Kossel diagram of an aligned cholesteric phase

Abstract

The selective reflection of light by the cholesteric phase has been examined using the Kossel technique. The Kossel diagram observed for an aligned monodomain sample, viewed down the helicoidal axis, using crossed polarizers, consists of a single bright annulus. Within this annulus two features are visible---a bright central ring and a pattern of four spiral arcs. When the analyzer is rotated from the crossed to the uncrossed position, one diagonally opposite pair of arcs remains stationary and the other pair rotates around the ring until it becomes superimposed on the stationary pair. The handedness of the spiral pattern is related to the chirality of the phase and offers a direct way of experimentally determining the twist sense---a cholesteric phase with a right-handed twist giving a clockwise spiral pattern. The presence of the more intense central ring and the spiral ``extinction'' pattern are in accordance with the analytical modeling of the selective reflection of cholesteric phases by Oldano et al.