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Today’s display technology benefits mainly from the development of the twisted nematic cells, which were introduced in the 1970’s. They dominate many aspects of our modern life from communication, business to entertainment. Liquid crystals are one of the base materials of these technologies due to their unique features. They behave like crystals showing Bragg reflections, which are usually common in ordered structures. On the other hand, liquid crystals are fluid like liquids. The anisotropic behavior and the relatively easy change of the optical properties due to electric fields led to the big market of display technologies. Therefore, fundamental research in both fields, the optical and the low frequency range, was necessary.
The investigation of materials in the THz frequency range is relatively new. Up to twenty years ago suitable THz emitters and detectors were hard to find. However, in the meantime a wide variety of different laboratory systems exist. Although several niche applications are currently successfully addressed with THz spectroscopy, the big breakthrough in terms of industrial application or even in daily life items is still lacking. Only a few companies use THz fields, for example for nondestructive testing and quality assurance. Besides the need of emitters and detectors, beam guide systems are also crucial for a more widespread exploitation of this technique. Here, liquid crystals are expected to play an important role in developing switchable devices at THz frequencies. For example phase shifters, switchable filters or lenses have been invented, whose functionality is mainly based on liquid crystals. Therefore, the investigation of the properties of liquid crystals at THz frequencies is necessary to optimize existing devices or even develop new ones based on liquid crystals.
First studies on liquid crystals at THz frequencies have already been performed by different groups. These reports mainly focus on established and easy to get liquid crystalline structures and lead to a first insight in the properties of liquid crystals at THz frequencies. Thus, in order to get deeper understandings of the mechanisms leading to the important properties at THz frequencies and to find structure dependent relations, systematic studies have been realized in the framework of this doctoral thesis. First the terminal position of a liquid crystal was systematically changed by implementing different atomic groups. The refractive indices and the absorption coefficients for both axes, parallel and perpendicular to the director of the liquid crystal molecules, were analyzed with a standard THz time domain setup. The birefringence was calculated and it was found that the isothiocyanato group leads to the highest values. As the rest of the molecular structure was kept identical, the most likely explanation is that the π electron system is more elongated compared to the other terminal groups. In a second study this was further investigated by changing the core structure. An additional aromatic ring was implemented. This leads to a longer elongated π electron structure along the long axis of the molecule. An increase in the birefringence was detected, which supports the assumed theory.
The so far realized studies are mainly limited to frequencies up to 3 THz. At such low frequencies the spectra lack strong resonant features. In order to get a better understanding and to compare these information with theoretical calculations, however, such features are important as they can easily be compared with the theoretical results. Measurements on a broadband THz spectrometer based on a completely novel THz generation and detection scheme based on air plasma were therefore performed at the technical university of Denmark leading to frequency information up to 15 THz. Here different absorption peaks could be identified which were compared to a calculated absorption spectrum. The calculations are based on a density functional approach and thus relate mainly on the molecular structure of the investigated liquid crystal. The calculated single molecule absorption spectrum agrees very well with the main features of the experimental data. This allows the conclusion that intramolecular modes play an important role in the absorption features at THz frequencies. The corresponding vibrations are not localized vibrations along a single bond between two atoms of the molecule but at these frequencies global motions of the entire molecule dominate. These latter vibrations cannot be considered as isolated without taking into account the influence of weak intermolecular interactions with the surrounding environment. The discrepancies found between the calculated and the experimental spectrum can thus result from the fact that no intermolecular forces were taken into account in the calculations.
Although liquid crystals are mainly used in the nematic phase most of them show more than one liquid crystalline phase. Therefore, one important parameter in characterizing liquid crystals are their phase transition temperatures, which are currently typically determined using DSC and optical analysis. The liquid crystal CE8 shows several distinct liquid crystalline phases and is an ideal candidate to analyze phase transitions. In order to analyze these transitions with THz time domain spectroscopy the liquid crystal is placed in a customized temperature controller unit and cooled from the isotropic phase to the crystalline phase. The temperature dependent refractive index at THz frequencies identifies most of the phase transitions by a step in the refractive index. Even the second order transition between the SmA* and SmC* phase is observed. This indicates that THz TDS could in this context become an alternative in particular to DSC.
So far we have applied liquid crystals in order to manipulate the THz wave. Due to a nonlinear effect, the so called Kerr effect, it is possible that a strong electric field induces a change in the properties of a material. Therefore, it is also possible to influence the properties of the liquid crystal with THz radiation. An amplifier system at the Max Planck Institute in Mainz is used to generate strong THz electric fields in a LiNbO3 crystal. These were focused collinear with a weak optical pulse on the liquid crystal 6CHBT in the isotropic phase. Due to the Kerr effect the THz field induces a birefringence in the liquid crystal which is detected by a change in the polarization of the optical pulse. This time dependent measurement shows a strong feature following nearly the square of the THz pulse and some delayed relaxation, where two different decay times can be observed. These are related to single molecule reorientations. The strong feature might be either some electronic or some molecular feature or a combination of both. However, compared to liquids the effect is sufficiently stronger leading to much higher nonlinear refractive index.
These fundamental investigations are followed by more application related considerations. Based on the results of the systematic spectroscopic studies of the THz properties of liquid crystals, high birefringent materials have been developed. The so far highest birefringence of a liquid crystal at THz frequencies was observed making these particular liquid crystals to one of the most promising candidates for future applications. Due to the thickness of the demonstrated devices so far, relaxation times are rather long. To enhance these two different ideas were considered. One is the use of a dual frequency liquid crystal. These show either a positive or negative dielectric anisotropy dependent on the frequency of the electric field which is used to align the molecules. Thus only one pair of electrodes is needed for an active switching, which is much faster than relaxation processes within the liquid crystal. The second approach is to create a directed polymer network inside the liquid crystal. Due to intermolecular forces the liquid crystals favor an alignment parallel to the network. An electric field can be used to reorient the liquid crystal molecules. The polymer network has only a small effect on the THz properties but allows a much faster relaxation compared to a relaxation of the liquid crystal alone. Therefore, faster switching speeds are possible and the design of the device can be simplified as only one pair of electrodes is needed. Even a continuously tuning from the ordinary to the extraordinary properties seen by the incoming THz wave is possible by a change of the applied voltage.
In conclusion, liquid crystals are expected to become one of the key elements in the development of switching devices at THz frequencies. The results of this work will not only help to gather a better and deeper understanding of the fundamental properties of liquid crystals at THz frequencies but novel concepts for faster switchable devices will also strongly benefit from the here presented investigations.