Light-induced structural changes in chiral liquid crystals

Novel light-sensitive chiral dopants are studied as a light-sensitive component in chiral liquid crystals which may be used in tunable optical devices. Light-induced cis-trans- isomerization of chiral dopants results in changes of helical twisting power which translates into variations of helical pitch. Due to the light absorption in the liquid crystal cell the pitch variation is non-uniform across the cell, which leads, at first, to a deformation of cholesteric layers, and then to the formation of cholesteric bubbles. The sequence of structural changes has a distinct visual pattern and occurs at the surface close to the UV light source. Small deformations of cholesteric layers and bubbles are unstable and disappear after removing UV irradiation. The increasing size of the cholesteric bubbles results in better stability; large bubbles do not disappear after removing UV light. A theoretical model is suggested to describe the undulations of cholesteric layers.