STRUCTURE AND OPTICAL BEHAVIOR OF CHOLESTERIC SOLITON LATTICES

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 diam agnetic anisotropy [chi(a) > 0] and a N-W soliton lattice for chi(a) < 0. We have also studied the optical reflection in such soliton lattic es. At fields close to the nematic cholesteric transition point the hi gher order reflections are more intense than the primary order and eac h Bragg band splits into three sub bands. In these regions the electro magnetic waves are very different as regards their polarizations, and angle between E and H. This is in contrast to the undistorted structur e where these waves are locally linearly polarized with E parallel H. In addition, optical diffraction pattern has been computed for propaga tion perpendicular to the twist axis. In the ferrocholesteric soliton lattice due to grain migration alone we predict a new type of diffract ion. This diffraction pattern for N-W lattice is very different from t hat of a pi soliton lattice.