A. Lapanik

Liquid Crystal-Reconfigurable Antenna Concepts for Space Applications at Microwave and Millimeter Waves

Novel approaches of tunable devices for millimeter wave applications based on liquid crystal (LC) are presented. In the first part of the paper, a novel concept of a tunable LC phase shifter realized in Low Temperature Cofired Ceramics technology is shown while the second part of the paper deals with a tunable high-gain antenna based on an LC tunable reflectarray. The reflectarray features continuously beam scanning in between . Also first investigations on radiation hardness of LCs are carried out, indicating that LCs might be suitable for space applications.

Sub-hertz relaxation process in chiral smectic mixtures doped with silver nanoparticles

A thermal activated sub-hertz frequency dielectric absorption process in both SmC* and SmA* phases along with characteristic Goldstone mode have been observed in both pure and silver nano particle doped liquid crystal mixtures. This process is well resolved, in particular, in the composites probably due to capturing of ions/charges inside the thiol group layer around the silver particles surface, thereby increasing the resistivity of the composites. The process is interpreted as space charge accumulation of ions/charges at the interface between liquid crystals and the cell polymer layer close to the electrodes. Improved switching characteristics and reduced spontaneous polarization have been detected.

Electrooptical and Dielectric Properties of Alkylthiol-Capped Gold Nanoparticle-Ferroelectric Liquid Crystal Nanocomposites: Influence of Chain Length and Tethered Liquid Crystal Functional Groups

The influence of gold nanoparticles dispersed in ferroelectric liquid crystals (FLCs) on the properties of the resulting nanocolloids has been investigated by thermoanalytical, electrooptical and dielectric methods. All gold nanoparticles were protected by thiolate capping layers either connected via alkyl groups of different chain length or via flexible spacers bridged to liquid crystalline p-cyano-biphenyl end groups. The colloidal stability of the gold nanocomposites with capping layers consisting of nonyl or octadecylthiolate groups is drastically reduced compared to the stability of those with p-cyano-biphenyl end groups. The present work describes the influence of the kind of capping layers around the gold nanoparticles and the effect of the amount of gold nanoparticles dispersed in such liquid crystal colloids on their electrooptical and dielectric parameters.

Induction and Enhancement of Ferroelectric Smectic C* Phase in Multi-Component Room Temperature Mixtures

Six multi-component room-temperature ferroelectric liquid crystal (FLC) mixtures have been formulated and investigated using optical polarizing microscopy, dielectric spectroscopy as well as electro-optic methods. In all the mixtures smectic C* phase is either induced or enhanced. Effect of various dopants on the occurrence and thermal stability of different phases has been discussed. Spontaneous polarizations, measured as a function of temperature by reversal current method, are found to vary between 13–62 nC/cm2 at room temperature in all the mixtures. Different types of dielectric absorption behavior were observed in planar geometry of the FLC cells. Critical frequencies of Goldstone and soft modes have been determined by frequency dependent dielectric study. Rigidity of the core structure, nature of chirality and extent of fluorination of the constituent molecules are found to have pronounced effect on the collective mode relaxation behavior of the mixtures.

Defect-Free FLCD's with High Optical Quality Based Upon New FLC's

We have designed new FLC compounds and new FLC materials based upon them, and we have found general criteria of the preparation of FLCDs with uniform and stable orientation of FLC materials [1]. The influence of the molecular structure on the quality of orientation, on thermal and mechanical stability and on the contrast ratio was investigated by using 4 types of aligning materials in the thin and thick testing cells. The correlation between the number, type and amount of achiral and 3-, 4-ring chiral compounds in the mixture and contrast ratio of our FLCDs was found. Finally we optimized the FLC-mixtures, alignment conditions and we prepared the defect-free samples with optical contrast about 700:1.

Nanomaterials dispersed in ferroelectric liquid crystals

Nanocomposites composed of a self prepared Ferroelectric Liquid Crystalline (FLC) matrix and different nanomaterials show pronounced reduction of important parameters like spontaneous polarization, switching time but no change of the tilt angle. As a typical subject silver nanoparticles where characterized. A pronounced increase of resistivity where described by comparing the pure FLC mixture with properties of FLC nanocomposites. Ion capturing due to silver particles has been discussed. Photoluminescence originated by the terphenyl compound and enhanced due to the Silver Surface Plasmon Resonance has been described.

W-Band Characterization of Anisotropic Liquid Crystals at Room Temperature

This paper describes the characterization of Liquid Crystals (LCs) at W-Band using the standard WR-10 rectangular waveguide. The reflection parameter of a 10 mm long waveguide - terminated by a short-circuit - is measured. The LC is oriented with a homogeneous magnetostatic field strength of 400 A/mm, generated with an air-cooled coil. The extracted complex permittivity of the LC K15 from Merck KgaA (also known as 5CB) is presented, showing permittivities well below 3 with a dielectric anisotropy of Deltaepsiv<0.19 (equivalent to tau=Deltaepsiv/epsivmax<8.5%) and dielectric losses less than tandelta<0.023, all at W-Band.

XRD Study of New FLC's: Correlation Between the Chemical Structure of Molecules, the Sign of the Optical Rotation and the Tilt Angle

New 4-ring FLCs with different positions of lateral substituents and bridge fragments in the central core were synthesized. These compounds have a wide temperature range of the smectic C* phase and similar length of the molecules. Different ways of the preparation of the FLC mixtures with a high tilt angle and the results of electrooptical and XRD measurements of the FLC mixtures based upon the chiral compounds are presented.

Highly birefringent nematic mixtures at room temperature for microwave applications

We present results on how the dielectric anisotropy in the low frequency region up to 1 GHz and the birefringence of the nematic liquid crystal (LC) phase at optical wavelength (589 nm) influences the performance of LC mixtures at microwave frequencies. For this, we prepared a wide range of mixtures being nematic at room temperature and containing different classes of promising compounds. Their quasistatic and frequency depended dielectric properties, their birefringence and the impact of those properties on their microwave behavior were investigated. The measurements were done using the cavity perturbation method at 30 and 38 GHz, respectively, in the temperature interval of -20°C and 135°C.

Shock-Free Ferroelectric Liquid Crystal Compositions: Optimized Chiral Compounds and Their Mixing Ratio with Non-Chiral Components

The best chiral compounds for the design of shock-free ferroelectric liquid crystal compositions with a wide temperature range of the chiral smectic C phase have been developed. For these experiments about 70 different 4-ring chiral compounds were synthesized and investigated. The influence of the chiral fragment, the mixing ratio between chiral and non-chiral molecules, and the pretilt angle of the alignment materials on the quality and on the mechanical stability of the orientation were investigated. New optimized ferroelectric liquid crystal compositions for the fabrication of shock-free ferroelectric liquid crystal displays with high contrast ratio are presented.