Molecular Terahertz Polarizability of PCH5, PCH7, and 5OCB
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Molecular Terahertz Polarizability of PCH5, PCH7, and 5OCB N. Vieweg & M. K. Shakfa & M. Koch
Received: 5 September 2011 / Accepted: 13 September 2011 / Published online: 22 September 2011 # Springer Science+Business Media, LLC 2011
Abstract In this letter we determine the principal and main polarizabilities of the nematic liquid crystals PCH5, PCH7 and 5OCB. The polarizabilities are calculated from the terahertz refractive indices using Vuks’ approximation. 5OCB, which has two phenyl rings in its core, shows the highest terahertz anisotropy.
Presently terahertz technology is developing at a rapid pace [1, 2]. Many applications are foreseen for THz systems, which range from non-destructive testing [3–5] and process monitoring in the industry [4, 5] to short-range communications with THz carrier waves [6, 7]. A mature THz technology not only requires THz sources and detectors but also devices to guide and manipulate THz waves. The latter include reflectors [8, 9], waveguides [10, 11], splitters [12], lenses [13] and modulators [14–16]. Modulators could be based on liquid crystals (LCs) as demonstrated by Pan et al. [17] who demonstrated for instance switchable phase shifters and filters. Wilk et al. presented an LC based switchable THz Bragg structure [18] and Khoo et al. [19] reported on a tunable metamaterial. Yet, in order to design efficient THz devices the dielectric THz properties of the LCs have to be know precisely [20]. Even more important is that the THz absorption is low but other key parameters such as the birefringence are high. The dielectric properties are determined by the molecular polarizability (MP) of the respective materials. Knowledge of the molecular polarizability and how it depends on the molecular structure is, hence, important for designing LC molecules with desired THz properties. In other words the structure-property relations which are well known in the optical frequency range have to be determined also for THz frequencies. The molecular terahertz polarizabilities of three different cyanobiphenyl (CB) molecules were recently presented by Vieweg et al. [21] and the impact of the alkyl chain length has been discussed. To investigate the influence of the core structure we present here the MPs of another three nematic molecules: PCH5, PCH7, and 5OCB.
N. Vieweg (*) : M. K. Shakfa : M. Koch Faculty of Physics, Philipps University of Marburg, Hans-Meerwein-Straße MZG C06, 35032 Marburg, Germany e-mail: [email protected]
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J Infrared Milli Terahz Waves (2011) 32:1367–1370
Nematic LCs are rod like molecules and possess a certain degree of orientational order in their nematic phase. Two different polarizabilities can be determined for the LCs i.e. the principal and the main polarizabilities. First, we discuss the principal polarizabilities K o and z e for ordinary and extraordinary polarization, respectively. The LC molecules prefer an orientation along a mean direction. The mean direction is described by the director ! n . The principal polarizabilities z o and z e are d
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