Pretransitional electro-optic response of 6CB liquid crystal in the isotropic phase

In this work, the electro-optic response of a 6CB liquid crystal layer is studied using a sensitive differential technique. The layer is held at a temperature just above the nematic to isotropic phase transition. Transverse magnetic (p) polarized light incident on the cell is coupled to guided modes in the liquid crystal layer using prism coupling. The modes manifest themselves as sharp dips in the reflectivity as the angle of incidence is scanned. When a low frequency sinusoidal voltage is applied to the cell, the resonant mode shapes and excitation angles are altered at a frequency which is twice that of the applied field, resulting in a modulation of the reflectivity for a given angle of incidence. By synchronous observation of the modulated signal, a differential signal is recorded. Comparing the data with modelling generated from multilayer optics theory, two effects are then quantified. The first of these is an induced birefringence, varying quadratically with applied voltage, which is well understood and can be expressed in terms of Landau-de Gennes theory. The second is a field induced perturbation in the imaginary part of the optical permittivity, δε_i, which implies a modification of the light scattering properties of the liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensitivity of the synchronous differential technique.     

Use of mode mixing to determine the optic tensor configuration of a thin ferroelectric liquid crystal layer

Abstract

It is possible to probe directly the optic tensor profile within a thin ferroelectric liquid crystal layer by the propagation of prism-coupled leaky guided modes. Plane parallel monochromatic radiation of single polarization is made incident through a prism into a planar ferroelectric liquid crystal layer. Monitoring the reflected signal as a function of incident angle results in a series of sharp dips at angles corresponding to the excitation of modes in the layer. For a ferroelectric layer in which there is in-plane or out-of-plane tilt, conversion of one linear polarization to the orthogonal polarization may occur, by detecting this conversion it has been possible to show the existence of thin boundary regions at the two surfaces of the thin ferroelectric liquid crystal layer.     

Guided modes and surface plasmon-polaritons observed with a nematic liquid crystal using attenuated total reflection

An aligned layer of nematic liquid crystal with suitable optical anisotropy and under the application of appropriate applied voltages, may support various guided modes of light. Here the attenuated total reflection technique is used to examine such guided modes allowing simultaneous observation of the surface plasmonpolariton (S.P.P.) at a metal/liquid crystal interface. We report observations of the interaction between the bulk guided modes and the S.P.P., an interaction made possible by the strong refractive index anisotropy of the chosen liquid crystal. The influence of applied voltage upon the guided modes, the S.P.P. and their interaction is studied. Detailed reflectivity results are compared with theory for layered uniaxial media.     

Observations on the voltage response of a 90° twisted nematic cell using guided modes

Using prism coupling to guided modes and surface plasmons we have examined in detail the director response of a 90° twisted nematic liquid crystal cell as a function of applied voltage. By careful comparison of angle scan reflectivity data with theoretical predictions generated from a combination of liquid crystal continuum theory and multilayer optics theory it has been possible to establish how the surface tilt changes with voltage, and also to observe changes in the optic constants due to changes in the order parameter with applied field.     

Optical guided-wave study of a homeotropically aligned ferroelectric liquid crystal

Abstract

The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have also been obtained.     

Pulsed voltage studies of electric field effects on the optical permittivity of a nematic liquid crystal

Prism coupling to resonant optical modes in a thin layer of homeotropically aligned nematic liquid crystal has been used to characterize the change in refractive indices which occurs when an electric field is applied. Reflectivity data, recorded over a range of angles of incidence for both TE and TM radiation show sharp minima corresponding to the excitation of optical modes in the liquid crystal layer. Application of a pulsed AC voltage, pulsed to avoid heating, while synchronously monitoring reflectivity changes allows detailed characterization of the shift in the position of these minima due to the influence of the electric field on both the ordinary and extraordinary refractive indices. By fitting theoretical predictions of Fresnel theory a complete characterization of change in both these parameters up to an applied field of some 5 × 10⁶ Vm^-1 is achieved. The refractive index changes recorded are compared with the director fluctuation order parameter theory with which good agreement is found.     

Multi-layer protruded electrodes for reducing the operating voltage and gamma shift of fringe-field switching LCDs

ABSTRACT

To reduce the operating voltage and gamma shift of the nematic liquid crystal display (LCD), a single-domain-protruded fringe-field switching (PFFS) electrode structure is designed. In this work, a kind of nematic liquid crystal (NLC) is introduced firstly. Then, the operating voltage and gamma shift of the proposed PFFS LCD are investigated under various electrodes’ parameters. Besides, its light leakage and contrast ratio are also discussed. The results show that the operating voltage of the PFFS LCD is only 1.40 V, here the electrodes’ width is 2 μm and electrodes’ gap is 4 μm. The gamma shift of the PFFS LCD can be reduced to the indistinguishable level under various electrodes’ sizes, if the height of the insulation layer is proper. For contrast ratio, it is larger than 200:1 at full viewing-angle, and the zone of 500:1 covers 60° polar angle.     

