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.     

Detailing smectic SSFLC director profiles by half-leaky guided mode technique and genetic algorithm

The genetic algorithm (GA), written to allow automatic analysis of optical reflectivity data obtained from liquid crystal cells using the half-leaky guided mode technique, has been developed to the point where liquid crystal cells can be analysed successfully giving greater detail of optical parameters and director profile than yielded by any other technique. The technique models the liquid crystal layer as a set of discrete, independent sub-layers which can map out the variation of the director through the thickness of the cell. Given sufficient high quality data, it is now possible automatically and accurately to fit the parameters of a complete liquid crystal cell. Using this highly adapted GA, half-leaky guided mode optical reflectivity data from the nematic, smectic A and smectic C* phases of SCE13 in a surface stabilized ferroelectric liquid crystal have been fitted to reveal director profiles and optical parameters of the cell in each phase.     

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.     

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.     

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.     

Groove depth dependence of the anchoring strength of a zero order grating-aligned liquid crystal

The influence of in-plane electric fields on the twist profile of a liquid crystal aligned by nondiffractive (zero order) gratings has been explored using an optical guided mode technique. The liquid crystal is studied in a novel cell geometry comprising one rubbed polyimide alignment layer and one zero order diffraction grating. Comparison of the director-distorted profiles with predictions from elastic continuum theory allow both the twist elastic constant of the bulk liquid crystal and the azimuthal anchoring strengths at the two surfaces to be determined. Experiments have been performed on different depth gratings. Good agreement with the Berreman topological alignment model has been achieved for fairly shallow gratings, but not for gratings with deeper groove profiles. This implies that the theory needs modification to account for deep gratings.     

Groove depth dependence of the anchoring strength of a zero order grating-aligned liquid crystal

The influence of in-plane electric fields on the twist profile of a liquid crystal aligned by nondiffractive (zero order) gratings has been explored using an optical guided mode technique. The liquid crystal is studied in a novel cell geometry comprising one rubbed polyimide alignment layer and one zero order diffraction grating. Comparison of the director-distorted profiles with predictions from elastic continuum theory allow both the twist elastic constant of the bulk liquid crystal and the azimuthal anchoring strengths at the two surfaces to be determined. Experiments have been performed on different depth gratings. Good agreement with the Berreman topological alignment model has been achieved for fairly shallow gratings, but not for gratings with deeper groove profiles. This implies that the theory needs modification to account for deep gratings.     

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 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.     

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.     

Quantification of the azimuthal anchoring of a homogeneously aligned nematic liquid crystal using fully-leaky guided modes

A novel optical guided mode technique, the fully-leaky guided mode technique, has been used to investigate the director distortion under the application of an in-plane electric field of a homogeneously aligned conventional cell filled with the nematic liquid crystal E7. The liquid crystal is aligned using polyimide rubbed along the direction of the gold electrode edges. A weak field is applied across a 3 mm gap between the gold electrodes to induce small changes in the twist angle of the director. These distortions are determined by fitting to the angledependent reflectivity and transmissivity data and are compared with continuum theory. From careful analysis of the results, both the twist elastic constant, k22, and the azimuthal anchoring strength, Wa, of the system are obtained. At 23.5 C for E7 on rubbed polyimide we find that k22=(6.50 +/- 0.05)x10-12N and Wa=(2.9 +/- 0.2)x10-5 J m-2.     

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.     

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.     

FT-IR study of liquid crystal molecules on alignment layers

Alignment control at the molecular level is crucial for realizing high-performance LCDs. In particular, the structure of the interface between the liquid crystal and the alignment layer must be clarified. By utilizing RAS (reflection-absorption spectroscopy), a highly sensitive FT-IR spectroscopic technique, we have obtained the following information about the orientation of liquid crystal molecules; (1) the cyano group in a liquid crystal molecule behaves like an electron donor with respect to an SiO layer, but like an electron acceptor with respect to a polyimide layer; (2) with the use of polarized IR spectroscopy, it was discovered that the liquid crystal 8CB is aligned with a slant away from the aligning direction, regardless of the type of alignment layer. The angle between the molecular short axis within the plane of the core and the surface normal was found to be smaller on polyimide layers than on SiO layers.     

