Optical Studies on Grandjean Planes in Cholesteric Liquid Crystals

Abstract

Optical investigations were performed to prove the idea of variable pitch in the Grandjean structure of cholesteric liquid crystals. According to this model, the helical structure is deformed in such a manner that with decreasing thickness of the wedge-shaped layer the pitch of the screw decreases, while the number of turns remains constant. At the next “step”, however, the pitch gets large again and the number of turns decreases by a half. From measurements on the Grandjean structure with several derivatives of cholesterol, the values of the pitch are determined as a function of temperature and found to be in good agreement with values obtained from the wavelength at maximum reflection. This is considered as a confirmation of the model used, in particular of the assumption that the gap width is essentially equal to an integer multiple of the half pitch and the number of turns changes by one half at the optically observed discontinuities.