A liquid crystal microlens obtained with a non-uniform electric field

Abstract

A homogeneously aligned nematic liquid crystal cell with a hole-patterned electrode and with an indium-tin oxide (ITO-) coated counter-electrode has been prepared. A non-uniform electric field can be produced by the asymmetrical electrode structure. The liquid crystal director can be reoriented by applying a voltage across the electrodes, and this produces an axially symmetrical profile of the refractive index. This liquid crystal cell is expected to have a lens effect and so its optical properties have been investigated. The profile of the output light intensity was measured by using a detecting system with an optical fibre. Some relationships between the lens properties, the diameter of the hole and the thickness of the liquid crystal layer have been examined. The liquid crystal cell becomes a convex (converging) lens with a relatively low voltage. A focal length of several millimetres can be obtained by applying voltages of 3-4 V. As the applied voltage increases, the focal length becomes longer, and the cell changes to a concave (diverging) lens when a high voltage is applied (? 20 V). These properties are discussed from the viewpoint of the director orientation effects resulting from the non-uniform electric fields in the cell.     

Accurate modelling of nematic liquid crystal under the combination of microwave signal and applied voltage

ABSTRACT

In this paper, a three-dimensional modelling of nematic liquid crystal (NLC) under the combined action of applied voltage and microwave signal is presented. The analytical method applied in the modelling is detailed. In previous research, the modelling of liquid crystal usually uses a small signal wave, and neglected its effect on the orientation of liquid crystal. In this paper, we take the microwave signal into consideration in the calculation of liquid crystal orientation, and get the influence of the power of microwave signal on the orientation. The variations of the relative permittivity of liquid crystal E7 with the power of microwave signals at 30 GHz are obtained. This method is applicable for the modelling of NLC under high power signals excitation.     

Physicochemical analysis of ion beam-induced surface modifications on polyethylene glycol films for liquid crystal alignment

ABSTRACT

We investigated the surface modification induced by the ion-beam (IB) irradiation of a polyethylene glycol (PEG) film and its liquid crystal (LC) alignment characteristics. The X-ray photoelectron spectroscopy analysis revealed the chemical modification; as the IB incidence angle increased, the number of surface C–O bonds decreased, inducing an anisotropic dipole moment on the PEG film surface. In addition, the physical modification was demonstrated via atomic force microscopy analysis using three-dimensional images as a function of the IB incidence angle. The surface roughness was analyzed; the modification with the smoothest surface was observed for an IB incidence angle of 45°. This modification affected the LC alignment state of the PEG film, as demonstrated by the polarized optical microscopy analysis with pre-tilt angle measurements. Furthermore, for the same IB incidence angle, the residual DC measured using the capacitance–voltage curves was extremely low. Hence, a PEG film irradiated with an IB incidence angle of 45° could be a suitable LC alignment layer.     

Momentum space imaging of a liquid crystal filled Fabry-Perot

Liquid crystal layer characteristics have for some time been determined by analysis of the optical guided modes supported within such layers. A novel technique for guided mode analysis, relying on scattering and re-radiation of light from a liquid crystal filled Fabry-Perot, is presented here and compared with a more standard technique, where the transmissivity of light through such a structure, as a function of the angle of incidence, is measured. The new technique which we label 'momentum space imaging' is found to hold some advantages over traditional methods, with data acquisition on the millisecond time-scale and full liquid crystal layer characterization being achievable with minor modification to existing theoretical multilayer modelling.     

The polar anchoring energy of nematic liquid crystals with polyvinylidene fluoride alignment layer

It has been proved that the high dielectric polyvinylidene fluoride (PVDF) could be used as an alignment layer in liquid crystal device. In this paper, the transmittance, pretilt angle and the polar anchoring energy of the substrates with PVDF alignment layer were researched. Theoretical results and the experimental results about the reflectivity R_SS (polarisation-conserving signals) recorded by full-leaky guided mode in liquid crystal technique are analysed to evaluate the anchoring energy of PVDF alignment layer. The result shows that the polar anchoring energy between PVDF alignment layer and liquid crystal molecules is 2.80 × 10^?4 J/m².     

The optical tensor configuration in a surface stabilized ferroelectric liquid crystal determined by using half leaky guided modes

The optic tensor configuration in a surface stabilized ferroelectric liquid crystal cell is investigated using optical excitation of half leaky guided modes. A thin ferroelectric liquid crystal layer is confined between a high index pyramid, with an index greater than the maximum of the liquid crystal, and a glass substrate having an index less than the minimum of the liquid crystal. Using standard attenuated total reflection experimental procedures, over a small angle range a series of sharp resonant peaks are recorded in the s-polarized reflectivity using p-polarized incident light. These peaks are extraordinarily sensitive to details of the optical tensor configuration within the cell. Fitting theoretically modelled reflectivities from multilayer Fresnel theory to the data allows determination of near surface alignment, bending of the chevron, surface tilt angle and biaxiality. To give a clear physical explanation for the great sensitivity of the technique, the electromagnetic field component distributions in the cell are also presented and analysed. The results confirm that the half leaky guided mode method has enormous potential for the study of the optic tensor configuration in liquid crystal layers.     

