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

Abstract

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.     

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.     

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.     

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 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.     

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.     

Anchoring Properties of Nematic Liquid Crystals Studied Using the Attenuated Total Reflection Method

By using the attenuated total reflection method associated with the excitation of surface plasmons, the tilt angle of the liquid crystal director and its gradient at the surface are measured in a planar nematic cell as a function of the applied voltage. The surface anchoring anisotropy δπ of the liquid crystal and the surface elastic constant k_s, are found to be δπ = 0.288 erg/cm and k_s, = 9·12 × 10^-11 erg, respectively, when the boundary condition suggested by Barbero et al is used. The theoretical and experimental values obtained with this boundary condition and that of Mada are discussed. The results show that the boundary condition proposed by Barbero et al is in better agreement with the experiment.     

A guided wave study of the voltage dependent director configuration in a nematic liquid crystal layer with finite surface tilt

Prism coupling techniques have been used to excite optical modes in a thin nematic liquid crystal with finite surface tilt in order to study the voltage dependent director profile. The surface tilts are opposite in character and it is found that at zero applied volts the stable configuration is the substantially horizontal state. On applying the field this state is broken, probably transforming to the twisted vertical state. By modelling all the data obtained, the detailed behaviour of the director profile has been fully characterized yielding much information, including the change of surface tilt with applied voltage. For the nematic liquid crystal E7, this gives a voltage induced surface tilt of approximately 0.67° V^-1 for a 5.65 μm thick cell. Also using a boundary layer model, it has been possible to analyse the free energy in the cell and hence show that the observed twisted vertical state is the expected stable state under the field applied.     

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.     

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

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.     

The optical response of liquid crystal cells to a low frequency driving voltage

This study investigates the optical response of liquid crystal cells to a low frequency square wave voltage of 0.1 Hz. It is found that there are three physical phenomena that dominate the overall properties of the device. The first is the discharging effect whereby the effective voltage over the liquid crystal layer decreases as a function of time; this occurs due to mobile ions being present within the liquid crystal material. The second is the charging-up of the cell where the effective voltage increases with time; this is attributed to charge separation taking place within the polyimide layer upon application of the d.c. voltage component. The third effect is cell asymmetry whereby the effective voltage depends upon the polarity of the externally applied field; this is the result of a locked-in d.c. holding voltage being present within the cell layers. These three effects are analysed in some detail with the view of developing a liquid crystal cell capable of being driven with a low frequency square wave voltage. A model of a liquid crystal cell in which the liquid crystal material can dissolve impurity ions from the alignment layers and in which the ions can then become re-adsorbed into the polyimide layer is deduced.     

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.     

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.     

The director structure and electroclinic fast optical switching in a grating-coupled ferroelectric smectic liquid crystal cell

Optical excitation of guided modes in a liquid crystal layer using grating-coupling gives sharp features in the angle-dependent reflectivity data. These features are strengthened by using a metallized grating to enhance coupling to the guided modes in the liquid crystal. In the present study the liquid crystal has a smectic A phase which exhibits fast electroclinic switching. Combining the sharp features in the reflectivity together with the electroclinic effect leads to fast, high contrast, optical switching which may open up potential for novel device structures.     

The director structure and electroclinic fast optical switching in a grating-coupled ferroelectric smectic liquid crystal cell

Optical excitation of guided modes in a liquid crystal layer using grating-coupling gives sharp features in the angle-dependent reflectivity data. These features are strengthened by using a metallized grating to enhance coupling to the guided modes in the liquid crystal. In the present study the liquid crystal has a smectic A phase which exhibits fast electroclinic switching. Combining the sharp features in the reflectivity together with the electroclinic effect leads to fast, high contrast, optical switching which may open up potential for novel device structures.     

A study of the electroclinic effect using half-leaky guided modes with a homeotropically aligned liquid crystal

High resolution voltage dependent tilt angle studies using optical excitation of half-leaky guided modes have been conducted on a homeotropically aligned ferroelectric liquid crystal mixture (Merck SCE13) in the S_A phase. Uniform homeotropic alignment is realised, with no surface aligning layer, by the application of an in-plane DC electric field when the liquid crystal is in the S_C* phase. The applied field unwinds the pitch of the S_C* chiral helix and gives a uniformly tilted homeotropic monodomain. On warming into the S_A phase, detailed studies of the voltage induced tilt, the electroclinic effect, are then conducted at various temperatures. Because there is no influence of surface anchoring forces, the linear relationship between the induced tilt angle and the DC field is obtained even under very weak fields. Further, the relationship between induced tilt and temperature confirms the predictions of a second order Landau mean-field theory with a coupling term between the tilt angle and the DC 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.     

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

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.