Characteristics of wide-band reflection of polymer-stabilised cholesteric liquid crystal cell prepared from an unsticking technique

An unsticking technique was applied for the fabrication of a polymer-stabilised cholesteric liquid crystal (PSCLC) cell. The template of polymer matrix in the planarly aligned cholesteric liquid crystal (Ch-LC) was perfectly preserved by lifting off the hydrophilic antisticking substrate. The desired PSCLC cell could then be obtained by injecting a third Ch-LC sample between two designed templates of PSCLC layer. By adjusting the pitch distribution in the two stacking templates, a reflectivity of nearly 50% incident light could be yielded and the bandwidth of the reflection spectrum could be controlled accurately in the PSCLC cell. Thus, a method to modulate the wide-band reflection of 50% visible light flux range was offered.     

Quantitative conoscopic imaging of cholesteric thin films

The periodic helical structure of an aligned polymerized cholesteric liquid crystal (ChLC) film gives rise to circular Bragg reflection within a narrow wavelength range, such that circularly polarized light of the same handedness as the helix is reflected, while counter circularly polarized light is transmitted. The ChLC is also strongly optically active with opposite rotatory power at either side of the circular Bragg zone. Both the selective reflection of circularly polarized light within the Bragg zone, as well as the optical activity of the cholesteric liquid crystal, are imaged with a new conoscopic technique based on a microscope equipped with the rotating polarizer-circular analyser MetriPol imaging system. The conoscopic images reveal the off-axis Bragg zone as an annulus at wavelengths below normal selective reflection. The annulus converges into a disk before disappearing at wavelengths above the region for normal selective reflection. The technique allows clear identification of the Bragg zone and the sign of the optical activity.     

Quantitative conoscopic imaging of cholesteric thin films

The periodic helical structure of an aligned polymerized cholesteric liquid crystal (ChLC) film gives rise to circular Bragg reflection within a narrow wavelength range, such that circularly polarized light of the same handedness as the helix is reflected, while counter circularly polarized light is transmitted. The ChLC is also strongly optically active with opposite rotatory power at either side of the circular Bragg zone. Both the selective reflection of circularly polarized light within the Bragg zone, as well as the optical activity of the cholesteric liquid crystal, are imaged with a new conoscopic technique based on a microscope equipped with the rotating polarizer-circular analyser MetriPol imaging system. The conoscopic images reveal the off-axis Bragg zone as an annulus at wavelengths below normal selective reflection. The annulus converges into a disk before disappearing at wavelengths above the region for normal selective reflection. The technique allows clear identification of the Bragg zone and the sign of the optical activity.     

Contrast ratio of twisted nematic liquid crystal cells and its improvement

The light leakage of twisted nematic (TN) liquid crystal displays is investigated. While the TN mode has a robust structure in regard to light scattering, due to thermal fluctuations of the liquid crystals and light leakage from imperfect rubbing defects, it maintains significant residual retardation of the liquid crystal layer in the black state. The residual retardation is induced by the azimuthally twisted liquid crystal alignment near substrate surfaces and causes major light leakage in the black state. We investigated several methods to compensate for the residual retardation, and the methods include the adjustment of the helical power of the liquid crystal material, the increase of driving voltage, the control of rubbing direction and a newly designed compensation film with a small angle twist. Though all these methods improve the contrast ratio by about 30%, each method has its own weaknesses and limitations. The development of a new compensation film would provide a good chance for future TN applications.     

Model to describe light scattering by polymer film containing droplets with inhomogeneous anchoring of liquid crystal molecules at the polymer–droplet interface: asymmetry effect in the angular distribution of light

A model to describe light scattering by polymer film containing of monolayer of liquid crystal droplets with inhomogeneous anchoring of liquid crystal molecules at the polymer-droplet interface is developed. It is based on the interference approximation of the wave scattering theory. The director field distribution in the droplet volume is determined by solving the free energy density minimization problem using the relaxation method. The spatial distribution of droplets in the layer is described by the hard disks model. The amplitude scattering matrices of individual droplets are found in the anomalous diffraction approximation. The algorithm for numerical analysis of the characteristics of light scattered in a polymer film containing droplets at homogeneous and inhomogeneous surface anchoring is described in terms of the partial filling factors of the monolayer film. Electrically controllable symmetry breaking effect of angular distribution of light scattered by films containing droplets with inhomogeneous anchoring at the polymer-droplet interface is described and experimentally confirmed.