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

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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.