Characterisation of highly anisotropic liquid crystals in the range of 2 to 8 THz

液晶（Liquid Crystals，LC）是近几十年来不断发展的研究课题，在过去的十年中，德国微波工程与光子学研究所（IMP）和德国默克公司（Merck KGaA，达姆施塔特）密切合作，对LC进行了研究和合成。已经非常成功地进行了对CPLCs的材料优化，产生了具有低损耗和相对高的各向异性（双折射，即一个光轴相对于另一个光轴的介电常数之间的差异）的新混合物。特别合成的LC混合物具有世界上最高的材料品质因数。基于这些材料，获得了世界范围内移相器的最高品质因数，在30 GHz时每dB超过180度。达姆施塔特工业大学是这一领域的国际领先研究机构。2012年10月，与澳大利亚卧龙岗大学合作，在室温下在300 GHz至1.5 THz的频率范围内对达姆施塔特Merck KGaA的四种LC混合物进行了测量。作为合作的一部分，傅里叶变换干涉仪（FTIR）已被评估为表征的方法。通常用于FIR测量。另一方面，在卧龙岗的设置，特别是在更高的太赫兹制度的光谱学修改，可以直接适应，从2到8 THz的吸收测量没有进一步的修改。在这里，一个意想不到的效果踢：两个四个LC混合物的吸收在约7 THz反转。在较低的频率，低吸收观察到在高介电常数的方向。上面，在高介电常数取向中观察到比在低介电常数情况下更高的吸收，与通常的预期相反。文献研究并没有在这些频率下提出类似的行为。独立于这种基本的物理效应，在这个频率范围内的研究是有趣的：有许多关于研究的出版物高达3 THz或在FIR范围内（从大约100 Hz开始）。30 THz），而在这两者之间，几乎没有或根本没有数据可用。此外，基于这种材料的连续可调谐太赫兹组件的应用潜力很大。特别是由于它们的低损耗，新的元件可能成为增强THz系统功能的关键。此外，使用谐振结构将比过去的项目更精确地测量LC混合物的损耗角。

Liquid Crystals (LC) have been subject of ongoing research for several decades.During the last decade, the Institute for Microwave Engineering and Photonics (IMP) and Merck KGaA, Darmstadt have studied and synthesized LCs in close co-operation. Material optimisation of nematic LCs that has been carried out very successfully resulted in new mixtures which feature low losses and relatively high anisotropy (birefringence, i.e. the difference between permittivity in one optical axis with respect to the other axis). The particularly synthesized LC mixtures feature the highest material quality factors in the world.Based on these materials, the highest figures of merit for phase shifters word-wide were obtained, with values of over 180 degrees per dB at 30 GHz. TU Darmstadt is the internationally leading research facility in this field.In October 2012, in co-operation with University of Wollongong, Australia, measurements of four LC mixtures by Merck KGaA, Darmstadt were carried out over a frequency range of 300 GHz to 1,5 THz at room temperature. Data of permittivity and loss angle were obtained for both optical axes.As part of the co-operation, Fourier Transform Intereometry (FTIR) has been assessed as a method of characterisation. Usually, they are used for FIR measurements. The set-up in Wollongong, on the other hand, is modified particularly for spectroscopy in the higher Terahertz regime and could be adapted directly and with no further modification for absorption measurements from 2 to 8 THz.Here, an unexpected effect kicked in: the absorption of two out of the four LC mixtures was inverted at about 7 THz. At lower frequencies, low absorption was observed in the high permittivity orientation. Above, higher absorption was observed in the high-permittivity orientation than in the low-permittivity case, contrary to common expectations. Literature research did not bring up similar behaviour at these frequencies.Independent of this fundamental physical effect, an investigation in this frequency range is interesting: there are many publications on research up to 3 THz or in the FIR range (starting from approx. 30 THz) while in between little or no data at all is available. Furthermore, there is high potential for application of continuously tuneable THz componentes based on such materials. Especially due to their low losses, novel components may become feasable which could enhance THz systems in their functionality.Furthermore, the loss angle of LC mixtures will be measured more precisely using resonant structures than it was in the past project.