Lizhen Ruan

Optical anisotropy and liquid‐crystal alignment properties of rubbed polyimide layers

The relation between the optical anisotropy of rubbed polyimide layers and the rubbing process is investigated using the recently developed polarization‐conversion guided mode technique. Results indicate that the effective optical anisotropy of the polyimide layers may be substantially changed by the rubbing process, although this does not significantly influence the ability of the polyimide layers to align liquid crystals.

The effect of an electric field on a homeotropically aligned smectic C liquid crystal

Abstract A homeotropically aligned layer of smectic C liquid crystal has been studied using optical excitation of half leaky guided modes. Under the application of an AC voltage, the director configuration changes over a time-scale of the order of seconds. Fitting model results to the recorded angle dependent reflectivity data indicates firstly a change of tilt angle of the primary director, suggesting perhaps a layer tilt, and secondly a reduction of the imaginary part of the optical permittivity, implying a suppression of fluctuations. There is no evidence for a Helfrich-like deformation in the liquid crystal layer. Detailed analysis finally shows that the results are correctly interpreted as an increase in cone angle, not layer tilt, with field. Then from the fitted data, the change of tilt angle with field is obtained and hence information on the mean-field theory parameters.

Electroclinic effects in a homogeneously aligned smectic A cell

Abstract The electroclinic effect, in a material having a first order SA to SC∗ transition, is studied using the half leaky guided mode geometry. Using an approximately 1 μm thick, homogeneously aligned cell, the voltage induced director twist is characterized at a few temperatures in the SA phase. The mean field theory readily explains the data recorded at low fields where a linear dependence on voltage is found. However, at higher fields, an unexpected saturation occurs which is most likely caused by the influence of strong surface anchoring forces.

Guided wave study of electroclinic effects in a homogeneously aligned smectic A liquid crystal

Abstract Using the half leaky guided mode geometry, we have studied the electroclinic effect using a thin homogeneously aligned layer of C8, a material having a first order SA to S∗C transition in the bulk. Application of voltage in the SA phase produces a small (< 12°), but easily measured director twist which is characterized over a range of temperatures in the SA phase. Mean field theory is used to interpret the data recorded. At low applied fields (6 V μm−1), a linear voltage dependence of induced twist is found. For higher applied fields there is a non-linear dependence which is caused by an induced first order SA to S∗C transition at these fields. Fitting the voltage dependent induced twist yields the first three Landau coefficients for this transition. The voltage dependent results also provide a procedure for determining the SA to S∗C phase transition.

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

Abstract 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.

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

Abstract 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.

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

Abstract 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.

A pulsed voltage study of the influence of an electric field on the optical permittivity of both smectic A and nematic phases of a liquid crystal

Abstract Guided optical wave techniques have been used to measure the changes in the optical permittivity of a thin, well aligned layer of a liquid crystal, 4-n-octyl-4′-cyanobiphenyl, as a function of applied voltage. The behaviour of both TE and TM guided modes has been studied, in the nematic and smectic A phases, using a pulsed voltage technique to avoid heating effects. Small changes of both e⊥ and e∥ have been detected, of the order of 10−5, and these changes can be interpreted, for both smectic A and nematic phases, in terms of the suppression of director fluctuations. Close to the smectic A–nematic transition the observed change in (e∥–e⊥) in the smectic A phase reverses in sign. This unexpected behaviour is discussed in terms of dipole-dipole interactions.

DIRECTOR CONFIGURATION OF A FERROELECTRIC LIQUID CRYSTAL IN A CELL WITH ANTIPARALLEL SURFACE TILT

Abstract Using the half-leaky guided wave technique we have characterized the optical director profile of a ferroelectric liquid crystal in a homogeneously aligned cell with rubbed nylon surfaces. At high temperatures the N phase is in the expected splayed state, with opposite surface tilts of the order 3°. On cooling into the SA phase, the director corresponds to that of an almost uniform slab with a tilt of 5°. Further cooling into the SC phase does not produce the expected chevron structure; rather a tilted-bookshelf structure is recorded with an initially, at high temperatures, increased tilt of 8°. As the sample is further cooled, this tilt diminishes and eventually at room temperature changes sign to -2°. In addition, the room temperature SC phase has a uniform twist of about 1.4° away from the initial alignment direction. This accords well with the polarization microscopy measurement of an extinction angle of about 1.5° for this cell.

Theory of nematic–smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic–smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic–smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic–smectic interface which is perpendicular to the smectic layers.