Reorientation Of Liquid Crystal Director Research Articles

Optical effects in liquid crystal cell with photosensitive chalcogenide glass substrate

We report on the observation of dynamic light-induced annular patterns formation in a planar liquid crystal (LC) cell with chalcogenide microfilm As2S3 as orientating layer. The annular patterns appear under the irradiation of the cell by the laser beam (λ = 473 nm) with Gaussian distribution of intensity. The experimental results are explained in terms of the liquid crystal director reorientation in the LC cell after absorption of light by chalcogenide film. The director reorientation results in the LC refractive index change and formation of annular patterns in LC cell. To theoretically model the system under study we first minimized the total free energy functional subject to Gaussian-type modulation of the pretilt angle at chalcogenide covered substrate. Having found the director spatial profile we studied the light beam propagation through the cell with a spatially modulated pretilt angle. We determined the number of the aberration rings depending on the parameters of the laser beam and LC cell. We believe that light-induced change in the LC director anchoring at chalcogenide film can be applied to the development of various optical and electro-optical elements, such as dynamic micro-lenses, light controlled switchers and triggers, etc.

Two beam energy exchange in hybrid liquid crystal cells with photorefractive field controlled boundary conditions

We develop a theory describing energy gain when two light beams intersect in a hybrid nematic liquid crystal (LC) cell with photorefractive crystalline substrates. A periodic space-charge field induced by interfering light beams in the photorefractive substrates penetrates into the LC layer and reorients the director. We account for two main mechanisms of the LC director reorientation: the interaction of the photorefractive field with the LC flexopolarization and the director easy axis at the cell boundaries. It is shown that the resulting director grating is a sum of two in-phase gratings: the flexoelectric effect driven grating and the boundary-driven grating. Each light beam diffracts from the induced gratings leading to an energy exchange between beams. We evaluate the signal beam gain coefficient and analyze its dependence on the director anchoring energy and the magnitude of the director easy axis modulation.

Forced convection in nanoparticles doped nematics without reorientation

The problem of forced convection in the cell of nanoparticles doped nematic liquid crystal with both boundaries being free, plane and isotherm is considered. These boundary conditions, offered by Rayleigh, allow us to get simple and exact solution for boundary-value problem, from which its most important peculiarities can be clearly seen. Particularly, there appears a possibility to induce convection without reorientation of liquid crystal director. It was shown that nanoparticles could have significant influence on the convection.

Thermomechanical mechanism of orientational optical nonlinearity in twisted nematic liquid crystals

In a nonuniform nematic liquid crystal director reorientation may take place as a result of a temperature gradient. We formulated a two-dimensional model of the temperature gradient caused by the light absorption of a Gaussian laser beam impinging normally on the liquid crystal cell. Within this model, we calculated the director reorientation and the subsequent optical nonlinearity of the liquid crystal.

Fiber-Optic Pressure Sensor Based on Tunable Liquid Crystal Technology

We propose a photoelastic pressure sensor using a liquid crystal (LC) tunable fiber polarizer. The polarizer contains an etched single-mode fiber sealed in an LC cell. A low-index LC is used as a tunable birefringent cladding of the fiber. Any electric-field-induced LC director reorientation may thus affect the fiber's light-guiding characteristics. Different polarization states experience different LC cladding indices. The transmittance contrast between the x- and y-polarized modes reaches 31.9 dB, which is sensitive enough to monitor any polarization state change in the fiber. A pressure sensor with the sensitivity of 0.25 rad/N is thus demonstrated. The corresponding stress-optic coefficient of the fiber is measured at 1550-nm telecomm band. The advantages and applications of the LC tuning and sensing technologies are discussed.

Finding exact spatial soliton profiles in nematic liquid crystals

Finding exact analytical soliton profile solutions is only possible for certain types of non-linear media. In most cases one must resort to numerical techniques to find the soliton profile. In this work we present numerical calculations of spatial soliton profiles in nematic liquid crystals. The nonlinearity is governed by the optical-field-induced liquid crystal director reorientation, which is described by a system of coupled nonlinear partial differential equations. The soliton profile is found using an iterative scheme whereby the induced waveguide and mode profiles are calculated alternatively until convergence is achieved. In this way it is also possible to find higher order solitons. The results in this work can be used to accurately design all-optical interconnections with soliton beams.

Experimental investigation of liquid crystal director reorientation by mechanical vibrations

Reorientation of a nematic liquid crystal undergoing mechanical vibrations of one of cell walls was studied experimentally. Orientational structures have been revealed, which have interesting properties, their characteristics and formation processes were investigated. It was shown that these structures have parameters that do not depend on the intensity and frequency of mechanical vibrations.

