H. A. van Sprang

A novel method for determining the anchoring energy function at a nematic liquid crystal‐wall interface from director distortions at high fields

We show that the anchoring energy function, i.e., the anisotropic part of the interfacial free energy, at a nematic liquid crystal‐wall interface can be determined uniquely without a numerical fitting procedure, when the integrated birefringence of a liquid crystal cell with a thickness much larger than the extrapolation length is measured as a function of an electric or magnetic field well above the Freedericksz threshold. The precision of the present method is closely argued, showing that the resulting anchoring energy function is reasonably insensitive to the uncertainties in the material parameters and in the cell thickness. As an example, the anchoring energy function at the interface between 5CB(pentylcyanobiphenyl) and an obliquely evaporated SiO was determined for the first time, by measuring the birefringence and the capacitance of a 56‐μm‐thick cell up to 150 V rms at 0.23 °C below the clearing temperature. A saturation of the field‐induced distortion was clearly observed at about 100 V rms. The...

Influence of the surface interaction on threshold values in the cholesteric‐nematic phase transition

The threshold values for the field‐induced cholesteric to nematic phase transition are described as crossover points of the free energy levels of the textures concerned. Known expressions for these energy levels as a function of an electric field are extended with a surface interaction term. By interpretation of experimental results in terms of the equations derived, the surface interaction energy is calculated both for planar and homeotropically aligned cholesteric liquid‐crystal mixtures. Besides differences between planar and homeotropic wall alignment a difference between two types of planar wall alignment can now be understood, too. Additionally, the method appears very useful for calculating the twist elastic constant K22.

The temperature dependence of liquid‐crystal tilt angles

The temperature behavior of various liquid crystals has been studied on obliquely evaporated SiOx (α°) layers. The structure of such layers is discussed and the competition between the effects of two inherent anisotropies of these layers is indicated as the possible origin of the observed temperature dependence of the tilt angle. Attention is also paid to the structure of liquid‐crystal layers close to the nematic to isotropic transition temperature (clearing point). Experimental results for tilt angles as a function of temperature have been adapted to a two‐torque model, which is suggested by the above considerations. This allows comparison between the structural data for SiOx and the parameters from the two‐torque model. It is found that indeed one of the directions of preference is connected roughly with the columnar axis of the SiOx.

Improved performance of polarization-stable VCSELs by monolithic sub-wavelength gratings produced by soft nano-imprint lithography

We present a novel method for fabricating polarization-stable oxide-confined single-mode GaAs based vertical cavity surface emitting lasers (VCSELs) emitting at 850 nm using a new soft-lithography nano-imprint technique. A monolithic surface grating is etched in the output mirror of the laser cavity using a directly imprinted silica-based sol-gel imprint resist as an etch mask. The opto-electronic performance of these devices is compared to VCSELs fabricated by state-of-the-art electron-beam lithography. The lasers made using the soft nano-imprint technique show single-mode TM lasing at a threshold and laser slope similar to that of devices made by e-beam lithography. The soft nano-imprint technique also enables the fabrication of gratings with sub-wavelength pitch, which avoids diffraction losses in the laser cavity. The resulting single-mode VCSEL devices exhibit 29% enhanced efficiency compared to devices equipped with diffractive gratings.

Hybrid genetic algorithm-tabu search approach for optimising multilayer optical coatings

Constructing multilayer optical coatings (MOCs) is a difficult large-scale optimisation problem due to the enormous size of the search space. In the present paper, a new approach for designing MOCs is presented using genetic algorithms (GAs) and tabu search (TS). In this approach, it is not necessary to specify how many layers will be present in a design, only a maximum needs to be defined. As it is generally recognised that the existence of specific repeating blocks is beneficial for a design, a specific GA representation of a design is used which promotes the occurrence of repeating blocks. Solutions found by GAs are improved by a new refinement method, based on TS, a global optimisation method which is loosely based on artificial intelligence. The improvements are demonstrated by creating a visible transmitting/infrared reflecting filter with a wide variety of materials.

An analytical expression for the optical threshold in highly twisted nematic systems with nonzero tilt angles at the boundaries

It has been shown recently that steep optical thresholds can occur in twisted nematic configurations with tilted boundary conditions. This is the case when the twist angle φ is between π and 2π. We have derived an analytical expression for this threshold, utilizing the results from a numerical evaluation of director patterns with a computer program. The consequences of the dependence of the threshold on the relevant physical parameters are discussed and the theory is compared with experimental data. Also we point out the differences with respect to the threshold equation for the conventional 90°‐twisted nematic configuration.

Surface Order in Nematics

Abstract Interfaces have been studied between a liquid crystal and either glass or an evaporated SiOX, (60°) layer. Experimental results are discussed from both residual birefringence measurements in the isotropic phase and studies of the influence on the director of an in-plane magnetic field at the phase transition from isotropic to nematic. These data show that on pyrex an ordered interfacial layer exists while on SiOx (60°) a multidomain structure is present at the surface. Comparison with literature data from nematic phase experiments shows that additional experiments with the isotropic phase and at the phase transition are vital for an understanding of the interfacial phenomena leading to liquid crystal anchorage.

Experimental and calculated results for the dynamics of oriented nematics with twist angles from 210° to 270°

A series of experimental data is presented on the switching of nematic liquid crystals with high twist, high tilt director configurations for step‐voltage and pulse‐voltage addressing. In order to describe the observed phenomena using a simple model containing a single viscosity coefficient only, we introduce an empirical correction to the rotational viscosity coefficient γ. The description appears adequate for step‐voltage addressing but less so in the case of decay under specific pulse‐voltage addressing conditions. Possible reasons are discussed.

Multilayer and Thin Film Analysis Using FPMulti Software

X-ray spectrometry combined with a fundamental parameter package can be used to obtain semiquantitative analytical results for thin layered samples. This paper explores the use of the program FPMulti for various applications. These applications are examples of how accurate this program is for multilayer and thin film analysis. After a short description of the analytical method, the range of applications is given with which the authors have experience. In the experimental section three applications are described in more detail: (i) an example to show the accuracy of the method, (ii) an example of the calculation of the layer thickness and (iii) the possibility of quantifying oxygen in 3d metal oxides.

Numerical calculations of director patterns in highly twisted nematic configurations with nonzero pretilt angles

The influence is described of several device parameters, i.e., the total director twist, the pretilt angle, and anchoring energy, on the director pattern in highly twisted samples with nonzero boundary tilt. A difference compared with previous approaches is the inclusion of a finite anchoring energy coefficient in the continuum equations. The results indicate a pronounced influence of the surface anchoring on the hysteresis in the midplane tilt angle as a function of an applied voltage.