Optical Techniques in the Determination of Pitch Lengths in the Cholesteric and Chiral Smectic C Phases

Abstract

There is no denying that the future of liquid crystals is in the display industry. The use of the twisted nematic (TN) and the superwisted nematic (STN) is widespread. The twisting in these devices is achieved either by use of optical components such as polarizers or the use of chiral dopants in a guest-host system. The later has gained much more attention. This is because not only are the accompanying optical components not needed, but any desired colour can be achieved by varying the pitch length of the helix formed by the chiral nematic (cholesteric) compound. In such application, the ability to determine the pitches of the resulting helical superstructure accurately and quickly is essential. Currently there are many methods for measuring the pitch of a helical super structure. These methods mostly employ optical or spectroscopic techniques. In this paper, four such methods namely, Fingerprint, Diffraction, Selective Reflection and Cano-wedge, are discussed in terms of their theoretical foundation, suitability to given situations and the validity of the results obtained. The suitability of the method depends among other things on the type of mesophase and the length of the pitch in relation to the wavelengths of the visible light. While the Fingerprint and Selective reflection may be used for both cholesteric and chiral smectic C (SmC*), Diffraction and Cano techniques are most suitable for the cholesterics.