UV tuning of the electro‐optical and morphology properties in polymer‐dispersed liquid crystals

Polymer‐dispersed liquid crystal (PDLC) films operating in reverse mode are transparent electro‐optical devices, which can be turned into an opaque state by application of a suitable electric field. The effect was investigated of different UV powers, used during the polymerization process, on the electro‐optical and morphology properties of PDLCs, working in reverse mode operation. Films were obtained by UV polymerization of mixtures of a low molecular weight nematic liquid crystal and a photopolymerizable liquid crystal monomer, homeotropically aligned by rough conductive surfaces. The electro‐optical and morphology properties of samples were related to the polymerization conditions. Samples polymerized by lower UV powers exhibited “polymer ball” morphology and an electro‐optical response due to the liquid crystal director reorientation, whereas samples obtained at higher UV powers showed a “Swiss cheese” morphology and an electro‐optical response due to dynamic scattering. In addition, we observed by conductivity and IR measurements that UV exposure induces a degradation of the nematic liquid crystal.

Spatially periodic reorientation of liquid crystal director at planar Freedericksz transition

Conditions for the formation of a spatially periodic structure in a liquid crystal cell at the threshold planar-planar director reorientation in a dc electric field are considered. The dependences of the threshold and the wave number of the arising structure on (i) the polar and azimuthal anchoring energy at the surface of the cell substrates, (ii) the ratio r of the elastic constants of the liquid crystal, and (iii) the value of the flexoelectric parameter v are calculated. It is shown that the range of the parameters r and v corresponding to the spatially periodic director reorientation depends strongly on the anchoring energy. The range of allowable values of the parameter r narrows with a decrease in the azimuthal anchoring energy and expands with a decrease in the polar anchoring energy, while the range of allowable values of the parameter v widens with decreasing azimuthal anchoring energy and depends nonmonotonically on the polar anchoring energy.

Surface Screening Layers and Dynamics of Energy Transfer in Photosensitive Polymer-Liquid Crystal Structures

ABSTRACT The dynamics of energy transfer in photoconductive polymer liquid crystal structures can contain important information on interface effects and surface electric fields contributing to the strength of liquid crystal reorientation gratings. The characteristic, transient effects observed during switching on and off of incident light or electric field can be explained by the presence of surface screening layers. Screening layers play an important role in the reorientation of liquid crystal director in cells with different alignment layers. Strong screening of external DC field is present not only in cells with a photoconductive polymer (56 V), but in standard cells with thicker (0.3 μm) polyimide, aligning layers.

Orienting Influence of Femtosecond Pulses on Nematic Liquid Crystals

ABSTRACT 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 azoxymolecules) 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.

Correlations between liquid crystal director reorientation and optical response time of a homeotropic cell

Correlations between the director reorientation time and its consequent optical response time (both decay and rise) of a homeotropic liquid crystal (LC) cell under crossed polarizers are derived theoretically based on small angle approximation. Results indicate that the optical response time is linearly proportional to the LC director reorientation time and is weakly dependent on the initial bias voltage. To validate the derived correlations, transient phase and transmittance responses at various bias voltages are analyzed numerically by solving the Erickson–Leslie equation. Pretilt angle is found to make an important contribution to the optical response time. Gray scale switching of the homeotropic cell is also investigated.

Photoinduced Alignment of Nematic Liquid Crystal on the Polymer Surface Microrelief

Polymer film surface modified in the holographic process under the laser illumination was used for nematic liquid crystal (LC) alignment. Azobenzene - containing polyurethanes were taken for a material of the film. A relief grating inscribed by laser illumination onto the surface of these polymers induced a reorientation of LC director from initial homeotropic alignment. Possible applications of described processes include command-surface creation for liquid crystal displays, reversible and adaptable in real-time liquid crystal structures for laser beam targeting and aiming, etc.

Observation of Nematic Liquid Crystal Director Reorientation at the Interface between Substrate and Liquid Crystal Layer by Total Reflection Ellipsometry

The liquid crystal (LC) director reorientation near the interface between the substrate and the LC layer was studied using total reflection ellipsometry (TRE). The dynamic and static responses of the ellipsometric angles to the electric field were measured by TRE. It was experimentally confirmed that total reflection occurs in the LC cell. The reflected light is dominantly modified by the interfacial LC director reorientation. The polar anchoring strength of the cell was estimated by analyzing the electrical response of the phase difference response. We confirm that the TRE is useful for studying the LC director reorientation near the interface between the substrate and the LC layer.

Letter surface and bulk reorientation in ferroelectric liquid crystals

The switching processes in ferroelectric liquid crystals are probed optically using a combination of transmission between crossed polarizers and surface evanescent field reflection techniques. This allows the behaviour of the liquid crystal director reorientation to be studied in the bulk of a device and near the surface simultaneously. Interesting behaviour is observed, and it is found that the processes can be adequately modelled using a simple one-dimensional model to represent the director structure across the thickness of the liquid crystal layer.