Dynamics of Selective Reflections of Cholesteric Liquid Crystals Subject to Electric Fields

In this paper we report on the time behavior of the apparent pitch of a cholesteric mixture undergoing a step-like dc electric field applied parallel to the helical axis. We show that the relative variation of the apparent pitch AA(t)/& decreases from the initial value to negative ones (XC&,) by two different characteristic times and tends finally to a saturation value with Ml&,<0 after a long time. An attempt to explain this phenomenon in a (semi-)quantitative way has been made by considering relaxation mechanisms on different spatial scales.