Wave mixing in electric field biased nematic thin films

Wave mixing due to molecular reorientation in electric field biased nematic thin films is studied. By using the continuum theory, general formulae have been derived for the orientational angle distribution. The concept of a thin film grating is utilized to describe the diffraction efficiency. Experimental results which exhibit the behaviours of the transmission beam and diffraction beams have been obtained as predicted by the numerical calculation.     

Wave mixing in electric field biased nematic thin films

Abstract

Wave mixing due to molecular reorientation in electric field biased nematic thin films is studied. By using the continuum theory, general formulae have been derived for the orientational angle distribution. The concept of a thin film grating is utilized to describe the diffraction efficiency. Experimental results which exhibit the behaviours of the transmission beam and diffraction beams have been obtained as predicted by the numerical calculation.     

Determination of the surface anchoring potential of a nematic in contact with a substrate

We have developed a method for determining the surface anchoring potential for nematics in contact with a substrate that provides director alignment. Our main result is that the surface torque and hence the anchoring potential may be determined from either dielectric or optical phase response of a nematic undergoing a Freedericksz transition. The method is based on the Frank-Oseen continuum theory, and makes no assumptions about the functional form of the potential. We have measured the surface anchoring potential of two types of substrate in contact with the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl. The surfaces were ITO-coated float glass, coated either with obliquely evaporated SiO or a buffed polymer film. Comparison of the results obtained from capacitance and optical measurements provides an estimate of the goodness of the method.     

Transient holographic grating in azo-dye-doped liquid crystals with off-resonant light

Holographic gratings in nematic liquid crystals doped with Disperse Red 1 (DR1) were investigated using a linearly polarised He–Ne laser. Although the electric transitions of the samples were almost off-resonant, the diffraction of the transient holographic grating was found to exhibit a sinusoidal feature with changing the polarisation of recording beams, and the diffraction intensities and response time of the gratings were strongly dependent on DR1 concentration. The results revealed that the diffraction of holographic grating depends on the rate of trans–cis isomerisation of DR1 molecules. Moreover, the temperature dependence of diffraction indicated that the grating formation mechanism was attributed to photoinduced reorientation of photoisomerisation effect.     

Light-induced Freedericksz transition in a nematic liquid crystal with chiral dopant

The influence of phototransformed molecules with chiralproperties changing on the absorption of light field on the light-induced Freedericksz transition threshold in a homeotropically oriented nematic cell is considered. It is shown that the appearance of the light-induced chiral molecules can decrease or increase the Freedericksz threshold value depending on the chirality sign of the phototransformed molecules and of the initial chiral dopant. Expressions for the threshold are obtained for circular and linear polarization of the incident light. The dependence of the threshold on the periodicity of the spatially modulated light intensity is estimated for large periods of modulation. The dependence of dopant threshold chirality on the director anchoring energy has been found.     

Linear analysis of transient pattern evolution in the non-Freedericksz twist geometry

We present a study of the evolution of the transient periodic pattern in the nematic director field reorientation in the magnetic non-Fréedericksz twist geometry. The stability of the uniform director field reorientation with respect to periodic perturbations is studied as a function of the magnetic field H, the angle α between H and the initial homogeneous nematic director n₀ (H not normal to n₀) and the nematic viscoelastic parameters. The results predict that for α < π/2, the amplitude of the periodic modes becomes damped after a critical time and eventually fade away and consequently does not give way to periodic inversion walls as in the Freedericksz geometry (α = π/2). Also for α < π/2, it is predicted that the selected periodic modes have progressively smaller wave vectors as the director reorients back to equilibrium. The amplitude becomes damped earlier and the wave vector of the periodic pattern decreases faster with time when the magnetic field acts away from the normal to the initial director.     

Effect of an electric field on defects in a nematic liquid crystal with variable surface anchoring

We report experimental studies on defects in a nematic liquid crystal with negative dielectric anisotropy mounted in a cell with perfluoropolymer-coated surfaces. The sample exhibits a discontinuous anchoring transition from planar to homeotropic on cooling at zero or a small electric field, and above a cross-over voltage a continuous ‘inverse Freedericksz transition’, at which the director starts tilting in opposite directions at the two surfaces. Defects of strength ±1/2 are either annihilated or expelled when the director tilts. On the other hand, disclination lines of ±1 which end in partial point defects (boojums) at the surfaces in the planar alignment regime acquire point defects of strength ±1 at the midplane of the cell when the director tilts. At a low enough temperature, the homeotropic anchoring becomes strong, and an electric field above the Freedericksz threshold generates the usual umbilic defects, which follow the dynamic scaling laws found in earlier studies.     

