Lattice structure in liquid-crystal blue phase with various chiral concentrations

Abstract

Lattice structures, including reflection lattice planes and lattice constant, of liquid-crystal blue phase I (BPI) are studied via the measurements on reflection spectrum and Kossel diagram as concentration of a chiral dopant is changed. Peaks of the reflection wavelength in BPI are mainly dominated by the lattice plane and the lattice constant, which are affected by the chiral concentration. In the chiral nematic state, as decreasing the chiral concentration the reflection peak will shift to a longer wavelength because the helical pitch linearly depends on the chiral concentration and becomes longer. However, this dependence of the chiral concentration and reflection wavelength is broken in the BPI. The reflection peak of BPI moves to a short wavelength when the chiral concentration is less due to the contraction of the lattice constant as well as helical pitch. Moreover, when the concentration of the chiral dopant increases over a certain value, a discontinuous shift in reflection peak occurs due to the production of the different lattice planes. It means that the relationship between the chiral concentration and the helical pitch in BPI is not the same as it in the chiral nematic phase and should be reconsidered.