Optical Investigations of Chiral Liquid Crystals

Doctoral thesis, 2000

Optical investigations of course involve experimental work, but a theoretical basis is also needed. The first chapter can hopefully can give the non-expert an insight into the science of liquid crystals. Liquid crystals are optically birefringent, and a review of the phenomenon of birefringence follows in chapter 2. A wave equation is derived from Maxwell's equations and solved, thus relating birefringent properties to the dielectric tensor. This thesis mainly deals with spectrophotometer measurements, where the wavelength of the light is scanned, and the transmittance through the sample is registered. We have studied chiral smectic liquid crystals, with a helicoidal structure in some (the tilted) phases. The material washed between glass plates, or a free-standing film was prepared. The propagation of light coincides with the helix axis, and a reflex, "selective reflection", can be detected when the wavelength matches the helical pitch. To provide a firm theoretical basis for the interpretation of these experiments, a detailed theory of light propagation along helical axis is developed. The derivation is consistent with the previous work of Hl. de Vries, but the notation is somewhat different, and the theory is even slightly extended. An introduction to the 4X4 matrix method is also given. The use of this method is not pursued here, but since it is an important numerical method, something would be missing, if it were not included in a treatment of optical methods. Sample preparation and experimental methods are described. Spectrophotometer measurements, microscopy, including a special method with inclined incidence of light, and conoscopy are covered. Pitch measurements using light diffraction are also included. Results are presented in the included papers, and also discussed in the main text. These observations are related to the deformation andg of the helix, when an electric field is applied. When the helix of the ferroelectric phase is deformed, one can see additional reflexes corresponding to Fourier terms of the deformation. In the antiferroelectric phase, a widening of the reflection band has been observed, when a field is applied. Structure, optical properties and applications of antiferroelectric materials are also described and discussed.