Structure and optical behaviour of cholesteric soliton lattices

Abstract

We consider the elastic instability of a ferrocholesteric induced by a field. In a magnetic field acting perpendicular to twist axis, at low fields we get, a 2 $\pi$ soliton lattice. Above a threshold field, this will become unstable leading to a $\pi$ soliton lattice for positive diamagnetic anisotropy [ $\chi_{\rm a}>0$] and a N-W soliton lattice $\chi_{\rm a}<0$. We have also studied the optical reflection in such soliton lattices. At fields close to the nematic cholesteric transition point the higher order reflections are more intense than the primary order and each Bragg band splits into three sub bands. In these regions the electromagnetic waves are very different as regards their polarizations, and angle between E and H. This is in contrast to the undistorted structure where these waves are locally linearly polarized with E parallel H. In addition, optical diffraction pattern has been computed for propagation perpendicular to the twist axis. In the ferrocholesteric soliton lattice due to grain migration alone we predict a new type of diffraction. This diffraction pattern for N-W lattice is very different from that of a $\pi$ soliton lattice.