Calculation of chevron profiles in ferroelectric liquid crystal cells

Abstract

The energetic aspects of layer deformation in ferroelectric liquid crystal cells are discussed and the actual chevron shape is calculated in some situations. We emphasize two, in our view, essential energy contributions. One term considers the layer curvature. The other one refers to the variations in the distance between layers and the consequent changes of the smectic cone angle. In some simple cases we can determine optimal shapes of the chevron layer structure by analytical solutions, based on these two energy terms. In more complicated situations other contributions have to be considered and the chevron profiles are simulated numerically. The influence of the applied voltage and the choice of parameter values are studied.     

Characterization of the alignment of a nematic liquid crystal using half leaky guided modes

The excitation of half leaky guided optical modes to characterize fully the optical tensor profile in a thin liquid crystal layer has been used to evaluate the effect of rubbed polyimide aligning layers on the alignment of a nematic liquid crystal. A cell fabricated with rubbed polyimide alignment surfaces was studied at a wavelength of 632.8 nm. The liquid crystalline layer is sandwiched between a high refractive index top glass plate and a low refractive index glass substrate. Angular dependent reflectivities are recorded using a coupling prism and matching fluid with the same index as the top glass plate. Careful fitting of the predictions from multilayer optics theory to the observed angle dependent polarization conversion and reflectivity data yields the director profile within the liquid crystal layer in great detail.     

Experimental investigation of the compressible continuum theory of a homeotropically aligned ferroelectric liquid crystal

The optical tensor configuration in a homeotropically aligned ferroelectric liquid crystal (FLC), SCE13 cell, is investigated by means of optical excitation of half leaky guided modes. A thin slab of FLC is confined between a high index pyramid and a low index substrate whose indices bound those of the liquid crystal. In this geometry there exists a small angle range over which a series of sharp resonant modes may propagate in the liquid crystal. Detecting the angular dependent reflectivity for plane polarized radiation and subsequently fitting the data by iteratively modelling from multilayer Fresnel theory, a full characterization of the tilt and twist profile throughout the cell is achieved. The temperature dependence of the tilt of the principal director, which is related to the smectic cone angle, and of the optical permittivity, as well as the pitch have been obtained. The tilt director profile across the cell is interpreted using a compressible continuum theory for SmC* liquid crystals which includes the possibility of variable cone angle and layer spacing.     

PDLC composites based on polyvinyl boric acid matrix – a promising pathway towards biomedical engineering

ABSTRACT

Polymer dispersed liquid crystal (PDLC) systems based on a smectic liquid crystal embedded in polyvinylalcohol-boric acid (PVAB) as biocompatible carrying matrix were prepared and characterised. The smectic liquid crystal contains biologically friendly structural blocks and was designed to have a direct isotropic–smectic transition and a mesophase stability range at human body temperature. The resulted PDLCs were characterised from morphological and thermotropic aspects by polarised light microscopy (POM), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and Raman microspectroscopy, and their surface properties were determined by contact angle measurements and surface energy calculations.

It was concluded that the electron-deficient PVAB matrix constrains the ester liquid crystal to grow as spherical droplets with planar anchoring. The droplet diameter was comprised in the range 4–11 µm, with a predominant droplet population around 7 µm and a narrower polydispersity as the amount of the liquid crystal in the polymeric matrix increases. The resulted PDLC films exhibited versatile morphology and surface properties which allow targeting of their application.     

Use of mode mixing to determine the optic tensor configuration of a thin ferroelectric liquid crystal layer

It is possible to probe directly the optic tensor profile within a thin ferroelectric liquid crystal layer by the propagation of prism-coupled leaky guided modes. Plane parallel monochromatic radiation of single polarization is made incident through a prism into a planar ferroelectric liquid crystal layer. Monitoring the reflected signal as a function of incident angle results in a series of sharp dips at angles corresponding to the excitation of modes in the layer. For a ferroelectric layer in which there is in-plane or out-of-plane tilt, conversion of one linear polarization to the orthogonal polarization may occur, by detecting this conversion it has been possible to show the existence of thin boundary regions at the two surfaces of the thin ferroelectric liquid crystal layer.     

Optically compensated double-layer electrically controlled birefringence liquid crystal display with wide-viewing-angle cone

An electrically controlled birefringence liquid crystal display has a problem over narrow viewing angle. To solve this problem, the authors proposed a double-layer electrically controlled birefringence liquid crystal display with a wide-viewing-angle cone under an applied voltage. In this device, each liquid crystal layer compensates the variation of retardation as a function of viewing angle. However, the optical compensation occurs only when certain voltages are applied. The objective of this paper is to propose a novel film compensated double-layer electrically controlled birefringence liquid crystal display that has a wide cone of view in any state. This device is based on the concept of compensation of retardation.