical nonlinearity in a nematic liquid crystal doped with an azo-dye is investigated. The reorientational part of the nonlinearity changes from self-defocusing to self-focusing character while the intensity passes through the saturation value of trans-cis photoisomerization. This observation, in accordance with previous models, indicates that the optical torque generated by the trans-isomers is of opposite sign than the torque generated by the cis-isomers. At very low intensities a further reorientational nonlinearity was found, which is attributed to light-induced orientational redistribution of adsorbed molecules at the surface.

Optically induced periodic structures in smectic-C liquid crystals.

We explore periodic structures of smectic-C (SmC) liquid crystals, induced optically by a polarization grating. The studied cells contain a passive surface of rubbed polyimide and an active photosensitive substrate of azo-dye doped polyimide. In a nematic phase the director field can be periodic independent of the angle between the grating vector and the rubbing direction. In the SmA phase periodic structure can be induced only by layer undulations. The SmC behaves similarly to the nematic phase, but the director can rotate only on a cone, which results in a more complex geometry. The periodic pattern is superimposed with four different director and layer structures. In spite of the coexistence of the nonuniform structures the diffraction efficiency is better in the SmC, than in the nematic phase.

Kinetics of director gliding on a polymer-liquid-crystal interface.

Magnetic-field-induced surface reorientation of nematic liquid crystals on polyethyl-methacrylate films is studied. The experiments indicate that the time scale of director gliding expands from a fraction of a second to several hours. A power-law distribution function of relaxation times provides very good agreement between measurements and calculated gliding curves. From the model an upper limit of the extrapolation length can be extracted, which points to a surprisingly strong equilibrium azimuthal anchoring of the liquid crystal on the polymer layer.

Optical waveguiding in bent-core liquid-crystal filaments.

We demonstrate optical waveguiding in recently discovered free-standing bent-core liquid-crystal filaments. The bent-core liquid-crystal molecules in air self-assemble into a cylindrical geometry that is "solidlike" along the radial direction of the filament and liquid in the axial direction of the filament. These filaments are unique not only because they are fluids, but also because they are anisotropic. For this reason, their waveguiding properties not predictable need to be characterized. The light power transmitted through the filament was found to be independent of temperature from 180 degrees C to near room temperature. Initial defects of newly pulled filaments were found to self-anneal, thus leaving defect-free fibers, where light scattering was found to be insignificant. The absorbance was found to be strongly wavelength dependent in the visible regime and relatively small in the infrared range. A self-assembled optical waveguide with self-annealing fluid properties may have promising applications in optical communications or in optical microchips.

Anomalous wavelength dependence of the dye-induced optical reorientation in nematic liquid crystals.

We report on Z-scan measurements of the spectral dependence of dye-induced enhancement of optical torque in nematic liquid crystals. It is found that the enhancement factor changes sign from positive to negative as photon energy increases from 2.4 to 2.7 eV. Modification of our earlier model is proposed to describe the observed spectral dependence.

Influence of anthraquinone dyes on optical reorientation of nematic liquid crystals.

Optical reorientation measurements in dye-doped nematic liquid crystals are reported. The dye-induced optical torque exceeds significantly the normal optical torque for all anthraquinone dyes investigated. With one of the dyes, optical Freedericksz transition in a planar layer is demonstrated.