Field dependence of the optical activity of a chiral liquid crystal near the isotropic–smectic A transition

Abstract

Chiral liquid crystals exhibit molecular optical activity in the isotropic phase. We have studied the evolution of the optical activity as a function of an applied electric field on a 76·2 μm film of the chiral liquid crystal W7, which exhibits an isotropic–smectic A transition at approximately 40°C. We measured the optical activity by recording the rotation of the plane of polarization of an incident linearly polarized ray of light, provided by a He–Ne laser. The applied biasing electric field is parallel to the direction of the incident beam. We find that at 41·0°C, the plane of polarization shifts from ?1·1° for an applied voltage of 30 V to a maximum of ?4·0° at 70 V. The absolute value of the signal decreases beyond this voltage. These shifts are in the direction of the smectic A phase and are in general larger than those observed as a function of temperature. Close to the isotropic–smectic A phase transition, molecules inside the liquid coalesce to form dynamic coherent groups, which have smectic nature. These groups are randomly oriented with respect to each other in the absence of an electric field. The application of an electric field causes the molecules within these groups to align along the direction of the field and to contribute coherently to the optical activity of the system. The way the molecules align with the field depends on the relative values of the polarizability α, which contributes to the alignment of the long axis of the molecule, and the dipole moment p, which contributes to the alignment of the short axis of the molecule. Our preliminary results and calculations suggest that for small fields, the electric field couples with the dipole moment p, whereas for fields in excess of 70 V, the field couples with the polarizability of the long axis of the molecule, causing a rotational reorientation of the molecules in the isotropic phase. The value of the field at which this reorientation occurs may be controlled by temperature.     

Liquid crystal orientation by holographic phase gratings recorded on photosensitive Langmuir-Blodgett films

Holographic gratings were recorded on photosensitive Langmuir-Blodgett films characterized by UV spectroscopy, birefringence measurements and atomic force microscopy. Different polarizations of Ar laser writing beams create particular patterns of chromophore orientation in the diffraction spots. The gratings were shown to orient a nematic liquid crystal with the director parallel to the axes of the chromophores predetermined by film irradiation. In the case of the sp grating (recorded with laser beams polarized perpendicular to each other), two equivalent easy directions for the liquid crystal orientations at 90degree with respect to each other were observed; that is a quasi-bistable anchoring interface had been prepared. Measurements of the pretilt angles theta1s and anchoring energy Ws of 5CB on different holographic gratings show that this orientation technique is very promising for display technology.     

Influence of anchoring at a nematic cell surface on threshold spatially periodic reorientation of a director

We have analysed the influence of surface director anchoring in a planar flexoelectric nematic cell on the threshold spatially periodic reorientation of the director in an external dc electric field. By minimizing the free energy of the nematic cell we obtained the equations for a director and numerically solved them in the one elastic constant approximation. The dependences of the threshold electric field and the spatial period of director structure on the azimuthal and polar anchoring energy, as well as the flexoelectric parameters, are determined. It is shown that the domain of the flexoelectric parameter values, at which the spatially periodic reorientation of a director takes place, increases with decreasing azimuthal anchoring energy and increasing polar anchoring energy.     

Interaction of electromagnetic waves in planar waveguide with a nematic layer

This work is a theoretical study of energy exchange between two coupled TE-wave modes on director diffraction grating in a planar waveguide containing a layer of nematic liquid crystal. The diffraction grating is produced by an external electric field in the nematic layer with spatial periodic anchoring energy between director and waveguide surface. The intensity of a signal mode at the output of the nematic layer has been calculated in dependence of anchoring energy amplitude and modulation period, the size of nematic layer and electrical field value. The cases of co-propagating and oppositely propagating modes have been analysed. The analytical expressions that describe the maximum values of signal mode intensity have been derived. The maximum intensity value output from the nematic has been shown to depend monotonously on the anchoring energy parameters in the case of oppositely propagating wave modes and non-monotonously in the case of co-propagating wave modes. In both cases, the maximum value of signal mode intensity grows with the increase in electric field.     

Orientational optical nonlinearity of nematic liquid crystals induced by high-molecular-mass azo-containing compounds

Processes of light-induced reorientation of nematic liquid-crystalline molecules induced by the addition of low concentrations (0.1–2.0 wt %) of comb-shaped polymers and carbosilane dendrimers containing azobenzene fragments are studied. When the molecular structure of the above compounds becomes more complicated, the induced orientational nonlinearity increases. The introduction of 2G and 3G dendrimers into a nematic has for the first time made it possible to visualize and study a purely optical first-order Freedericksz transition in the field of a linearly polarized wave.     

Monodomain liquid crystalline networks: reorientation mechanism from uniform to stripe domains

Monodomain acrylate-based networks have been synthesized by a two-step cross-linking procedure using gamma-irradiation. Strain-induced reorientation of the nematic director is studied by X-ray diffraction measurements. The geometrical shape (aspect ratio) of the monodomain films is found to affect drastically the dynamics and mechanics of the reorientation transition. Uniform continuous rotation of the director occurs in narrow samples with an aspect ratio AR=12 when the external mechanical field is applied perpendicular to the initial orientation. Under the same conditions, films having a lower aspect ratio (AR=2.5) demonstrate the formation of stripe domains with an alternating sense of director rotation (clockwise and counter-clockwise towards the stress axis). Deformation of a square film (AR=1) generates stripe domains in the geometrical centre of the sample, whereas a uniform continuous rotation is observed in other regions of the film. Finally, a comparison of experimental data and theoretical predictions is presented and discussed.     

