ON THE OPTICS OF TWIST GRAIN-BOUNDARY SMECTICS

We have studied theoretically the optical properties of twist grain boundary smectics. We find many reflection bands even at normal incidence. In some of the reflection bands an incident light in any state of polarization gets strongly reflected while in some others the strongly reflected state is of a circular polarization with the same or the opposite handedness as that of the structure. At oblique incidence, depending upon the screw symmetry, a reflection band either has three sub-bands of different polarizations or is a single band of a particular polarization. We find optical diffraction for light incident perpendicular to the twist axis. The diffraction pattern is completely different for TGB(A) and TGB(C). In addition in absorbing TGB(C) the pattern can even become asymmetric. From a Fourier inversion of the complex diffracted amplitudes we can evaluate in some cases the sizes of the smectic blocks and the grain boundaries.