Causes for the Appearance of Fringes in Cholesterics, Williams Domains and Chiral Smectic C Liquid Crystals

Fringes observed in cholesterics (Ch), Williams domains (WD) and chiral smectic C liquid crystals (Sm C * ) were studied systematically. Generally the fringes are clearly focused as real and imaginary images on planes just above the upper surface and near the lower surface of the samples. Additional images, sometimes defocused, may also be observed inside the samples. Three causes for this have been considered: (1) wave surface distrotion, (2) ray deflection from the wave normal that occurs in anisotropic media and (3) the effect of optical neutral lines. The fringes in Ch are due to cause (1) and those in WD to causes (1) and (2). All the causes, especially (2) and (3) apply to Sm C * . The importance of (2) is shown systematically for the first time.