Monodomain liquid crystalline networks: reorientation mechanism from uniform to stripe domains

Monodomain acrylate-based networks have been synthesized by a two-step cross-linking procedure using gamma-irradiation. Strain-induced reorientation of the nematic director is studied by X-ray diffraction measurements. The geometrical shape (aspect ratio) of the monodomain films is found to affect drastically the dynamics and mechanics of the reorientation transition. Uniform continuous rotation of the director occurs in narrow samples with an aspect ratio AR=12 when the external mechanical field is applied perpendicular to the initial orientation. Under the same conditions, films having a lower aspect ratio (AR=2.5) demonstrate the formation of stripe domains with an alternating sense of director rotation (clockwise and counter-clockwise towards the stress axis). Deformation of a square film (AR=1) generates stripe domains in the geometrical centre of the sample, whereas a uniform continuous rotation is observed in other regions of the film. Finally, a comparison of experimental data and theoretical predictions is presented and discussed.     

Threshold voltage of the Freedericksz transition in a hexatic smectic I phase

Using the linearized equations of the elastic continuum theory, simple formulae for the threshold voltage for the Freedericksz transition in planar films of the hexatic smectic I phase are derived. In contrast to the Freedericksz transition of a nematic film, this threshold strongly depends on the film thickness. The behaviour of the film in the weakly non-linear region slightly above the threshold is also considered to characterize the film instability.     

Viscoelastic coefficients of glass-forming nematics

In this paper, we report measurements of the viscoelastic properties of nematic liquid crystals which exhibit a glass transition in the nematic phase. We have studied the Freedericksz transition in planar cells with a magneto-optical method. K₁ was determined from the critical field, and the rotational viscosity, γ₁, from the response time for the director orientation by the external field. We found a temperature dependence of γ₁ of the Vogel type, with absolute values ranging over several orders of magnitude, and K₁ values similar to those of conventional thermotropic low molar mass nematics.     

Droplet shape and reorientation fields in nematic droplet/polymer films

Polymer films containing droplets of nematic liquid crystal form an important class of new electro-optic light valves and displays. While previous work has shown that the nematic droplet size is an important factor in the electro-optic properties of these films, here we report that the droplet shape is equally important in determing the electro-optics of the film. Electron micrographs show that for films using polyvinyl alcohol as the polymeric binder the cavities formed by the polymer matrix are oblate in nature, and aligned with the minor axis perpendicular to the film plane. In oblate cavities the elastic-deformation free energy is minimized when the director field in the droplet is aligned along a major axis of the spheroid; the electric field performs work on the nematic in reorienting the nematic into a higher-energy state, equal to the elastic-free-energy difference between the two configurations. Calculations and experiment are used to estimate the elastic and electric field free-energy-density changes that occur upon reorientation of the nematic droplet. The general agreement between these two values is used to indicate that droplet shape anisotropy is a major factor in determining the electrooptic properties of these films.     

Droplet shape and reorientation fields in nematic droplet/polymer films

Polymer films containing droplets of nematic liquid crystal form an important class of new electro-optic light valves and displays. While previous work has shown that the nematic droplet size is an important factor in the electro-optic properties of these films, here we report that the droplet shape is equally important in determing the electro-optics of the film. Electron micrographs show that for films using polyvinyl alcohol as the polymeric binder the cavities formed by the polymer matrix are oblate in nature, and aligned with the minor axis perpendicular to the film plane. In oblate cavities the elastic-deformation free energy is minimized when the director field in the droplet is aligned along a major axis of the spheroid; the electric field performs work on the nematic in reorienting the nematic into a higher-energy state, equal to the elastic-free-energy difference between the two configurations. Calculations and experiment are used to estimate the elastic and electric field free-energy-density changes that occur upon reorientation of the nematic droplet. The general agreement between these two values is used to indicate that droplet shape anisotropy is a major factor in determining the electrooptic properties of these films.     

Nematic and smectic liquid single crystal elastomers: Influence of external stress parallel and perpendicular to the director

The influence of uniaxial mechanical stress applied parallel or perpendicular to the director axis (optical axis) of nematic and smectic-A elastomers having a permanent macroscopically ordered monodomain structure is investigated. Due to the different phase structures applying an external mechanical stress leads to completely different responses with respect to the young moduli and reorientation processes of the director. Nematic elastomers exhibit a weak anisotropy of the moduli parallel (μ₁₁ and perpendicular (μ) to the optical axis with μ₁₁ / μ = 1.5. The moduli are mainly determined by the rubber elasticity of the poly(siloxane) network. Applying the mechanical stress perpendicular to the initial director axis causes a rotation of the director axis that is connected with a periodic pattern formation and a ‘soft elastic’ response. For smectic-A elastomers μ₁₁/μ is in the order of 10². While μ reflects a rubber elastic response similar to that of the nematic system, μ₁₁ indicates an enthalpy-elastic behaviour of the one-dimensional long range order of the smectic layers. In this direction reorientation of the phase structure with a deformation of the smectic layers occurs above a threshold elongation. In contrast to the nematic networks, a deformation perpendicular to the optical axis causes no director reorientation, displaying the liquid-like properties within the smectic layers.